CHAPTER V.
ANCIENT GLACIERS IN THE WESTERN HEMISPHERE.
_New England._
In North America all the indubitable signs of glacial action are found over the entire area of New England, the southern coast being bordered by a double line of terminal moraines. The outermost of these appears in Nantucket, Martha's Vineyard, No Man's Land, Block Island, and through the entire length of Long Island--from Montauk Point, through the centre of the island, to Brooklyn, N. Y., and thence across Staten Island to Perth Amboy in New Jersey. The interior line is nearly parallel with the outer, and, beginning at the east end of Cape Cod, runs in a westerly direction to Falmouth, and thence southwesterly through Wood's Holl, and the Elizabeth Islands--these being, indeed, but the unsubmerged portions of the moraine. On the mainland this interior line reappears near Point Judith, on the south shore of Rhode Island, and, running slightly south of west, serves to give character to the scenery at Watch Hill, and thence crops out in the Sound as Fisher and Plum Islands, and farther west forms the northern shore of Long Island to Port Jefferson.
In these accumulations bordering the southern shore of New England, the characteristic marks of glacial action can readily be detected even by the casual observer, and prolonged examination will amply confirm the first impression. The material of which they are composed is, for the most part, foreign to the localities, and can be traced to outcrops of rock at the north. The boulders scattered over the surface of Long Island, for example, consist largely of granite, gneiss, hornblende, mica slate, and red sandstone, which are easily recognised as fragments from well-known quarries in Connecticut, Rhode Island, and Massachusetts; yet they have been transported bodily across Long Island Sound, and deposited in a heterogeneous mass through the entire length of the island. Not only do they lie upon the surface, but, in digging into the lines of hills which constitute the backbone of Long Island, these transported boulders are found often to make up a large part of the accumulation. Almost any of the railroad excavations in the city of Brooklyn present an interesting object-lesson respecting the composition of a terminal moraine.
All these things are true also of the lines of moraine farther east, as just described. Professor Shaler has traced to its source a belt of boulders occurring extensively over southern Rhode Island, and found that they have spread out pretty evenly over a triangular area to the southward, in accordance with the natural course to be pursued by an ice-movement. Nearly all of Plymouth County, in southeastern Massachusetts, is composed of foreign material, much of which can be traced to the hills and mountains to the north. Even Plymouth Rock is a boulder from the direction of Boston, and the "rock-bound" shores upon which the Pilgrims are poetically conceived to have landed are known, in scientific prose, as piles of glacial rubbish dumped into the edge of the sea by the great continental ice-sheet.
The whole area of southeastern Massachusetts is dotted with conical knolls of sand, gravel, and boulders, separated by circular masses of peat or ponds of water, whose origin and arrangement can be accounted for only by the peculiar agency of a decaying ice-front. Indeed, this whole line of moraines, from the end of Cape Cod to Brooklyn, N. Y., consists of a reticulated network of ridges and knolls, so deposited by the ice as to form innumerable kettle-holes which are filled with water where other conditions are favourable. Those which are dry are so because of their elevation above the general level, and of the looseness of the surrounding soil; while many have been filled with a growth of peat, so that their original character as lakelets is disguised.
As already described, these depressions, so characteristic of the glaciated region, are, in the majority of cases, supposed to have originated by the deposition of a great quantity of earthy material around and upon the masses of ice belonging to the receding front of the glacier, so that, when at length the ice melted away, a permanent depression in the soil was left, without any outlet.
To some extent, however, the kettle-holes may have been formed by the irregular deposition of streams of water whose courses have crossed each other, or where eddies of considerable force have been produced in any way. The ordinary formation of kettle-holes can be observed in progress on the foot of almost any glacier, or, indeed, on a small scale, during the melting away of almost any winter's snow. Where, from any cause, a stratum of dirt has accumulated upon a mass of compact snow or ice, it will be found to settle down in an irregular manner; furrows will be formed in various directions by currents of water, so that the melting will proceed irregularly, and produce upon a miniature scale exactly what I have seen on a large scale over whole square miles of the decaying foot of the great Muir Glacier in Alaska. The effects of similar causes and conditions we can see on a most enormous scale in the ten thousand lakes and ponds and peat-bogs of the whole glaciated area both in North America and in Europe.
In addition to these two lines of evidence of glacial action in New England, we should mention also the innumerable glacial grooves and scratches upon the rocks which can be found on almost any freshly uncovered surface. In New England the direction of these grooves is ordinarily a little east of south. Upon the east coast of Massachusetts and New Hampshire the scratches trend much more to the east than they do over most of the interior. This is as it should be on the glacial theory, since the ice would naturally move outwards in the line of least resistance, which would, of course, be towards the open sea wherever that is near. In the interior of New England the scratches upon the rocks indicate a more southerly movement in the Connecticut Valley than upon the mountains in the western part of Massachusetts. This also is as it should be upon the glacial theory. The scratches upon the mountains were made when the ice was at its greatest depth and when it moved over the country in comparative disregard of minor irregularities of surface, while in the valleys, at least in the later portion of the Ice age, the movement would be obstructed except in one direction. In the interpretation of the glacial grooves and scratches it should be borne in mind that they often represent the work done during the closing stages of the period. Just as the last shove of the carpenter's plane removes the marks of the previous work, so the last rasping of a glacial movement wears away the surfaces which have been previously polished and striated.
In various places of New England it is interesting as well as instructive to trace the direction of the ice-movement by the distribution of boulders. My own attention was early attracted to numerous fragments of gneiss in eastern Massachusetts containing beautiful crystals of feldspar, which proved to be peculiar to the region of Lake Winnepesaukee, a hundred miles to the north, and to a narrow belt stretching thence to the southwestward. In ascending almost any of the lower summits of the White Mountains one's attention can scarcely fail of being directed to the difference between the material of which the mountains are composed and that of the numerous boulders which lie scattered over the surface. The local geologist readily recognises these boulders as pilgrims that have wandered far from their homes to the northward.
Trains of boulders, such as those already described in Rhode Island, can frequently be traced to some prominent outcrop of the rock in a hill or mountain-peak from which they have been derived. One of the earliest of these to attract attention occurs in the towns of Richmond, Lenox, and Stockbridge, in the western part of Massachusetts. Here a belt of peculiar boulders about four hundred feet wide is found to originate in the town of Lebanon, N. Y., and to run continuously to the southeast for a distance of nine miles. West of Fry's Hill, where the outcrop occurs, no boulders of this variety of rock are to be found, while to the southeast the boulders gradually diminish in size as their distance from the outcrop increases. Near the outcrop boulders of thirty feet in diameter occur, while nine miles away two feet is the largest diameter observed.
Sir Charles Lyell endeavoured to explain this train of boulders by the action of icebergs during a period of submergence--supposing that, as icebergs floated past or away from this hill in Lebanon, N. Y., they were the means of the regular distribution described. It is needless to repeat the difficulties arising in connection with such a theory, since now both by observation and experiment we have become more familiar with the movement of glacial ice. What we have already said about the transportation of boulders over Switzerland by the Alpine glaciers, and what is open to observation at the present time upon the large glaciers of Alaska, closely agree with the facts concerning this Richmond train of boulders, and we have no occasion to look further for a cause.
Indeed, trains of boulders ought to appear almost everywhere over the glaciated area; and so they do where all the circumstances are favourable. But, readily to identify the train, requires that to furnish the boulders there should be in the line of the ice-movement a projecting mass of rock hard enough to offer considerable resistance to the abrading agency of the ice and characteristic enough in its composition to be readily recognised. Ship Rock, in Peabody, Mass., weighing about eleven hundred tons, and Mohegan Rock, in Montville, Conn., weighing about ten thousand tons, have ordinarily been pointed to as boulders illustrating the power of ice-action. Their glacial character, however, has been challenged from the fact that the variety of granite to which they belong occurs in the neighbourhood, and indeed constitutes the bed-rock upon which they rest.[AV] Some would therefore consider them, like some of which we have already spoken, to be boulders which have originated through the disintegration of great masses of rock, of which these were harder nuclei that have longer resisted the ravages of the tooth of time. It must be admitted that possibly this explanation is correct; but it is scarcely probable that, in a region where there are so many other evidences of glacial action, these boulders could have remained immovable in presence of the onward progress of the ice-current that certainly passed over them.
[Footnote AV: Popular Science Monthly, vol. xxxvii, pp. 196-201.]
However, as already seen, we are not left to doubt as to the movement of some boulders of great size. That which now claims the reputation of being the largest in New England is in Madison, N. H., and measures thirty by forty by seventy-five feet. This can be traced to ledges of Conway granite, about two miles away.[AW] Many boulders in the vicinity of New Haven, Conn., can be identified, as from well-known trap-dykes, sixteen miles or more to the north. The so-called Judge's Cave, on West Rock, 365 feet above the adjoining valley and weighing a thousand tons, is one of these. Professor Edward Orton[AX] describes a mass of Clinton limestone near Freeport, Warren County, Ohio, as covering an area of three-fourths of an acre, and as sixteen feet in thickness. It overlies glacial clays and gravels, and must have been transported bodily from the elevations containing this rock several miles to the northwest.
[Footnote AW: See W. 0. Crosby's paper in Appalachia, vol. vi, pp. 59-70.]
[Footnote AX: Geological Survey of Ohio, vol. iii, p. 385,]
Portions of New England present the best illustrations anywhere afforded in America of what are called "drumlins." These are "lenticular-shaped" hills, composed of till, and containing, interspersed through their mass, numerous scratched stones of all sizes. They vary in length from a few hundred feet to a mile, and are usually from half to two-thirds as wide as they are long. In height they vary from twenty-five to two hundred feet.
But, according to the description of Mr. Upham, whatever may be their size and height, they are singularly alike in outline and form, usually having steep sides, with gently sloping, rounded tops, and presenting a very smooth and regular contour. From this resemblance in shape to an elliptical convex lens, Professor Hitchcock has called them lenticular hills to distinguish these deposits of till from the broadly flattened or undulating sheets which are common throughout New England.
