CHAPTER IV.
SIGNS OF PAST GLACIATION.
The facts from which we draw the inference that vast areas of the earth's surface which are now free from glaciers were, at a comparatively recent time, covered with them, are fourfold, and are everywhere open to inspection. These facts are: 1. Scratches upon the rocks. 2. Extensive unstratified deposits of clay and sand intermingled with scratched stones and loose fragments of rock. 3. Transported boulders left in such positions and of such size as to preclude the sufficiency of water-carriage to account for them. 4. Extensive gravel terraces bordering the valleys which emerge from the glaciated areas. We will consider these in their order:
1. The scratches upon the rocks.
Almost anywhere in the region designated as having been covered with ice during the Glacial period, the surface of the rocks when freshly uncovered will be found to be peculiarly marked by grooves and scratches more or less fine, and such as could not be produced by the action of water. But, when we consider the nature of a glacier, these marks seem to be just what would be produced by the pushing or dragging along of boulders, pebbles, gravel, and particles of sand underneath a moving mass of ice.
Running water does indeed move gravel, pebbles, and boulders along with the current, but these objects are not held by it in a firm grasp, such as is required to make a groove or scratch in the rock. If, also, there are inequalities in the compactness or hardness of the rock, the natural action of running water is to hollow out the soft parts, and leave the harder parts projecting. But, in the phenomena which we are attributing to glacial action, there has been a movement which has steadily planed down the surface of the underlying rock; polishing it, indeed, but also grooving it and scratching it in a manner which could be accomplished only by firmly held graving-tools.
This polishing and scratching can indeed be produced by various agencies; as, for example, by the forces which fracture the earth's crust, and shove one portion past another, producing what is called a _slicken-side_. Or, again, avalanches or land-slides might be competent to produce the results over limited and peculiarly situated areas. Icebergs, also, and shore ice which is moved backwards and forwards by the waves, would produce a certain amount of such grooving and scratching. But the phenomena to which we refer are so extensive, and occur in such a variety of situations, that the movement of glacial ice is alone sufficient to afford a satisfactory explanation. Moreover, in Alaska, Greenland, Norway, and Switzerland, and wherever else there are living glaciers, it is possible to follow up these grooved and striated surfaces till they disappear underneath the existing glaciers which are now producing the phenomena. Thus by its tracks we can, as it were, follow this monster to its lair with as great certainty as we could any animal with whose footprints we had become familiar.
2. The till, or boulder-clay.
A second sign of the former existence of glaciers over any area consists of an unstratified deposit of earthy material, of greater or less depth, in which scratched pebbles and fragments of rock occur without any definite arrangement.
Moving water is a most perfect sieve. During floods, a river shoves along over its bed gravel and pebbles of considerable size, whereas in time of low-water the current may be so gentle as to transport nothing but fine sand, and the clay will be carried still farther onwards, to settle in the still water and form a delta about the river's mouth. The transporting capacity of running water is in direct ratio to the sixth power of its velocity. Other things being equal, if the velocity be doubled, the size of the grains of sand or gravel which it transports is increased sixty-four fold.[AS] So frequent are the changes in the velocity of running water, that the stratification of its deposits is almost necessary and universal. If large fragments of rocks or boulders are found embedded in stratified clay, it is pretty surely a sign that they have been carried to their position by floating ice. A small mountain stream with great velocity may move a good-sized boulder, while the Amazon, with its mighty but slow-moving current, would pass by it forever without stirring it from its position. But the vast area which is marked in our map as having been covered with ice during the Glacial period is characterised by deep and extensive deposits of loose material devoid of stratification, and composed of soil and rock gathered in considerable part from other localities, and mixed in an indiscriminate mass with material which has originated in the disintegration of the underlying local strata.
[Footnote AS: Le Conte's Geology, p. 19.]
