Part 7

_Abandoned gaps._ Many abandoned water-gaps must exist among the hills of the state. Cook’s Gap, through which the New York and New England Railroad crosses the trap ridge, three miles west of New Britain, has already been discussed. It must not be confounded with the majority of the other gaps in the trap ridge, which are oblique, break the alignment of the ridge, and are due to faults.

The New York and New England Railroad in ascending to the eastern plateau passes through Bolton Notch, a few miles east of Manchester. This notch, also, is an abandoned river bed but, as it seems, abandoned at a later date and for another reason than that assigned for Cook’s Gap. The drift is very heavy in this region, and the most probable explanation is that the post-glacial streams do not altogether follow pre-glacial valleys. This gap, used by turnpike and railroad, testifies of another and older drainage system.

That in this brief article all the problems connected with the Connecticut rivers have been solved, or even noted, is not to be expected. It is hoped, however, that the work done may prove a help to further study of the same regions, and that the tentative conclusions advanced may be substantiated by further investigation.

HENRY B. KUMMEL.

FOOTNOTES

[33] The author desires to express his obligation to Professor W. M. Davis for aid in the preparation of this article. It was first written under his direction and with the help of his suggestions when the author was in the graduate school of Harvard University. Prof. Davis is not responsible, however, for the statement of the views herein advanced, although in general it is believed that he is in accord with them.

[34] Amer. Jour. Sci. 3d ser., vol. xxxvii, 1889, p. 423. Bull. Geol. Soc. Amer., vol. ii, p. 545.

[35] The rough diagrams accompanying this paper may aid the reader who is unacquainted with the details of the region under discussion. The abbreviations on the above figure are as follows: C. The Connecticut. Cr. Pl. Crystalline plateau (the shaded area). F. The Farmington. H. Hartford. Ho. The Housatonic. Lm. Limestone area. M. Meriden. Mi. Mill River. Mt. Middletown. N. The Naugatuck. N. H. New Haven. No. The Norwalk. Q. The Quinnipiac. Qg. The Quinnebaug. S. The Scantic. Sa. The Saugatuck. T. Tariffville. Th. The Thames. The unshaded area is the Triassic sandstone lowland, and the blackened areas represent the ridges of the faulted trap sheets.

[36] Am. Jour. of Sci., 3d ser., vol. xxxvii, p. 430.

[37] Bulletin of Geol. Soc. of Amer., vol. ii, p. 554.

[38] It is not desired to affirm that these periods of erosion and elevation began and ended promptly with the beginning or end of a period. The time statements must be considered as only approximate.

[39] An exaggerated idea must not be had of the steepness and narrowness of these crystalline valleys. The valley of the Farmington, five miles up from where it opens into the Triassic sandstone, is 400 to 500 feet deep, and a mile and a half wide at the top. The Connecticut valley, just below Middletown, is about 400 feet deep and two miles wide at the top. These are fair representatives of the valleys in the crystalline rocks in the central part of the state.

[40] Amer. Jour. of Sci., vol. x, 3d ser., 1875, p. 506.

[41] McGee. Amer. Jour. Sci., 3d ser., vol. xxxv, p. 376.

[42] Davis and Wood, Geographic Development of Northern New Jersey, pp. 413, 414.

[43] “Rivers and Valleys of Pennsylvania,” Davis, W. M., published in The National Geographic Magazine, in 1889.

[44] Davis, W. M., “Geographic Development of Northern New Jersey,” p. 397–8.

[45] Davis, W. M., Amer. Jour. of Sci., 3d ser. vol. xxxvii., 1889, p. 432.

[46] Geog. Devel. of Northern New Jersey, p. 404 et seq. Proc. Bos. Soc. Nat. Hist. Also Rivers of Northern New Jersey, p. 11 et seq. National Geographic Magazine, vol. ii, p. 93.