The trend, or direction of the longer axis, of these lenticular hills is nearly the same for all of them comprised within any limited area, and is approximately like the course of the striæ or glacial furrows marked upon the neighbouring ledges. In eastern Massachusetts and New Hampshire, within twenty-five miles of the coast, it is quite uniformly to the southeast, or east-southeast. Farther inland, in both of these States, it is generally from north to south, or a few degrees east of south; while in the valley of the Connecticut River it is frequently a little to the west of south. In New Hampshire, besides its accumulation in these hills, the till is frequently amassed in slopes of similar lenticular form. These have their position almost invariably upon either the south or north side of the ledgy hills against which they rest, showing a considerable deflection towards the southeast and northwest in the east part of the State. It cannot be doubted that the trend of the lenticular hills, and the direction taken by these slopes, have been determined by the glacial current, which produced the striæ with which they are parallel.[AY]
[Footnote AY: Proceedings of the Boston Society of Natural History, vol. xx, pp. 224, 225.]
Drumlins are abundant in the vicinity of Boston, and constitute nearly all the islands in Boston Harbour. On the mainland, Beacon Hill, Bunker Hill, Green Hill, Powderhorn Hill, Tufts College Hill, Winter Hill, Mount Ida, Corey Hill, Parker Hill, Wollaston Heights, Prospect Hill, and Telegraph Hill are specimens.
The northeastern corner of Massachusetts and the southeastern corner of New Hampshire are largely covered with these peculiar-shaped glacial deposits, while they are numerous as far west as Fitchburg, in Massachusetts, and Ware, N. H., and in the northeastern part of Connecticut. A little later, also, we shall refer to an interesting line of them in central New York. Elsewhere in America, except in a portion of Wisconsin, they rarely occur in such fine development as in New England. In Europe they are best developed in portions of Ireland.
One's first impression in examining an exposed section of a drumlin would lead him to think that the mass was entirely unstratified; but closer examination shows that there is a coarse stratification, but evidently not produced by water-action. The accumulation has probably taken place gradually by successive deposits underneath the glacier itself. Professor William M. Davis has suggested a plausible explanation which we will briefly state.
The frequency with which drumlins are found to rest upon a mass of projecting rock, the general co-ordination of the direction of their axes with the direction of the scratches upon the underlying rock, and the abundance of scratched stones in them, all support the theory that drumlins are formed underneath the ice-sheet, somewhat in the way that islands and bars of silt are formed in the delta of a great river. The movement of ice seems to have been concentrated in pretty definite lines, often determined by the contour of the bottom, leaving a slacker movement in intervening areas, which were evidently protected in some cases by projecting masses of rock. In these areas of slower movement there was naturally an accumulation at the same time that there was vigorous erosion in the lines of more rapid movement.
There was doubtless a continual transfer of material from the end of the drumlin which abutted against the moving mass of ice to the lower end, as there is in the formation of an island in a river. If time enough had elapsed, the whole accumulation would have been levelled by the glacier and spread over the broader area where the more rapid lines of movement became confluent, and where the differential motion was less marked. Drumlins are thus characteristic of areas in the glaciated region whose floor was originally only moderately irregular, and where there was an excessive amount of ground-moraine to be transported, and where the movement did not continue indefinitely. It has been suggested, also, that some of the long belts of territory in New England and central New York covered by drumlins may represent old terminal moraines which were subsequently surmounted by a readvance of the ice, and partially wrought over into their present shape.
It is in New England, also, that kames are to be found in better development than anywhere else in America. These interesting remnants of the Glacial age are clearly described by Mr. James Geikie. His account will serve as well for New England as for Scotland.
The sands and gravels have a tendency to shape themselves into mounds and winding ridges, which give a hummocky and rapidly undulating outline to the ground. Indeed, so characteristic is this appearance, that by it alone we are often able to mark out the boundaries of the deposits with as much precision as we could were all the vegetation and soil stripped away and the various subsoils laid bare. Occasionally, ridges may be tracked continuously for several miles, running like great artificial ramparts across the country. These vary in breadth and height, some of the more conspicuous ones being upward of four or five hundred feet broad at the base, and sloping upward at an angle of twenty-five or even thirty-five degrees, to a height of sixty feet and more above the general surface of the ground. It is most common, however, to find mounds and ridges confusedly intermingled, crossing and recrossing each other at all angles, so as to enclose deep hollows and pits between. Seen from some dominant point, such an assemblage of kames, as they are called, looks like a tumbled sea--the ground now swelling into long undulations, now rising suddenly into beautiful peaks and cones, and anon curving up in sharp ridges that often wheel suddenly round so as to enclose a lakelet of bright clear water.[AZ]
[Footnote AZ: The Great Ice Age, pp. 210, 211.]
In New England attention was first directed to kames in 1842, by President Edward Hitchcock, in a paper before the American Association of Geologists and Naturalists, describing the gravel ridges in Andover, Mass. In the accompanying plate is shown a portion of this kame system, which has a double interest to me from the fact that it was while living upon the banks of the Shawshin River, near where the kames and the river intersect, that I began, in 1874, my special study of glacial deposits. The Andover ridges are composed of imperfectly stratified water-worn material, and are very sharply defined, from the town of Chelsea, back from the coast into New Hampshire, for a distance of twenty-five miles. The base of the ridges does not maintain a uniform level, but the system descends into shallow valleys, and rises over elevations of one hundred to two hundred feet, without interruption. This indifference to slight changes of level is specially noticeable where the system crosses the Merrimac River, just above the city of Lawrence. It is also represented in the accompanying plate, where the base of the ridges in the immediate valley of the Shawshin is fifty feet lower than the base of those a short distance to the north, at the points marked _a_, _b_, and _c_. The ridges here terminate at the surface in a sharp angle, and are above their base forty-one feet at _a_, forty-nine feet at _b_, and ninety-one feet at _c_. Between _c_ and _b_ there is an extensive peat-swamp, filling the depression up to the level of an outlet through which the surplus water has found a passage.
Several systems of kames approximately parallel to this have been traced out in Massachusetts and New Hampshire, while the remnants of a very extensive system are found in the Connecticut Valley above the Massachusetts line. But they abound in greatest profusion in the State of Maine, where Professor George H. Stone has plotted them with much care. The accompanying map gives only an imperfect representation of the ramifying systems which he has traced out, and of the extent to which they are independent of the present river-channels. One of the longest of these extends more than one hundred miles, crossing the Penobscot River nearly opposite Grand Lake, and terminating in an extensive delta of gravel and sand in Cherryfield, nearly north of Mount Desert. This is represented on our map by the shaded portion west of the Machias River. Locally these ridges are variously designated as "horsebacks," "hogbacks," or "whalebacks," but that in Andover, Mass., was for some reason called "Indian Ridge." Nowhere else in the world are these ridges better developed than in New England, except it be in southern Sweden, where they have long been known and carefully mapped.
The investigations of Mr. W. 0. Crosby upon the composition of till in eastern Massachusetts is sufficiently important in its bearings upon the question of glacial erosion to merit notice at this point.[BA] The object of his investigations was to determine how much of the so-called ground moraine, or till, consisted of material disintegrated by mechanical action, and how much by chemical action. The "residuary clay," which has arisen from chemical decomposition, would properly be attributed to the disintegrating agencies of preglacial times, while the clay, which is strictly mechanical in its origin, remains to represent the true "grist" or "rock flour" of the Glacial period.
[Footnote BA: Proceedings of the Boston Society of Natural History, vol. xxv (1890), pp. 115-140.]
The results of Mr. Crosby's investigations show that "not more than one-third of the _detritus_ composing the till of the Boston Basin was in existence before the Ice age, and that the remaining two-thirds must be attributed to the mechanical action of the ice-sheet and its accompanying torrents of water. In other words, if we assume the average thickness of the drift as thirty feet, the amount of glacial erosion can scarcely fall below twenty feet. After scraping away the residuary clays and half-decomposed material, the ice-sheet has cut more than an equal depth into the solid rocks."
Mr. Crosby's investigations also convinced him that the movement of the till, or ground moraine, underneath the ice was not _en masse_, but that "it must have experienced differential horizontal movements or flowing, in which, normally, every particle or fragment slipped or was squeezed forward with reference to those immediately below it, the velocity diminishing downward through the friction of the underlying ledges.... The glaciation was not limited to masses which were firmly caught between the ice and the solid ledges, and it was in every case essentially a slipping and not a rolling movement.... These differential horizontal movements mean that the till acted as a lubricant for the ice-sheet; and the clayey element, especially, co-operating in many cases with the pent-up subglacial waters, must have greatly facilitated the onward progress of the ice." He concludes, therefore, that the onward movement of the vast ice-sheet greatly exceeded that of the main part of the ground moraine, the ice-sheet slipping over the till, the whole being in some degree analogous to that of a great land-slip. "In both cases the progress of a somewhat yielding and mobile mass is facilitated by an underlying clayey layer saturated with water."
_New York, New Jersey, and Pennsylvania._
West of New England the glacial phenomena over the northern part of the United States are equally marked all the way to the Missouri River, and the boundary-line of the glaciated region can be traced with little difficulty. It emerges from New York Bay on Staten Island and enters New Jersey at Perth Amboy. A well-formed moraine covers the northern part of Staten Island, and upon the mainland marks the boundary from Perth Amboy, around through Raritan, Plainfield, Chatham, Morris, and Hanover, to Rockaway, and thence in a southwesterly direction to Belvidere, on the Delaware River. That portion of New Jersey lying north of this serpentine line of moraine hills is characterised by the presence of transported boulders, by numerous lakes of evident glacial origin, and by every other sign of glacial action, while south of it all these peculiar characteristics are absent. The observant passenger upon the railroad trains between New York and Philadelphia can easily recognise the moraine as it is passed through on the Pennsylvania Railroad at Metuchen and on the Bound Brook Railroad at Plainfield. Near Drakestown, in Morris County, there is a mass of blue limestone measuring, as exposed, thirty-six by thirty feet, and which was quarried for years before discovering that it was a boulder brought with other drift material from many miles to the northwest and lodged here a thousand feet above the sea.