3. Transported boulders.
Where there is a current of water deep enough to float large masses of ice, there is scarcely any limit to the size of boulders which may be transported upon them, or to the distance to which the boulders may be carried and dropped upon the bottom. The icebergs which break off from the glaciers of Greenland may bear their burdens of rock far down into the Atlantic, depositing them finally amidst the calcareous ooze and the fine sediment from the Gulf Stream which is slowly covering the area between Northern America and Europe. Northern streams like the St. Lawrence, which are deeply frozen over with ice in the winter, and are heavily flooded as the ice breaks up in the spring, afford opportunity for much transportation of boulders in the direction of their current. In attributing the transportation of a boulder to glacial ice, it is necessary, therefore, to examine the contour of the country, so as to eliminate from the problem the possibility of the effects having been produced by floating ice.
Another source of error against which one has to be on his guard arises from the close resemblance of boulders resulting from disintegration to those which have been transported by ice from distant places. Owing to the fact that large masses of rocks, especially those which are crystalline, are seldom homogeneous in their structure, it results that, under the slow action of disintegrating and erosive agencies, the softer parts often are completely removed before the harder nodules are sensibly affected, and these may remain as a collection of boulders dotting the surface. Such boulders are frequent in the granitic regions of North Carolina and vicinity, where there has been no glacial transportation. Several localities in Pennsylvania, also, south of the line of glacial action as delineated by Professor Lewis and myself, had previously been supposed to contain transported boulders of large size, but on examination they proved in all cases to be resting upon undisturbed strata of the parent rock, and were evidently the harder portions of the rock left in loco by the processes of erosion spoken of. In New England, also, it is possible that some boulders heretofore attributed to ice-action may be simply the results of these processes of disintegration and erosion. Whether they are or not can usually be determined by their likeness or unlikeness to the rocks on which they rest; but oftentimes, where a particular variety of rock is exposed over a broad area, it is difficult to tell whether a boulder has suffered any extensive transportation or not.
One of the most interesting and satisfactory demonstrations of the distribution of boulders by glacial ice was furnished by Guyot in Switzerland in 1845. His observations and argument will be most readily understood by reference to the accompanying map, taken from Lyell's clear description.[AT] The Jura Mountains are separated from the Alps by a valley, about eighty miles in width, which constitutes the main habitable portion of Switzerland, and they rise upwards of two thousand feet above it. But large Alpine boulders are found as high as two thousand feet above the Lake Neufchâtel upon the flanks of the Jura Mountains beyond Chasseron (at the point marked G on the map), and the whole valley is dotted with Alpine boulders. Upon comparing these with the native rocks in the Alps, Guyot in many cases was able to determine the exact centres from which they were distributed, and the distribution is such as to demonstrate that glacial ice was the medium of distribution.
[Footnote AT: Antiquity of Man, p. 299.]
For example, the dotted lines upon the map indicate the motion of the transporting medium. On ascending the valley of the Rhône to A, the diminutive representative of the ancient glacier is still found in existence, and is at work transporting boulders and moraines according to the law of ice-movement. Following down the valley from A, boulders from the head of the Rhône Valley are found distributed as far as B at Martigny, where the valley turns at right angles towards the north. It is evident that floating ice in a stream of water would by its momentum be carried to the left bank, so that if icebergs were the medium of transportation we should expect to find the boulders from the right-hand side of the Rhône Valley distributed towards the left end of the great valley of Switzerland--that is, in the direction of Geneva. But, instead, the boulders derived from C, D, and E, on the Bernese Oberland side, instead of crossing the valley at B, continue to keep on the right-hand side and are distributed over the main valley in the direction of the river Aar.
As is to be expected also, the direct northward motion of the ice from B is stronger than the lateral movement to the right and left after it emerges from the mouth of the Rhône Valley, at F, and consequently it has pushed forwards in a straight line, so as to raise the Alpine boulders to a greater height upon the Jura Mountains at G than anywhere else, the upper limit of boulders at G being 1,500 feet higher than the limits at I or K on the left and right, points distant about one hundred miles from each other. All the boulders to the right of the line from B to G have been derived from the right side of the Rhône, while all the boulders to the left of that line have been derived from its left side.