[47] Faults in the Triassic Formation near Meriden, Conn. Bulletin of the Mus. Comp. Zoöl. Harvard Univ. vol. xvi. No. 4, p. 82.

[48] J. D. Dana, Amer. Jour. Sci. 3d. ser., vol. xv, p. 506.

[49] J. D. Dana, Amer. Jour. of Sci. 3d ser. vol. xxv, p 446.

[50] J. D. Dana, Amer. Jour. of Sci., 3d ser., vol. xxiii, p. 198.

[51] J. D. Dana. Amer. Jour. Sci., 3d ser., vol. xxv, p. 441.

_STUDIES FOR STUDENTS._

GEOLOGICAL HISTORY OF THE LAURENTIAN BASIN.

The study of the Pleistocene history of the basin drained by the St. Lawrence has been fragmentary and is still far from being complete. There is a lack of agreement in the interpretation of observations already made, due in part to the comparatively limited portion of the field examined even by those who have given the subject most attention, and in part to lack of uniformity in the standards of comparison used. It is with the hope of assisting in reaching more harmonious results that attention is here invited to methods of study.

In the present treatment of the subject it may be advantageously subdivided, and the facts and hypotheses relating to each division separately considered. Of the divisions that may be suggested the following seem the most important:

1. Character of the sub-morainal or hard-rock topography in the Laurentian basin.

2. Origin of the basin.

3. Sedimentary deposits.

4. Shore markings left by former water-bodies.

5. Fossils in ancient sediments, shore ridges, terraces, etc.

6. Fauna of the present lakes.

7. Changes in elevations of the land.

8. Former outlets.

9. Probable effects of an ice sheet on drainage.

10. Probable effects of a subsidence which would make the basin an arm of the sea.

1. _Character of the hard-rock topography._ In order to learn the character of the Laurentian basin it is necessary to examine the rock surface beneath the general covering of glacial débris and stratified sediments which partially fill it. To do this, those areas in which rock in place forms the surface require to be mapped and their elevations noted; the records of wells and other excavations which pass through the superficial deposits should also be obtained and the character of the underlying rock ascertained, as far as is practicable. When sufficient data of this nature shall have been recorded, a contour map of the basin can be drawn that will reveal the shape of the depression with which the student has to deal. The depth of the present lakes plus an estimated thickness of clay and morainal material covering their bottoms, will probably furnish the only means of sketching contours over the deeper portions of the basin. Even an approximately accurate map of this character cannot be constructed for a long time to come, but every advance towards it will serve to make the problems to be studied more and more definite.

Something of the form of the rock-basin is already known and several deep channels in its borders, now filled with drift, have been discovered. The courses of buried channels connecting the basins of some of the present lakes have also been approximately determined. It is not necessary at this time to refer specifically to the discoveries that have been made, but it may be stated that enough is known to assure us that the basin is a depression in solid rock, the bottom of which is below sea level.

2. _Origin of the basin._ The rocks in which the Laurentian basin is situated are, with the exception of the Lake Superior region, nearly horizontal and belong almost wholly to the Paleozoic. The basin is essentially a depression in undisturbed strata, and all who have considered its origin seem agreed that it has been formed by excavation. A vast mass of horizontal strata has been removed, leaving an irregular rim of undisturbed rocks on all sides. The form of the depression is now obscured by drift; the deeper portions contain stratified sediments which have been deposited within it and it has been warped somewhat by orographic movement.

The manner in which the excavation was formed has been explained principally in two ways. One hypothesis is that it owes its origin to a time of subaërial denudation preceding the Glacial epoch, during which a valley, or series of valleys, was worn out by stream erosion; and that the depression thus produced has been but slightly modified by ice action. The closing of the ancient valley has been referred to orographic movements and to the filling of its outlet by glacial débris. Another hypothesis is to the effect that the excavation is mainly due to ice erosion during the Glacial epoch, without special reference to previous topographic relief. A warping of the earth’s crust so as to produce a true orographic basin does not seem to require consideration, for the same reason as already stated, that the rocks in which the basin lies have been but little disturbed from their original horizontal position. Future study of the region must determine which of the two hypotheses outlined above best suits the facts; or if each hypothesis has something in its favor, what combination of the two may be accepted as the final explanation.