Across Pennsylvania the glacial boundary passes through Northampton, Monroe, Luzerne, Columbia, Sullivan, Lycoming, Tioga, and Potter Counties, where it enters the State of New York, running still in a northwest direction through Allegany and Cattaraugus Counties to the vicinity of Salamanca. Here it turns to the south nearly at a right angle, running southwestward to Chautauqua County and re-entering Pennsylvania in Warren County, and thence passing onward in the same general direction through Crawford, Venango, Mercer, Butler, and Lawrence Counties to the Ohio line in Columbiana County, about ten miles north of the Ohio River.
The occurrence of a well-defined terminal moraine to mark the glacial boundary eastward from Pennsylvania led Professor Lewis and myself, who made the survey of that State in 1880, to be rather too sanguine in our expectations of finding an equally well-marked moraine everywhere along the southern margin of the glaciated area; still, the results are even more interesting than would have been the exact fulfilment of our expectations, since they more fully revealed to us the great complexity of effect which is capable of being brought about by ice-action. Before proceeding farther with the details, therefore, it will be profitable at this point to pause in the narrative and briefly record a few generalisations that have forced themselves into prominence during the years in which field-work has been in progress.
Previous to our explorations in Pennsylvania it had been thought that the indications of ice-action would extend much farther south in the valleys than on the mountains, and this indeed would have been the case if the glaciers in northern Pennsylvania had been of local origin; but our experience very soon demonstrated that the great gathering-place of the snows which produced the glacial movement in northern Pennsylvania could not have been local, but that over the northern part of that State there was distinct evidence of a continental movement of ice whose centre was far beyond the Alleghanies.
For example, we found that the evidences of direct glacial action extended farther south upon the hills and plateaus than they did in the narrow valleys, while everywhere on the very southern border of glacial indications we found boulders that had been brought from the granitic region of northern New York or central Canada. In eastern Pennsylvania we found indeed a terminal moraine more or less distinctly marking the southern border over the highlands. This was more specially true in Northampton and Monroe Counties.
In Northampton County it was very interesting to see long lines of hills, a hundred or more feet in height and lying several hundred feet above the Delaware River, composed entirely of glacial _débris_, much of which had been brought bodily over the sharp summit of the Blue Ridge, or Kittatinny Mountain, which rises as a continuous wall to the northwest and is everywhere several hundred feet higher than the moraine in Northampton County. The summit of Blue Ridge, also, as far south as the glacial movement extended, shows evident signs of glacial abrasion, some hundreds of feet evidently having been removed by that means, leaving a well-defined shoulder, marking the limits of its southwestern border. Resting upon the summit of the glaciated portion of the Blue Ridge, there are also numerous boulders of Helderberg limestone, which must have been brought from ledges at least five hundred feet lower than the places upon which they now lie.
In Monroe County the terminal moraine marking there the extreme limit of the ice-movement is upon an extensive plateau of Pocono sandstone, about eighteen hundred feet above sea-level, and five or six hundred feet lower than the crest of the Alleghany Mountains, a short distance to the north. The moraine hills are here well marked by the occurrence of circular lakelets and kettle-holes (such as have been described as characteristic of the shores and islands bordering the south of New England); by the occurrence of granitic boulders, which must have been brought from the Adirondacks or Canada; and by the various other indications referred to on a previous page.
As already intimated, the instructive point in our observations is the fact that, between Kittatinny Mountain, in Northampton County, and Pocono plateau, in Monroe County, there is a longitudinal depression, running northeast by southwest, parallel with the ranges of the mountain system, which is here about a thousand feet below the respective ridges on either side. This, therefore, is one of the places where we should have expected a considerable southern extension of the ice, if it had been largely due to local causes. Now, while there is indeed a gradual southern trend down the flanks of the mountain, yet, upon reaching the axis of the valley, there appears at once a very marked change in the character of the deposit, and the influence of powerful streams of water becomes manifest, and it is evident, upon a slight inspection, that we have come upon a line of drainage which sustained a peculiar relation to the continental ice-sheet.
From Stroudsburg, near the Delaware Water-Gap, to Weissport, on the Lehigh River, a distance of about thirty miles, the valley between the mountains is continuous, and the elevation at each end very nearly the same. But about half-way between the two places, near Saylorsburg, there is a river-parting from which the water now runs on the one hand north to Stroudsburg, and thence to the Delaware River, and on the other hand south, through Big and Aquonchichola Creeks, to the Lehigh River. The river-parting is formed by a great accumulation of gravel, whose summit is about two hundred feet above the level of the valleys into which the creeks empty at either end; and there are numerous kettle-holes and lakelets in the vicinity, such as characterize the glacial region in general.
In short, we are, without doubt, here on a well-marked terminal moraine much modified by strong water-action in a valley of glacial drainage. The gravel and boulders are all well water-worn, and the material is of various kinds, including granite boulders from the far north, such as characterise the terminal moraine on the highlands; but the pebbles are not scratched, and the gravel is more or less stratified. It is evident that we are here where a violent stream of water poured forth from that portion of the ice-front which filled this valley, and which found its only outlet in the direction of the Lehigh River. The gravel can be traced in diminishing quantities to the southward, in accordance with this theory, while to the northward there extends a series of gravel ridges, or kames, such as we have shown naturally to owe their origin to the accumulations taking place in ice-channels formed near the front of a glacier as it slowly melts away.
From similar occurrences of vast gravel accumulations in other valleys stretching southward from the glacial margin, we came to expect that, wherever there was an open, line of drainage from the glaciated region southward, the point of intersection between the glacial margin and the drainage valley would be marked by an excessive accumulation of water-worn gravel, diminishing in coarseness and abundance down the valleys in proportion to the distance from the glacial margin.
For example, the Delaware River emerges from the glaciated region at Belvidere, and there are there vast accumulations of gravel rising a hundred or more feet above the present level of the river, while gravel terraces, diminishing in height, mark the river below to tide-water at Trenton. The Lehigh River leaves the glaciated region at Hickory Run, a few miles above Mauch Chunk, but the gorge is so steep that there was little opportunity either for the accumulation of gravel there or for its preservation. Still, the transported gravel and boulders characteristic of the melting floods pouring forth from a glacier, are found lining the banks of the Lehigh all along the lower portion of its course. In the Susquehanna River we have a better example at Beach Haven, in Luzerne County, where there are very extensive accumulations of gravel resting on the true glacial deposits of the valley, and extending down the river in terraces of regularly diminishing height for many miles, and merging into terraces of moderate elevation which line the Susquehanna Valley throughout the rest of its course. Above Beach Haven the gravel deposits in the trough of the river valley are more irregular, and betray the modifying influence of the slowly decaying masses of ice which belonged to the enveloping continental glacier.
Westward from the north fork of the Susquehanna, similar extensive accumulations of gravel occur at the intersection of Fishing Creek in Columbia County, Muncy, Loyalsock, Lycoming, and Pine Creeks in Lycoming County, all tributary to the Susquehanna River, and all evidently being channels through which the melting floods of the ice-sheet brought vast quantities of gravel down to the main stream. Williamsport, on the West Branch of the Susquehanna, is built upon an extensive terrace containing much granitic material, brought down from the glaciated region by Lycoming Creek, when it was flooded with the waters melted from the continental ice-sheet which had here surmounted the Alleghanies and invaded the valley of the Susquehanna.
Analogous deposits of unusual amounts of gravel, occurring in streams flowing southward from the glaciated region, occur at Great Valley, Little Valley, and Steamburg in Cattaraugus County, New York, and at Russelburg and Garland in Warren County, Pennsylvania, also at Titusville and Franklin in Venango County, and at Wampum in Lawrence County, of the same State.
As a rule, Professor Lewis and myself found it more difficult to determine with accuracy the exact point to which the ice extended in the axis of these south-flowing valleys than we did upon the highlands upon either side; and, in looking for the positive indications of direct ice-action in these lines of drainage, we were almost always led up the valley to a considerable distance inside of the line. This arose from our inexperience in interpreting the phenomena, or rather from our inattention to the well-known determining facts in the problem. On further reflection it readily appeared that this was as it should be. The ice-front, instead of extending farther down in a narrow valley than on the adjoining highlands (where they are of only moderate elevation) ought not to extend so far, for the subglacial streams would not only wear away the ice of themselves, but would admit the air into the tunnels formed by them so as to melt the masses both from below and from above, and thus cause a recession of the front. If we had understood this principle at the beginning of our survey, it would have saved us much perplexity and trouble.
A single further illustration of this point will help to an understanding of many references which will hereafter be made to the water deposits which accumulated in the lines of drainage running southward from the glaciated area. At Warren, Pa., Conewango Creek, which is the outlet from Chautauqua Lake, enters the Alleghany River after flowing for a number of miles in a deep valley with moderate slopes. In ascending the creek from Warren, the gravel terraces, which rise twenty-five or thirty feet above high-water mark, rapidly increase in breadth and height, and the pebbles become more and more coarse. After a certain distance the regular terraces begin to give place to irregular accumulations of gravel in ridges and knobs. In the lower portion of the valley no pebbles could be found which were scratched. Up the valley a few miles pebbles were occasionally discovered which showed some slight indications of having been scratched, but which had been subjected to such an amount of abrasion by water-action as almost to erase the scratches. On reaching Ackley's Station, the stream is found to be cutting through a regular terminal moraine, extending across the valley and full of clearly marked glaciated stones. Above this terminal moraine the terraces and gravel ridges which had characterised the valley below disappear, giving place to long stretches of level and swampy land, which had been subject to overflow.