A boulder of talcose granite containing 61,000 French cubic feet, measuring about forty feet in one direction, came, according to Charpentier, from the point _n_, near the head of the Rhône Valley, and must have travelled one hundred and fifty miles to reach its present position.
It scarcely needs to be added that the grooves and scratches upon the rocks over the floor of this great valley of Switzerland indicate a direction of the ice-movement corresponding to that implied in the distribution of boulders. Thus, at K upon the map referred to, Lyell reports that the abundant grooves and striæ upon the polished marble all trend down the valley of the Aar.[AU]
[Footnote AU: Antiquity of Man, p. 305.]
Similar facts concerning the transportation of boulders have been observed at Trogen, in Appenzel, where boulders derived from Trons, one hundred miles distant, are found to keep upon the left bank of the Rhine, however much the valley may wind about; and in some places, as at Mayenfeld, it turns almost at right angles, as did the Rhône at Martigny. Upon reaching the lower country at Lake Constance, these granite blocks from the left side of the valley deploy out upon the same side and do not cross over, as they would inevitably have done had they been borne along by currents of water.
In America Ave do not have quite so easy a field as is presented in Switzerland for the discovery of crucial instances showing that boulders have been transported by glacial ice rather than by floating ice, for in Switzerland the glaciated area is comparatively small and the diminutive remnants of former glaciers are still in existence, furnishing a comprehensive object-lesson of great interest and convincing power. Still, it is not difficult to find decisive instances of glacial transportation even in the broad fields of America which now retain no living remnants of the great continental ice-sheet.
As every one who resides in or who visits New England knows, boulders are scattered freely over all parts of that region, but for a long time the theory suggested to account for their distribution was that of floating ice during a period of submergence. One of the most convincing evidences that the boulders were distributed by glacial ice rather than by icebergs is found in Professor C. H. Hitchcock's discovery of boulders on the summit of Mount Washington (over 6,000 feet above the sea), which he was able to identify as derived from the ledges of light grey Bethlehem gneiss, whose nearest outcrop is in Jefferson, several miles to the northwest, and 3,000 or 4,000 feet lower than Mount Washington. However difficult it may be to explain the movement of these boulders by glacial ice, it is not impossible to do so, but the attempt to account for their transportation by floating ice is utterly preposterous. No iceberg could pick up boulders so far beneath the surface of the water, and even if it could advance thus far in its work it could not by any possibility land them afterwards upon the summit of Mount Washington.
Among the most impressive instances of boulders evidently transported by glacial ice, rather than by icebergs, were some which came to my notice when, in company with the late Professor H. Carvill Lewis, I was tracing the glacial boundary across the State of Pennsylvania. We had reached the elevated plateau (two thousand feet above the sea) which extends westwards and southwards from the peak of Pocono Mountain, in Monroe County. This plateau consists of level strata of sandstone, the southern part of which is characterised by a thin sandy soil, such as is naturally formed by the disintegration of the underlying rock, and there is no foreign material to be found in it. But, on going northwards to the boundary of Tobyhanna township, we at once struck a large line of accumulations, stretching from east to west, and rising to a height of seventy or eighty feet. This was chiefly an accumulation of transported boulders, resembling in its structure the terminal moraines which are found at the front of glaciers in the Alps and in Alaska, and indeed wherever active glaciers still remain. But here we were upon the summit of the mountain, where there are no higher levels to the north of us, down which the ice could flow. Besides, among these boulders we readily recognised many of granite, which must have come either from the Adirondack Mountains, two hundred miles to the north, or from the Canadian highlands, still farther away.