It is a suggestive fact in connection with the first of these hypotheses, that the youngest rocks in the region antedating the Pleistocene belong to the Carboniferous. This seems to show that the land has not been submerged since at least the close of the Paleozoic. If not a region of sedimentation during this vast interval, it must have been subjected to erosion. The erosion of an ancient land surface might result in the production of topographic forms of diverse character, depending on its altitude, on the length of time it was exposed to atmospheric agencies during various stages of elevation, and on climatic and other conditions. The study of topographic forms is now sufficiently advanced to enable one to predict somewhat definitely what features would appear under certain conditions. We also know the characteristics of topographic forms due to glacial erosion. It seems evident, therefore, that a knowledge of the hard-rock topography in the Laurentian basin, would enable one to draw definite conclusions in reference to the part that ice and water each had in shaping the forms now found there.

The conclusion that the region under consideration has been glaciated is well established; it remains, therefore, to determine what topographic forms, if any, due to pre-glacial stream erosion can be recognized. As an example of this kind of evidence desired, attention may be directed to the northward facing rock escarpments which follow the southern shores of lakes Erie and Ontario for a large part of their courses and at varying distances up to several miles. These escarpments are composed of the edges of nearly horizontal strata, mostly of Paleozoic limestone, and their bases are buried beneath glacial débris and stratified clays so deeply that in some instances, at least, they do not reveal half of their actual height. These escarpments not only have Pleistocene deposits banked against them, but their faces and summits are polished and grooved, showing how stubbornly they resisted the invasion of the ice which impinged against them from the north. South of lake Ontario especially, the trend of the escarpment referred to is directly athwart the course of the ancient glaciers. The entire history of these escarpments cannot be discussed here, as my desire is simply to call attention to the fact that they existed before the Glacial epoch, and are relics of a strongly accented pre-glacial topography. They are within the southern border of the Laurentian basin, and hence afford means of determining, in part, what was the form of that basin before it was modified by ice action. Other similar escarpments exist in the northern and western portions of the same great basin, and as this study progresses it is to be expected that still other features of the pre-glacial land will be revealed. It is perhaps too early to decide what were the special topographic forms which gave character and expression to the St. Lawrence basin before the ice invasion, but the Erie and Ontario escarpments and some other similar features now recognized, suggest that in Tertiary times it resembled the present condition of the upper portion of the Mississippi valley, where bold, rock escarpments border wide stream-worn depressions.

Deep drift-filled channels are known to cut across the Erie and Ontario escarpments. These seem to have been formed by streams tributary to the main drainage line to the north. If this conclusion is well founded, a study of the hard-rock topography should reveal other similar channels and finally indicate a well matured drainage system. If even the broader and stronger features of the pre-glacial surface can be determined, then the modifications due to glacial abrasion will become conspicuous, and the amount that glaciers have broadened and deepened the basin be determinable.

A study of the lithological character of the drift south of the present lakes should show, at least in a rough way, what portion of it was derived from the waste of rocks within the Laurentian basin. This inquiry has already been undertaken by at least two geologists, and estimates of the quantity of material removed from the basins of lakes Michigan and Erie respectively, have been made. This method may be extended so as to embrace a larger area, or some special portion of the great depression best suited for the trial may be selected. If the material removed from the basin or re-distributed within it by glacial action can be shown to be approximately equivalent in volume to the amount of rock excavated in order to form the depression, it would evidently tend to support the hypothesis of glacial erosion. If, on the contrary, the amount of débris derived from the basin should fall far short of what would be requisite to refill it, no very definite conclusion would seem to be indicated unless account could also be taken of the fine material carried away by glacial streams.