Something similar to this so often appears, that there can be no question as to its meaning, which is, that during the farthest extent of the ice the front rested for a considerable period of time along the line marked by the terminal moraine. During this period there occurred both the accumulation of the moraine and of the gravel terraces in the valley below, due to the vast flow of water emerging from the ice-front, especially during the period when it was most rapidly melting away. Upon the retreat of the ice, the moraine constituted a dam which has not yet been wholly worn away. For a while the water was so effectually ponded back by this as to form a lake, which has since become filled up with sediment and accumulations of peat. From this it is evident, also, that when the ice began to retreat, the retreat was so continuous and rapid that no parallel terminal moraines were formed for many miles.
Before leaving this section we will summarise the leading facts concerning the glacial phenomena north of Pennsylvania and New Jersey. From the observations of Professor Smock, it appears that, from the southern margin the ascent to the summit of the ice-sheet was pretty rapid; the depth one mile back from the margin being not much less than a thousand feet. "Northward the angle of the slope diminished, and the glacier surface approximated to a great level plain. The distance between the high southwestern peaks of the Catskills and Pocono Knob in Pennsylvania is sixty miles. The difference in the elevation of the glacier could not have exceeded a thousand feet,"[BB] that is, the slope of the surface was about seventeen feet to the mile.
[Footnote BB: American Journal of Science, vol. cxxv, 1883, p. 339 _et seq._]
Professor Dana estimates the thickness of the ice in southern Connecticut to have been between fifteen hundred and two thousand feet. Attempts to calculate the thickness of the ice farther north, except from actual discovery of glacial action on the summits of the mountains, are based upon uncertain data with reference to the slope necessary to secure glacial movement. In the Alps the lowest mean slopes down which glaciers move are about two hundred and fifty feet to a mile; but in Greenland, Jensen found the slope of the Frederickshaab Glacier to be only seventy-five feet to the mile, while Helland found that of the Jakobshavn Glacier to be only forty-five feet.
It is doubtful if even that amount is necessary to secure a continental movement of ice, since, as already remarked, it is unsafe to draw inferences concerning the movements of large masses of ice from those of smaller masses in more constricted areas. We have seen, from the glacial deposits on the top of Mount Washington, that over the northern part of New England the ice was more than a mile in depth. We have no direct evidence of the depth of the stream which surrounded the Adirondack Mountains. Nor, on the other hand, are we certain that the Catskills were not completely enveloped in ice, though most observers, reasoning from negative evidence, have supposed that to be the case. But from the facts stated concerning the boulders along the glacial boundary in Pennsylvania, it is certain that the ice was deep enough to surmount the ridge of the Alleghanies where they are two thousand and more feet in height. At the least calculation the ice must have been five hundred feet thick, in order to secure the movement of which there is evidence across the Appalachian range. Supposing this to be the height of the ice above the sea on the crest of the Alleghanies, and that the slope of the surface of the ice-sheet was as moderate as Professor Smock has estimated it (namely seventeen feet to the mile), the ice would be upwards of six thousand feet in thickness in the latitude of the Adirondacks, which corresponds closely with the positive evidence Ave have from the mountains in New England.
A study of the map of New York will make it easy to understand the distribution of some interesting glacial marks over the State. The distance along the Hudson from the glacial boundary in the vicinity of New York to the valley of the Mohawk is about one hundred and sixty miles. Prom the glacial boundary at Salamanca, N. Y., to the same valley, is not over eighty miles. It is easy to see, therefore, that when, in advancing, the ice moved southward past the Adirondacks, the east end of the valley of the Mohawk was reached and closed by the ice, while at the west end of Lake Ontario the ice-front was still in Canada. Thus the drainage, which naturally followed the course of the St. Lawrence, would first be turned through the Mohawk. Afterwards, when the Mohawk had been closed by ice, the vast amount of ponded water was compelled to seek a temporary outlet over the lower passages leading into the Susquehanna or into the Alleghany.
A number of such passages exist. One can be traced along the line of the old canal from Utica to Binghamton, whose highest level is not far from eleven hundred feet. Another lies in a valley leading south of Cayuga Lake, whose highest point, at Wilseyville, is nine hundred and forty feet above tide. Another leads south to the Chemung River from Seneca Lake, whose highest point, at Horseheads, is less than nine hundred feet above tide. The cols farther west are somewhat more elevated; the one at Portage, leading from the Genesee River into the Canisteo, being upwards of thirteen hundred feet, and that of Dayton, leading from Cattaraugus Creek into the Conewango, being about the same. Of other southern outlets farther west we will speak later on.
Fixing our minds now upon the region under consideration, in the southern part of the State of New York, we can readily see that a glacial lake must have existed in front of the ice while it was advancing, until it had reached the river-partings between the Mohawk and the St. Lawrence Rivers on the north and the Susquehanna and Alleghany Rivers on the south. After the ice had attained its maximum extension, and was in process of retreat, there would be a repetition of the phenomena, only they would occur in the reverse order. The glacial markings which we see are, of course, mainly those produced during the general retreat of the ice.
The Susquehanna River stretching out its arms--the Chenango and Chemung Rivers--to the east and the west, evidently serves as a line of drainage for the vast glacial floods. These floods have left, along their courses, extensive elevated gravel terraces, with much material in them which is not local, but which has been washed out of the direct glacial deposits from the far north. The east-and-west line of the water-parting throughout the State is characterised by excessive accumulations of glaciated material, forming something like a terminal moraine, and is designated by President Chamberlin as "the terminal moraine of the second Glacial epoch," corresponding, as he thinks, to the interior line already described as characterising the south shore of New England.
In the central part of New York the remarkable series of "Finger Lakes," tributary to Lake Ontario and emptying into it through the Oswego and Genesee Rivers, all have a glacial origin. Probably, however, they are not due in any great degree to glacial erosion, but they seem to occupy north-and-south valleys which had been largely formed by streams running towards the St. Lawrence when there was, by some means (probably through the Mohawk River), a much deeper outlet than now exists, but which has been filled up and obliterated by glacial _débris_. The ice-movement naturally centred itself more or less in these north-and-south valleys, and hence somewhat enlarged them, but probably did not deepen them. The ice, however, did prevent them from becoming filled with sediment, and on its final retreat gave place to water.
Between these lakes and Lake Ontario, also, and extending east and west nearly all the way from Syracuse to Rochester, there is a remarkable series of hills, from one hundred to two or three hundred feet in height, composed of glacial _débris_. But while the range extends east and west, the axis of the individual hills lies nearly north and south. These are probably remnants of a morainic accumulation which were made during a pause in the first advance of the ice, and were finally sculptured into their present shape by the onward movement of the ice. These are really "drumlins," similar to those already described in northeastern Massachusetts and southeastern New Hampshire. In the valley of central New York these have determined the lines of drainage of the "Finger Lakes," and formed dams across the natural outlets of nearly all of them.
North of the State of New York the innumerable lakes in Canada are all of glacial origin, being mostly due to depressions of the nature of kettle-holes, or to the damming up of old outlets by glacial deposits. A pretty well-marked line of moraine hills, formed probably as terminal deposits in the later stages of the Ice age, runs from near the eastern end of Lake Ontario to the Georgian Bay, passing south of Lake Simcoe.
_The Mississippi Basin._
The physical geography of the glaciated region north of the Ohio River is so much simpler than that of New England and the Middle States, that its characteristics can be briefly stated. Ohio, Indiana, and Illinois are covered with nearly parallel strata of rock mostly of the Carboniferous age. In general, the surface slopes gently to the west; the average elevation of Ohio being about a thousand feet above tide, while that of the Great Lakes to the north and of the middle portion of the Mississippi Valley is less than six hundred feet. The glacial deposits are spread in a pretty even sheet over the area which was reached by the ice in these States, and the lines of moraine, of which a dozen or more have been partially traced in receding order, are much less clearly marked than they are in New England, or in Michigan, and the States farther to the northwest.
The line marking the southern limit attained by the ice of the Glacial period in these three States is as follows: Entering Ohio in Columbiana County, about ten miles north of the Ohio River, the glacial boundary runs westward through New Lisbon to Canton in Stark County, and thence to Millersburg in Holmes County. A few miles west of this place it turns abruptly south, passing through Danville in Knox County, Newark in Licking County, Lancaster in Fairfield County, to Adelphi in Ross County. Thence bearing more westward it passes through Chillicothe to southeastern Highland County and northwestern Adams, reaching the Ohio River near Ripley, in Clermont County. Thence, following the north bank of the Ohio River to Cincinnati, it crosses the river, and after extending through the northern part of Boone County, Kentucky, and recrossing the river to Indiana, not far from Rising Sun, it again follows approximately the north bank of the river to within about ten miles of Louisville, Ky., where it bends northward running through Clarke, Scott, Jackson, Bartholomew, and Brown Counties to Martinsville, in Morgan County, where it turns again west and south and follows approximately the West Branch of the White River through Owen, Greene, and Knox Counties, where it crosses the main stream of White River, and, continuing through Gibson and Posey Counties, crosses the Wabash River near New Harmony.
In Illinois the line still continues southwesterly through White, Gallatin, Saline, and Williamson Counties, where it reaches its southern limit near Carbondale, in latitude 37° 40', and from this point trends northwestward, approximately following the northeastern bluff of the Mississippi River, to the vicinity of Carondelet, Mo., a short distance south of St. Louis.
Beyond the Mississippi the line follows approximately the course of the Missouri River across Missouri, and continues westward to the vicinity of Manhattan, in Kansas, where it turns northward, keeping about a hundred miles west of the Missouri River, through eastern Kansas and Nebraska, and striking the river near the mouth of the Niobrara, in South Dakota. From there the line follows approximately the course of the Missouri River to the vicinity of Fort Benton, in northwestern Montana, where the line again bears more northward, running into British America.