Limiting our observations simply to the boulders, we should indeed have been at liberty to suppose that they had been transported across the valley of the Mohawk or of the Great Lakes by floating ice during a period of submergence. But we were forbidden to resort to this hypothesis by the abrupt marginal line, running east and west, upon Pocono plateau, along which these northern boulders ceased. South of this evident terminal moraine there was no barrier, and there were no northern boulders. On the theory of submergence, there was no reason for the boundary-line so clearly manifested. Ice which had floated so far would have floated farther.
Still further, on going a few miles east of the Pocono plateau, one descends into a parallel valley, lying between Pocono Mountain and Blue Mountain, and one thousand feet below their level. But our marginal southern boundary of transported granite rocks did not extend much farther south in the valley than it did on the plateau, except where we could trace the action of a running stream, evidently corresponding to the subglacial rivers which pour forth from the front of every extensive glacier. In these facts, therefore, we had a crucial test of the glacial hypothesis, and, in view of them, could maintain, against all objectors, the theory of the distant glacial transportation of boulders, even over vast areas of the North American continent.
Since that experience, I have traced this limit of southern boulders for thousands of miles across the continent, according to the delineation which may be seen in the map in a later chapter. If necessary, I could indicate hundreds of places where the proof of glacial transportation is almost as clear as that on the Pocono plateau in Pennsylvania. One of the most interesting of these is on the hills in Kentucky, about twelve miles south of the Ohio River, at Cincinnati, where I discovered boulders of a conglomerate containing many pebbles of red jasper, which can be identified as from a limited formation cropping out in Canada, to the north of Lake Huron, six hundred or seven hundred miles distant. That this was transported by glacial ice, and not by floating ice, is evident from the fact that here, too, there was no barrier to the south, requiring deposits to cease at that point, and from the further fact that boulders of this material are found in increasing frequency all the way from Kentucky to the parent ledges in Canada. With reference to these boulders, as with reference to those found on the summit of Mount Washington, we can reason, also, that any northerly subsidence permitting a body of water to occupy the space between Kentucky and Lake Superior, and deep enough to facilitate the movement across it of floating ice, would render it impossible for the ice to have loaded itself with them.
The same line of reasoning is conclusive respecting the innumerable boulders which cover the northern portion of Ohio, where I have my residence. The whole State of Ohio, and indeed almost the entire Mississippi basin between the Appalachian and the Rocky Mountains, is completely covered, and to a great depth, with stratified rocks which have been but slightly disturbed in the elevation of the continent; yet, down to an irregular border-line running east and west, granitic boulders everywhere occur in great numbers. In the locality spoken of in northern Ohio the elevation of the country is from two hundred to five hundred feet above the level of Lake Erie. The nearest outcrops of granitic rock occur about four hundred miles to the north, in Canada. After the meeting of the American Association for the Advancement of Science in Toronto in the summer of 1889, I had the privilege of joining a company of geologists in an excursion, conducted by members of the Canadian Survey, to visit the region beyond Lake Nipissing, north of Lake Huron, where the ancient Laurentian and Huronian rocks are most typically developed. I took advantage of the trip to collect specimens of a great variety of the granites and gneisses and metamorphic schists and trap-rock of the region. On bringing them home I turned them over to the professor of geology, who at once set his class at work to see if they could match my fragments from Canada with corresponding fragments from the boulders of the vicinity. To the great gratification, both of the pupils and myself, they were able to do so in almost every case; and so they might have done in any county or township to the south until reaching the limit of glacier action which I had previously mapped. Here, at Oberlin, on the north side of the water-shed, it is possible to imagine that we are on the southern border of an ancient lake upon whose bosom floating ice had brought these objects from their distant home in Canada. But this theory would not apply to the portion of the State which is south of the water-shed and which slopes rapidly towards the Gulf of Mexico. Yet the distribution of boulders is practically uniform over the glaciated area on both sides of the water-shed, constituting thus an indisputable proof of the glacial theory.
4th. As the significance of the gravel terraces which mark the lines of outward drainage from the glaciated area cannot well be indicated in a single paragraph, the reader is referred for further information upon this point to the general statements respecting them throughout the next chapter.