It is still in dispute whether the ice extended from the eastern centre far enough west to join the ice-movement from the Rocky Mountain plateau. Dr. George M. Dawson[BC] is of the opinion that it did not, but that there was a belt of a hundred miles or more, east of the Rocky Mountains, which was never covered by true glacial ice. Mr. Upham[BD] is equally confident that the two ice-movements became confluent, and united upon the western plateau of Manitoba. The opportunity for such a difference of opinion arises in the difficulty sometimes encountered of distinguishing between a direct glacial deposit and a deposit taking place in water containing boulder-laden icebergs. Where Mr. Upham supposes the ice-fields of the east and of the west to have been confluent in western Manitoba, Dr. Dawson supposes there was an extensive subsidence of the land sufficient to admit the waters of the ocean. Leaving this question for the present undetermined, we will now rapidly summarise the glacial phenomena west of the third meridian from Washington (which corresponds nearly with the western boundary of Pennsylvania), and east of the Rocky Mountains.
[Footnote BC: Transactions of the Royal Society of Canada, vol. viii, sec. iv, pp. 54-74.]
[Footnote BD: American Geologist, vol. vi, September, 1890; Bulletin of the Geological Society of America, vol. ii, pp. 243-276.]
That the glacial movement extended to the southern boundary just delineated is established by the presence down to that line of all the signs of glacial action, and their absence beyond. Glacial groovings are found upon the freshly uncovered rock surfaces at frequent intervals in close proximity to the line all along its course, while granitic boulders from the far north are scattered, with more or less regularity, over the whole intervening space between this line and the Canadian highlands. I have already referred to a boulder of jasper conglomerate found in Boone County, Kentucky, which must have come from unique outcroppings of this rock north of Lake Huron. Granitic boulders from the Lake Superior region are also found in great abundance at the extreme margin mentioned in southern Illinois. West of the Missouri River it is somewhat more difficult to delineate the boundary with accuracy, on account of an enveloping deposit of fine loam, technically called "loess." Loess is very abundant in the whole valley of the Missouri River below Yankton, South Dakota, being for a long distance in the vicinity of the river a hundred feet or more in depth. Over northern Missouri and southern Illinois the deposit is nearly continuous, but less in depth, and everywhere in that region tends to hide from view the unstratified glacial deposit continuously underlying it.
A single instance of personal experience will illustrate the condition of things. While going south from Chicago, in search of the southern limit of glacial action, I stopped off from the train at Du Quoin, about forty miles north of where I subsequently found the boundary. Here the whole surface was covered with loess, two or three feet in depth. Below this was a gravelly soil, three or four feet in thickness, which contained many scratched pebbles of granite. A well which had recently been dug, reached the rock at a depth of twenty feet, and revealed a beautifully polished and scratched surface, betraying, beyond question, the action of glacial ice. As we shall show a little later, it is probable that, about the time the ice of the Glacial period had reached its maximum development, this area, which is covered with loess, was depressed in level, and remained under water during a considerable portion of the period when the ice-front was retreating.
To such an extent is this portion of the area included in southern Iowa, northern Missouri, southern Illinois, and the extreme southern portions of Indiana and Ohio covered with loess, that it has been difficult to determine the relation of its underlying glacial deposits to the more irregular deposits found farther north. At an early period of recent investigations, while making a geological survey of the State of Wisconsin, President T. C. Chamberlin fixed upon the line of moraine hills, which can be seen upon our map, running southward between Green Bay and Lake Michigan, and sweeping around in a curve to the right, passing south of Madison and northward along the line of Wisconsin River, and in another curve to the left, around the southern end of Lake Michigan, as the "terminal moraine of the second Glacial epoch." In Wisconsin the character of this line of moraine hills had been discovered and described by Colonel Charles Whittlesey, in 1866. It was first named the "kettle-moraine," because of the frequent occurrence in it of "kettle-holes." This line of moraine hills has been traced with a great degree of confidence across the entire glaciated area, as shown upon our map, but it is not everywhere equally distinct, and, as will be observed, follows a very irregular course.
Beginning in Ohio we find it coinciding nearly with the extreme glacial boundary until it reaches the valley of the Scioto River, on the sixth meridian west from Washington, where it begins to bear northward and continues in that direction for a distance of sixty or seventy miles, and then turns southward again in the valley of the Miami, having formed between these two valleys a sort of medial moraine.[BE] A similar medial moraine had also been noted by President Chamberlin between the valleys of the Grand and Cuyahoga Rivers, in the eastern part of Ohio. Indeed, for the whole distance across Ohio and Indiana, this moraine occurs in a series of loops pointing to the south, corresponding in general to the five gentle valleys which mark the territory, namely, those of the Grand and Mahoning Rivers; the Sandusky and Scioto Rivers; the Great Miami River; the White River; and the Maumee and Wabash Rivers. Everywhere, however, over this area these morainic accumulations approximate pretty closely to the extreme boundary of the glaciated region.
[Footnote BE: See map at the beginning of the chapter.]
In Illinois President Chamberlin's original determination of the moraine fixed it near the southern end of Lake Michigan, as shown upon our map, but Mr. Frank Leverett has subsequently demonstrated that there is a concentric series of moraines south of this, reaching across the State, (but somewhat obscured by superficial accumulations of loess referred to) and extending nearly to the latitude of St. Louis.
West of Wisconsin President Chamberlin's "terminal moraine of the second Glacial epoch" bends southward through eastern Minnesota, and, sweeping down through central Iowa, forms, near the middle of the northern part of that State, a loop, having its southern extremity in the vicinity of Des Moines. The western arm of this loop runs through Minnesota in a northwesterly direction nearly parallel with the upper portion of the valley of the Minnesota, until reaching the latitude of the head-waters of that river, where, in the vicinity of the Sisseton Agency, in Dakota, it turns to the south by an acute angle, and makes a loop in that State, extending to the vicinity of Yankton, and with the valley of the James River as its axis. The western arm of this loop follows pretty closely the line of the eastern edge of the trough of the Missouri River, constituting what is called the "Missouri Coteau," which continues on as a well-marked line of hills running in a northwesterly direction far up into the Dominion of Canada.
One of the most puzzling glacial phenomena in the Mississippi Valley is the driftless area which occupies the southeastern portion of Minnesota, the southwestern part of Wisconsin, and the northwestern corner of Iowa, as delineated upon our map. This is an area which, while being surrounded on every side by all the characteristic marks of glaciation, is itself conspicuous for their entire absence. Its rocks preserve no glacial scratches and are covered by no deposits of till, while northern boulders avoided it in their journey to more southern latitudes.
The reason for all this is not evident in the topography of the region. The land is not higher than that to the north of it, nor is there any manifest protection to it by the highlands south of Lake Superior. Nor yet is there any reason to suppose that any extensive changes of level in former times seriously affected its relations to the surrounding country. Professor Dana, however, has called attention to the fact that even now it is in a region of comparatively light precipitation, suggesting that the snow-fall over it may always have been insignificant in amount. But this could scarcely account for the failure of the great ice-wave of the north to overrun it. We are indebted again to the sagacity of President Chamberlin in suggesting the true explanation.
By referring to the map it will be noticed that this area sustains a peculiar relation to the troughs of Lake Michigan and Lake Superior, while from the arrangements of the moraines in front of these lakes it will be seen that these lake basins were prominent factors in determining the direction of the movement of the surplus ice from the north. It is the more natural that they should do so because of their great depth, their bottoms being in both cases several hundred feet below the present water-level, reaching even below the level of the sea.
These broad, deep channels seem to have furnished the readiest outlet for the surplus ice of the North, and so to have carried both currents of ice beyond this driftless area before they became again confluent. The slight elevation south of Lake Superior served to protect the area on account of the feebleness of direct movement made possible by the strength of these diverging lateral ice-currents. The phenomenon is almost exactly what occurs where a slight obstruction in a river causes an eddy and preserves a low portion of land below it from submergence. A glance at the map will make it easily credible that an ice-movement south of Manitoba, becoming confluent with one from Lake Superior, pushed far down into the Missouri Valley and spread eastward to the Mississippi River, south of the unglaciated driftless area, and there became confluent with a similar movement which had been directed by the valleys of Lake Michigan and Lake Erie. There can be little doubt that President Chamberlin's explanation is in the main correct, and we have in this another illustration of the analogy between the behaviour of moving ice and that of moving water.
The accompanying illustrations will give a better idea than words can do of the celebrated glacial grooves on the hard limestone islands near Sandusky, in the western part of Lake Erie. Through the interest aroused in them by an excursion of the American Association for the Advancement of Science, while meeting in Cleveland, Ohio, in 1888, the Kelly Island Lime and Transport Company, of which Mr. M. C. Younglove is the president, has been induced to deed to the Western Reserve Historical Society for preservation a portion of one of the most remarkable of the grooves still remaining.
The portion of the groove preserved is thirty-three feet across, and the depth of the cut in the rock is seventeen feet below the line, extending from rim to rim. Originally there was probably here a small depression formed by preglacial water erosion, into which the ice crowded the material, which became its graving-tool, and so the rasping and polishing went on in increasing degree until this enormous furrow is the result. The groove, however, is by no means simple, but presents a series of corrugations merging into each other by beautiful curves. When exposed for a considerable length it will resemble nothing else so much as a collection of prostrate Corinthian columns lying side by side on a concave surface.
The direction of these grooves is a little south of west, corresponding to that of the axis of the lake. This is nearly at right angles to the course of the ice-scratches on the summit of the water-shed south of this, between the lake and the Ohio River. The reason for this change of direction can readily be seen by a little attention to the physical geography. The highlands to the south of the lake rise about seven hundred feet above it. When the Ice period was at its climax and overran these highlands, the ice took its natural course at right angles to the terminal moraine and flowed southeast according to the direction indicated by the scratches on the summit; but when the supply of ice was not sufficient to overrun the highlands, the obstruction in front turned the course and the resultant was a motion towards Toledo and the Maumee Valley, where in the vicinity of Fort Wayne an extensive terminal moraine was formed.
The much-mooted question of a succession of glacial epochs finds the most of its supporting facts in the portion of the glaciated area lying west of Pennsylvania. That there have been frequent oscillations of the glacial front over this area is certain. But it is a question whether the glacial deposits south of this distinct line of moraine hills are so different from those to the north of it as to necessitate the supposition of two entirely distinct glacial epochs. This can be considered most profitably here.
The following are among the points with reference to which the phenomena south of the moraine just delineated differ from those north of the line:
1. The glacial deposits to the south appear to be distributed more uniformly than those to the north. To the north the drift is often accumulated in hills, and is dotted over with kettle-holes, while to the south these are pretty generally absent. Any one travelling upon a line of railroad which traverses these two portions of the glaciated area as indicated upon our map can easily verify these statements.
2. The amount of glacial erosion seems to be much less south of the line of moraine hills delineated than north of them. Still, glacial striæ are found, almost everywhere, close down to the extreme margin of the glaciated area.
3. The gravel deposits connected with the drainage of the Glacial period are much less abundant south of the so-called "terminal moraine of the second Glacial period" than they are north of it. South of this moraine the water deposits attributed to the Glacial period are of such fine silt as to indicate slow-moving currents over a gentle low slope of the surface.
4. The glacial deposits to the south are more deeply coloured than those to the north, showing that they have been longer exposed to oxidising agencies. Even the granitic boulders show the marks of greater age south of this line, being disintegrated to a greater extent than those to the north.
5. And, finally, there occur, over a wide belt bordering the so-called moraine hills of the second Glacial epoch, extensive intercalated beds of vegetal deposits. Among the earliest of these to be discovered were those of Montgomery County, Ohio, where, in 1870, Professor Orton, of the Ohio Survey, found at Germantown a deposit of peat fourteen feet thick underneath ninety-five feet of till, and there seem also to be glacial deposits underneath the peat as well as over it. The upper portion of the peat contains "much undecomposed sphagnous mosses, grasses, and sedges, and both the peat and the clayey till above it" contain many fragments of coniferous wood which can be identified as red cedar (_Juniperus Virginianus_). In numerous other places in that portion of Ohio fresh-appearing logs, branches, and twigs of wood are found underneath the till, or mingled with it, much as boulders are. Near Darrtown, in Butler County, Ohio, red cedar logs were found under a covering of sixty-five feet of till, and so fresh that the perfume of the wood is apparently as strong as ever. Similar facts occur in several other counties in the glaciated area of southern Ohio and southern Indiana. Professor Collett reports that all over southwestern Indiana peat, muck, rotted stumps, branches, and leaves of trees are found from sixty to one hundred and twenty feet below the surface, and that these accumulations sometimes occur to a thickness of from two to twenty feet.
Farther to the northwest similar phenomena occur. Professor N. H. Winchell has described them most particularly in Fillmore and Mower Counties, Minnesota, from which they extend through a considerable portion of Iowa. In the above counties of Minnesota a stratum of peat from eighteen inches to six or eight feet in thickness, with much wood, is pretty uniformly encountered in digging wells, the depth varying from twenty to fifty feet. This county is near the highest divide in the State of Minnesota, and from it "flow the sources of the streams to the north, south, and east." The wood encountered in this stratum indicates the prevalence f coniferous trees, and the peat mosses indicate a cool and moist climate.
Nor are intercalated vegetable deposits absent from the vast region farther north over the area that drains into Hudson Bay. At Barnesville, in Clay County, Minnesota, which lies in the valley of the Red River of the North, and also in Wilkin County in the same valley, tamarack wood and sandy black mud containing many snail-shells have been found from eight to twelve feet below a surface of till; and Dr. Robert Bell reports the occurrence of limited deposits of lignite between layers of till, far to the northwest, in Canada, and even upon the southern part of Hudson Bay; while Mr. J. B. Tyrrell reports[BF] many indications of successive periods of glaciation near the northern end of the Duck Mountain. The most characteristic indications which he had witnessed consisted of stratified beds of silt, containing fresh-water shells, with fragments of plants and fish similar to those living in the lakes of the region at the present time.
[Footnote BF: Bulletin of the Geological Society of America, vol. i, pp. 395-410.]
Reviewing these facts with reference to their bearing upon the point under consideration, we grant, at the outset, that they do indicate a successive retreat and readvance of the ice over extensive areas. This is specially clear with respect to the vegetal deposits interstratified with beds of glacial origin. But the question at issue concerning the interpretation of these phenomena is, Do they necessarily indicate absolutely distinct glacial epochs separated by a period in which the ice had wholly disappeared from the glaciated area to the north? That they do, is maintained by President Chamberlin and many others who have wide acquaintance with the facts. That they do not certainly indicate a complete disappearance of the ice during an extensive interglacial epoch, is capable, however, of being maintained, without forfeiting one's rights to the respect of his fellow-geologists. The opposite theory is thus stated by Dr. Robert Bell: "It appears as if all the phenomena might be referred to one general Glacial period, which was long continued, and consequently accompanied by varying conditions of temperature, regional oscillations of the surface, and changes in the distributions of sea and land, and in the currents in the ocean. These changes would necessarily give rise to local variations in the climate, and might permit of vegetation for a time in regions which need not have been far removed from extensive glaciers."[BG]
[Footnote BG: Bulletin of the Geological Society of America, vol. i, pp. 287-310.]
At my request, Professor J. E. Todd, of Iowa, whose acquaintance with the region is extensive, has kindly written out for me his conclusions upon this subject, which I am permitted to give in his own words:
"I am not prepared to write as I would like concerning the forest-beds and old soils. I will, however, offer the following as a partial report. I have come to think that there is considerable confusion on the subject. I believe there are five or six different things classed under one head.
"1. _Recent Much and Soils._--The finest example I have found in the whole Missouri Valley was twenty feet below silt and clay, in a basin inside the outer moraine, near Grand View, South Dakota. From my examination of the reported old soil near Albia, Iowa, I think the most rational way of reconciling the conflicting statements concerning it is that it also belongs to this class.
"2. _Peat or Soil under Loess._--This does not signify much if the loess was formed in a lake subject to orographic oscillations, or if, as I am coming to believe, it is a fluviatile deposit of an oscillating river like the Hoang-Ho on the great Chinese plain. It at least does not mean an interglacial epoch.
"3. _Wood and Dirt rearranged, not in situ._--This occurs either in subaqueous or in subglacial deposits. I have found drift-wood in the lower layers of the loess here, but not _in situ_. I have frequently found traces of wood in till in Dakota, but always in an isolated way. I think, from reading statements about the deposits in eastern Iowa, that most if not all of the cases are of this sort. Two things have conspired to lead to this error: one, the influence of Croll's speculation; and the other, the easy inference of many well-diggers, and especially well-borers, that what they pass through are always in layers. In this way a log becomes a forest-bed. Scattered logs and muck fragments occurring frequently in a region, though at different levels, are readily imagined by an amateur geologist to be one continuous stratum antedating the glacier or floods (as the case may be in that particular region), when, in fact, it has been washed down from the margin of the transporting agent and is contemporaneous with it. I suspect the prevalence of wood in eastern Iowa may be traced to a depression of the driftless region during the advance of the glacier, so as to bring the western side of that area more into the grasp of glacial agencies.
"4. _Peat between Subglacial Tills._--If cases of this sort are found, they are in Illinois, Indiana, and Ohio. Professor Worthen insisted that there were no interglacial soils or forest-beds in Illinois; and in the cases mentioned in the State reports he repeatedly explains the sections given by his assistants, so as to harmonize them with that statement. I think he usually makes his explanations plausible. He was very confident in referring most of them, to preglacial times. His views, I suppose, will be published in the long-delayed volume, now about to be issued.
"5. _Vegetable Matter between Glacial Till and Underlying Berg Till or other Drift Deposits._--When one remembers that the front of the great ice-sheet may have been as long in reaching its southern boundary as in receding from it, and with as many advance and retrograde movements, we can easily believe that much drift material would have outrun the ice and have formed deposits so far ahead of it that vegetation would have grown before the ice arrived to bury it.
"6. _Preglacial Soils, etc._--I believe that this will be found to include most in southern Ohio, if not in Illinois, as Worthen claimed."
The phenomena of the Glacial period are too vast either to have appeared or to have disappeared suddenly. By whatever cause the great accumulation of ice was produced, the advance to the southward must have been slow and its disappearance must have been gradual, though, as we shall show a little later, the final retreat of the ice-front occupied but a short time relatively to the whole period which has elapsed since. As we shall show also, the advent of the Ice period was probably preceded and accompanied by a considerable elevation of the northern part of the continent Whether this elevation was contemporaneous upon both sides of the continent is perhaps an open question; but with reference to the area east of the Rocky Mountains, which is now under consideration, the centre of elevation was somewhere south of Hudson Bay. Putting together what we know, from the nature of the case, concerning the accumulation and movement of glacial ice, and what we know from the relics of the great glacial invasion, which have enabled us to determine its extent and the vigour of its action, the course of events seems to have been about as follows:
Throughout the Tertiary period a warm climate had prevailed over British America, Greenland, and indeed over all the lands in proximity to the north pole, so far as explorers have been able to penetrate them. The vegetation characterizing these regions during the Tertiary period indicates a temperature about like that which now prevails in North Carolina and Virginia. Whatever may be said in support of the theory that the Glacial period was produced by astronomical causes, in view of present facts those causes cannot be regarded as predominant; at most they were only co-operative. The predominant cause of the Glacial period was probably a late Tertiary or post-Tertiary elevation of the northern part of the continents, accompanied with a subsidence in the central portion. Of such a subsidence in the Isthmus of Panama indications are thought to be afforded by the occurrence of late Tertiary or, more probably, post-Tertiary sea-shells at a considerable elevation on the divide along the Isthmus of Panama, between the Atlantic and Pacific Oceans. Of this we shall speak more fully in a later chapter.
Fixing our thoughts upon what is known as the Laurentian plateau, which, though now in the neighbourhood of but two thousand feet above the sea, was then much higher, we can easily depict in imagination the beginnings of the great "Laurentide Glacier," which eventually extended to the margin already delineated on the south and southwest in the United States, and spread northward and eastward over an undetermined area. Year after year and century after century the accumulating snows over this elevated region consolidated into glacial ice and slowly pushed outward the surplus reservoirs of cold. For a long time this process of ice-accumulation may have been accompanied by the continued elevation of the land, which, together with the natural effect of the enlarging area of ice and snow, would tend to lower the temperature around the margin and to increase still more the central area of accumulation.
The vigour of movement in any direction was determined partly by the shape of the valleys opening southward in which the ice-streams would naturally concentrate, and partly by those meteorological conditions which determine the extent of snow-fall over the local centres of glacial dispersion. For example, the general map of North America in the Ice period indicates that there were two marked subcentres of dispersion for the great Laurentide Glacier, the eastern one being in Labrador and the western one north of Lake Superior. In a general way the southern boundary of the glaciated region in the United States presents the appearance of portions of two circumferences of circles intersecting each other near the eastern end of Lake Erie. These circles, I am inclined to believe, represent the areas over which a semi-fluid (or a substance like ice, which flows like a semi-fluid) would disperse itself from the subcentres above mentioned.
A study of the contour of the country shows that that also, in a general way, probably had something to do with the lines of dispersion. The western lobe of this glaciated area corresponds in its boundary pretty closely with the Mississippi Valley, having the Ohio River approximately as its eastern arm and the Missouri as its western, with the Mississippi River nearly in its north and south axis. The eastern lobe has its farthest extension in the axis of the Champlain and Hudson River Valleys, its western boundary being thrown more and more northward as the line proceeds to the west over the Alleghany Mountains until reaching the longitude of the eastern end of Lake Erie; but this southern boundary is by no means a water-level, nor is the contour of the country such that it could ever have been a water-level. But it conforms in nearly every particular to what would be the resultant arising from a pretty general southward flow of a semi-fluid from the two subcentres mentioned, meeting with the obstructions of the Adirondacks in northern New York and of the broader Appalachian uplift in northern Pennsylvania.
How far south the area of glacial accumulation may have extended cannot be definitely ascertained, but doubtless at an early period of the great Ice age the northern portions of the Appalachian range in New York, New England, New Brunswick, and Nova Scotia became themselves centres of dispersion, while only at the height of the period did all their glaciers become confluent, so that there was one continuous ice-sheet.
In the western portion of the area covered by the Laurentide Glacier, the depression occupied by the Great Lakes, especially Lakes Michigan and Superior, evidently had a marked influence in directing the flow of ice during the stages which were midway between the culmination of the Ice period and both its beginning and its end. This would follow from the great depth of these lakes, the bottom of Lake Michigan being 286 feet below sea-level, and that of Lake Superior 375 feet, making a total depth of water of about 900 and 1,000 feet respectively. Into these oblong depressions the ice would naturally gravitate until they were filled, and they would become the natural channels of subsequent movement in the direction of their longest diameters, while the great thickness of ice in them would make them the conservative centres of glacial accumulation and action after the ice had begun to retreat.
These deductions from the known nature of ice and the known topography of the region are amply sustained by a study of the detailed map showing the glacial geology in the United States. But on this we can represent indeed only the marks left by the ice at various stages of its retreat, since, as already remarked, the marks of each stage of earlier advance would be obliterated by later forward movements. We may presume, however, that in general the marks left by the retreating ice correspond closely with those actually made and obliterated by the advancing movement.
From observations upon the glaciers of Switzerland and of Alaska, it is found that neither the advance nor the retreat of these glaciers is constant, but that, in obedience to meteorologic agencies not fully understood, they advance and retreat in alternate periods, at one time receding for a considerable distance, and at other times regaining the lost ground and advancing over the area which has been uncovered by their retreat.
"M. Forel reports, from the data which he has collected with much care, that there have been in this century five periods in the Alpine glaciers: of enlargement, from 1800 (?) to 1815; of diminution, from 1815 to 1830; of enlargement, from 1830 to 1845; of diminution, from 1845 to 1875; and of enlargement again, from 1875 onward. He remarks further that these periods correspond with those deduced by Mr. C. Lang for the variations for the precipitations and temperature of the air; and, consequently, that the enlargement of the glaciers has gone forward in the cold and rainy period, and the diminution in the warm and the dry."[BH]
[Footnote BH: American Journal of Science, vol. cxxxii, 1886, p. 77.]
When, now, we attentively consider the combination of causes necessary to produce the climatic conditions of the great Ice age of North America, we shall be prepared to find far more extensive variations in the progress of the continental glacier, both during its advance and during its retreat, than are to be observed in any existing local glaciers.
With respect to the arguments adduced in favor of a succession of glacial epochs in America the following criticisms are pertinent:
1. So far as we can estimate, a temporary retreat of the front, lasting a few centuries, would be sufficient to account for the vegetable accumulations that are found buried beneath the glacial deposits in southern Ohio, Indiana, central Illinois, and Iowa, while a temporary readvance of the ice would be sufficient to bury the vegetable remains beneath a freshly accumulated mass of till. Thus, as Dr. Bell suggested, the interglacial vegetal deposits do not necessarily indicate anything more than a temporary oscillation of the ice-front, and do not carry with them the necessity of supposing a disappearance of the ice from the whole glaciated area. Thus the introduction of a whole Glacial period to account for such limited phenomena is a violation of the well-known law of parsimony, which requires us in our explanations of phenomena to be content with the least cause which is sufficient to produce them. In the present instance a series of comparatively slight oscillations of the ice-front during a single glacial period would seem to be sufficient to account for all the buried forests and masses of vegetal _débris_ that occur either in the United States or in the Dominion of Canada.
2. Another argument for the existence of two absolutely distinct glacial periods in North America has been drawn from the greater oxidation of the clays and the more extensive disintegration of certain classes of the boulders found over the southern part of the glaciated area of the Mississippi Valley, than has taken place in the more northerly regions. Without questioning this statement of fact (which, however, I believe to be somewhat exaggerated), it is not difficult to see that the effects probably are just what would result from a single long glacial period brought about by such causes as we have seen to be probably in operation in America. For if one reflects upon the conditions existing when the Glacial period began, he will see that, during the long ages of warm climate which characterised the preceding period, the rocks must have been extensively disintegrated through the action of subaërial agencies. The extent to which this disintegration takes place can be appreciated now only by those who reside outside of the glaciated area, where these agencies have been in uninterrupted action. In the Appalachian range south of the glaciated region the granitic masses and strata of gneiss are sometimes found to be completely disintegrated to a depth of fifty or sixty feet; and what seem to be beds of gravel often prove in fact to be horizontal strata of gneiss from which the cementing material has been removed by the slow action of acids brought down by the percolating water.
Now, there can be no question that this process of disintegration had proceeded to a vast extent before the Glacial period, so that, when the ice began to advance, there was an enormous amount of partially oxidised and disintegrated material ready to be scraped off with the first advance of ice, and this is the material which would naturally be transported farthest to the south; and thus, on the theory of a single glacial period, we can readily account for the greater apparent age of the glacial _débris_ near the margin. This _débris_ was old when the Glacial period began.
3. With reference to the argument for two distinct glacial periods drawn from the smaller apparent amount of glacial erosion over the southern part of the glaciated area, we have to remark that that would occur in case of a single ice-invasion as well as in case of two distinct ice-invasions, in which the later did not extend so far as the former.
From the very necessity of the case, glacial erosion diminishes as the limit of the extent of the glaciation is approached. At the very margin of the glacier, motion has ceased altogether. Back one mile from the margin only one mile of ice-motion has been active in erosion, while ten miles back from its front there has been ten times as much moving ice actually engaged in erosion, and in the extreme north several hundred times as much ice, Thus it is evident that we do not need to resort to two glacial periods to account for the relatively small amount of erosion exhibited over the southern portion of our glaciated area.
At the same time, it should be said that the indications of active glacial erosion near the margin are by no means few or small. In Lawrence County, Pennsylvania, on the very margin of the glaciated area, Mr. Max Foshay[BI] has discovered very extensive glacial grooves, indicating much vigour of ice-action even beyond the more extensive glacial deposits which Professor Lewis and myself had fixed upon as the terminal moraine. In Highland and Butler Counties, Ohio, and in southwestern Indiana and southern Illinois, near the glacial margin, glacial grooves and striæ are as clear and distinct in many cases as can anywhere be found; while upon the surface of the limestone rocks within the limits of the city of St. Louis, where the glacial covering is thin, and where disintegrating agencies had had special opportunities to work, I found very clear evidences of a powerful ice-movement, which had planed and scratched the rock surface; and at Du Quoin, Illinois, as already related, the fragments thrown up from the surface of the rock, fifty or sixty feet below the top of the soil, were most beautifully planed and striated. It should be observed, also, that this whole area is so deeply covered with _débris_ that the extent of glacial erosion underneath is pretty generally hid from view.
[Footnote BI: Bulletin of the Geological Society, vol. ii, pp. 457-464.]
4. The uniformity of the distribution of the glacial deposits over the southern portion of the glaciated area in the Mississippi Valley is partly an illusion, due to the fact that there was a vast amount of deposition by water over that area during the earlier stages of the ice-retreat. This has been due partly to the gentler slope which would naturally characterise the borders of an area of elevation, and partly to an extensive subsidence which seems to have begun soon after the ice had reached its farthest extent of motion.
It should be borne in mind that at all times a glacier is accompanied by the issue of vast streams of water from its front, and that these of course increase in volume when the climax has been reached and the ameliorating influences begin to melt away the accumulated mass of ice and to add the volume of its water to that produced by ordinary agencies. As these subglacial streams of water poured out upon the more gentle slopes of the area in front of the ice, they would distribute a vast amount of fine material, which would settle into the hollow places and tend to obscure the irregularities of the previous direct glacial deposit.
Such an instance came clearly under my own observation in the vicinity of Yankton, in South Dakota, where, upon visiting a locality some miles from any river, and to which workmen were resorting for sand, I found that the deposit occupied a kettle-hole, filling it to its brim, and had evidently been superimposed by a temporary stream of water flowing over the region while the ice was still in partial occupation of it. Thus, no doubt, in many cases, the original irregularities of the direct glacial deposits have been obliterated, even where there has been no general subsidence.
But, in the area under consideration, the loess, or loam, is so extensive that it is perhaps necessary to suppose that the central portions of the Mississippi Valley were subjected to a subsidence amounting to about five hundred feet; so that the glacial streams from the retreating ice-front met the waters of the ocean in southern Illinois and Indiana; thus accounting for the extensive fine silt which has done so much over that region to obscure the glacial phenomena.
_West of the Rocky Mountains._
The glacial phenomena in the United States west of the Rocky Mountains must be treated separately, since American geologists have ceased to speak of an all-pervading ice-cap extending from the north pole. But, as already said, the glaciation of North America has proceeded from two definite centres of ice-accumulation, one of which we have been considering in the pages immediately preceding. The great centre of glacial dispersion east of the Rocky Mountains is the region south of Hudson Bay, and the vast ice-field spreading out from that centre is appropriately named the Laurentide Glacier. The movement of ice in this glacial system was outward in all directions from the Laurentian hills, and extended west several hundred miles, well on towards the eastern foot of the Rocky Mountains.
The second great centre of glacial dispersion occupies the vast Cordilleran region of British Columbia, reaching from the Rocky Mountains on the northeast to the Coast Range of the Pacific on the southwest, a width of four hundred miles. The length is estimated by Dr. Dawson to be twelve hundred miles. The principal centre of ice-accumulation lies between the fifty-fifth and the fifty-ninth parallel. From this centre the movement was in all directions, but chiefly to the northwest and to the south. The movement of the Cordilleran glaciers extended northwest to a distance of three hundred and fifty miles, leaving their moraines far down in the Yukon Valley on the Lewes and Pelly Rivers.[BJ] Southward the Cordilleran Glacier moved to a distance of six hundred miles, extending to the Columbia River, in the eastern part of the State of Washington.
[Footnote BJ: See George M. Dawson, in Science, vol. xi, 1888, p. 186, and American Geologist, September, 1890, pp. 153-162.]
From this centre, also, the ice descended to the sea-level upon the west, and filled all the channels between Vancouver's Island and the mainland, as well as those in the Alexander Archipelago of Alaska. South of Vancouver's Island a glacier pushed out through the straits of Juan de Fuca to an unknown distance. All the islands in Puget Sound are composed of glacial _débris_, resembling in every respect the terminal moraines which have been described as constituting many of the islands south of the New England coast. The ice-movement in Puget Sound, however, was probably northward, resulting from glaciers which are now represented by their diminutive descendants on the flanks of Mount Rainier.
South of the Columbia River the country was never completely enveloped by the ice, but glaciers extended far down in the valleys from all the lofty mountain-peaks. In Idaho there are glacial signs from the summit of the Rocky Mountains down to the westward of Lake Pend d'Oreille. In the Yellowstone Park there are clear indications that the whole area was enveloped in glacial ice. An immense boulder of granite, resting upon volcanic deposits, may be found a little west of Inspiration Point, on the Yellowstone Cañon. Abundant evidences of glacial action are also visible down the Yellowstone River to the vicinity of Livingston, showing that that valley must have been filled with glacial ice to a depth of sixteen hundred feet. To the west the glaciers from the Yellowstone Park extended to the border of Idaho, where a clearly marked terminal moraine is to be found in the Tyghee Pass, leading over from the western fork of the Madison River into Lewis Fork of the Snake River. South of Yellowstone Park the Teton Mountains were an important centre for the dispersion of local glaciers, but they did not descend upon the western side much below the 6,000-foot level, and only barely came to the edge of the great Snake River lava plains. To the east the movement from the Teton Mountains joined that from various other lofty mountains, where altogether they have left a most intricate system of glacial deposits, in whose reticulations Jackson's Lake is held in place.
In Utah extensive glaciers filled all the northern valleys of the Uintah Mountains, and extended westward in the Wahsatch range to the vicinity of Salt Lake City. The mountain region of Colorado, also, had its glaciers, occupying the head-waters of the Arkansas, the Platte, the Gunnison, and the Grand Rivers. The most southern point in the Rocky Mountains at which signs of local glaciers have been noted is near the summits of the San Juan range, in southwestern Colorado. Here a surface of about twenty-five square miles, extending from an elevation of 12,000 feet down to 8,000 feet, shows every sign of the former presence of moving ice. The greater part of the glaciation in Colorado is confined to elevations above 10,000 feet.
The whole range of the Sierra Nevada through Oregon, and as far south as the Yosemite Valley in California, formerly sustained glaciers of far greater size than any which are now found in those mountains. In general these glaciers were much longer on the western side of the Sierra Nevada than on the eastern. On the eastern side glaciers barely came down to Lake Tahoe and Lake Mono in California. The State of Nevada seems to have been entirely free from glaciers, although it contains numerous mountain-peaks more than ten thousand feet high. In the Yosemite Cañon glaciers extended down the Merced River to the mouth of the cañon; while in the Tuolumne River, a few miles to the north, the glaciers which still linger about the peaks of Mount Dana filled the valley for a distance of forty miles.
It is a question among geologists whether or not the glaciation west of the Rocky Mountains was contemporaneous with that of the eastern part of the continent. The more prevalent opinion among those who have made special study of the phenomena is that the development of the Cordilleran glaciers was independent of that of the Laurentide system. At any rate, the intense glaciation of the Pacific coast seems to have been considerably later than that of the Atlantic region. Of this we will speak more particularly in discussing the questions of the date and the cause of the Glacial period. It is sufficient for us here simply to say that, from his extensive field observations throughout the Cordilleran region, Dr. George M. Dawson infers that there have been several successive alternations of level on the Pacific coast corresponding to successive glacial and interglacial epochs, and that when there was a period of elevation west of the Rocky Mountains there was a period of subsidence to the east, and _vice versa_. In short, he supposes that the east and west for a long time played a game of seesaw, with the Rocky Mountains as the fulcrum. We give his scheme in tabulated form.
_Scheme of Correlation of the Phenomena of the Glacial Period in the Cordilleran Region and in the Region of the Great Plains._
CORDILLERAN REGION. REGION OF THE GREAT PLAINS.
Cordilleran zone at a high Correlative subsidence and elevation. Period of most severe submergence of the great plains, glaciation and maximum development with possible contemporaneous of the great Cordilleran Glacier. increased elevation of the Laurentian axis and maximum development of ice upon it. Deposition of the lower boulder-clay of the plains.
Gradual subsidence of the Correlative elevation of the Cordilleran region and decay of the western part, at least, of the great glacier, with deposition of great plains, which was probably the boulder-clay of the interior more or less irregular and led to plateau and the Yukon basin, of the the production of extensive lakes lower boulder-clay of the littoral in which interglacial deposits, and probably also, at a later stage including peat, were formed. (and with greater submergence), of the interglacial silts of the same region.
Re-elevation of the Cordilleran Correlative subsidence of the region to a level probably as high plains, which (at least in the as or somewhat higher than the western part of the region) present. Maximum of second period exceeded the first subsidence and of glaciation. extended submergence to the base of the Rocky Mountains near the forty-ninth parallel. Formation of second boulder-clay, and (at a later stage) dispersion of large erratics.
Partial subsidence of the Correlative elevation of the Cordilleran region, to a level plains, or at least of their about 2,500 feet lower than the western portion, resulting in a present. Long stage of stability. condition of equilibrium as Glaciers of the second period between the plains and the considerably reduced. Upper Cordillera, their _relative_ boulder-clay of the coast probably levels becoming nearly as at formed at this time, though perhaps present. Probable formation of the in part during the second maximum Missouri coteau along a shore-line of glaciation. during this period of rest.
Renewed elevation of the Simultaneous elevation of the great Cordilleran region, with one plains to about their present well-marked pause, during which the level, with final exclusion of littoral stood about 200 feet lower waters in connection with the sea. than at present. Glaciers much Lake Agassiz formed and eventually reduced, and diminishing in drained towards the close of this consequence of general amelioration period. This simultaneous movement of climate towards the close of the in elevation of both great areas Glacial period. may probably have been connected with a more general northern elevation of land at the close of the Glacial period.
In New Zealand the marks of the Glacial period are unequivocal The glaciers which now come down from the lofty mountains upon the South Island of New Zealand to within a few hundred feet of the sea then descended to the sea-level. The longest existing glacier in New Zealand is sixteen miles, but formerly one of them had a length of seventy-eight miles. One of the ancient moraines contains a boulder from thirty to forty feet in diameter, and the amount of glacial _débris_ covering the mountain-sides is said to be enormous. Reports have also been recently brought of signs of ancient glaciers in Australia.
According to Darwin, there are distinct signs of glaciation upon the plains of Patagonia sixty or seventy miles east of the foot of the mountains, and in the Straits of Magellan he found great masses of unstratified glacial material containing boulders which were at least one hundred and thirty miles away from their parent rock; while upon the island of Chiloe he found embedded in "hardened mud" boulders which must have come from the mountain-chains of the continent. Agassiz also observed unquestionable glacial phenomena on various parts of the Fuegian coast, and indeed everywhere on the continent south of latitude 37°. Between Concepcion and Arauco, in latitude 37°, Agassiz observed, near the sea-level, a glacial surface well marked with furrows and scratches, and as well preserved, he says, "as any he had seen under the glaciers of the present day."