As the case stands at present it appears that there is evidence of a pre-glacial valley or series of valleys as has been claimed by several geologists, and that all but the boldest features of the old topography have been obliterated or greatly modified by glacial erosion followed by glacial and other sedimentation. Additional observations should show somewhat definitely the amount of work assignable to particular portions of the history. How far the results of subaërial and of glacial erosion have been modified by other agencies, more especially by orographic movements, has also to be considered.

If the St. Lawrence basin shall be shown to be largely the result of subaërial erosion it will follow, unless it is found that the deeper portions are the result of glacial action, that the land at the time the streams did their work, must have stood higher than at present, for the reason that the bottom of the depression is now below sea level. Some idea of the smallest amount of elevation necessitated by this hypothesis might be obtained by estimating the gradients of the ancient streams and the amount of elevation required to bring the bottom of the depression up to sea level.

A study of the hard-rock topography in the valleys of the Ottawa and St. Lawrence and of the present submerged Atlantic border of the continent would also be instructive in this connection. The strict correlation of the topographic history of the interior and of the continent’s margin may be difficult, but as the two regions are directly connected, valuable results should follow their comparative study.

The hypothesis that the Laurentian basin is due largely to pre-glacial erosion, necessitates also that the ancient system of river valleys should have been closed in some way so as to form the basins of the present and of former lakes. The closing has been referred to several agencies. An unequal subsidence following the period of stream erosion has been postulated. During the Glacial epoch the entire region was ice-covered and only glacial streams of one kind or another could have existed. On the retreat of the ice, when portions of the basin were abandoned, the drainage is supposed to have been obstructed by the ice itself, as will be noticed below. When the glaciers melted, a vast sheet of débris was left which in many instances filled or obstructed previous drainage lines. Old channels, now deeply buried, have been reported to connect the basins of the various existing lakes, as has already been mentioned, but no similar channel which could have afforded an escape for the waters of the entire basin has been discovered. Here again an acquaintance with the hard-rock topography should give assistance and indicate either that such a channel existed or that orographic movements have taken place which have obstructed the former drainage system. The glacial hypothesis assumes that the basin was excavated mainly by glacial abrasion and does not require that the land should be either higher or lower than at present. The study in this direction merges with that of the general glaciation of the northeastern part of the continent, and cannot be treated at this time.

3. _Sediments._--Regularly stratified deposits of clay and sand occur along many portions of the borders of the present Laurentian lakes. These were clearly formed in water bodies which formerly existed within the Laurentian basin, and which in certain directions, at least, were of wider extent than the present lakes. The areas occupied by these deposits have been partially mapped, but much remains to be done in this direction. Fresh sections, particularly of the stratified clays, are exposed from time to time by artificial excavations, in which much of their history may be learned. Not only should records be made of the facts noted at special excavations, but the extent and character of the stratified deposits in one area should be determined and compared with similar data obtained in other areas. For example: the clays covering large tracts on the west shore of Lake Michigan and on the southern and western border of Lake Superior are of a red color, while other areas bordering Lake Erie are covered with blue clay. These two deposits have been supposed to have been laid down at the same time and in the same lake. The definite correlation of the clays of these two areas by direct contact, however, does not seem to have been made, and there are reasons for thinking that they may be quite distinct and that they originated in separate lakes.

The outer limits of the deposits of clay and sand here referred to are known in some instances to be determined by ancient beaches and terraces. Such associations of deep and of shallow water deposits require special attention, as the study of one may assist in interpreting the significance of the other. The fine, evenly stratified clays frequently contain large angular bowlders, which appear to have been dropped from floating ice and to show an intimate connection between the ancient lakes and neighboring glaciers. The possibility, however, of the bowlders having been brought into the ancient water bodies by rivers, or floated outwards from the shore by lake ice, should also be considered. Huge angular masses of limestone have been reported as occurring in southern Michigan especially, which rest on superficial deposits and are thought to have been carried northward by lake ice. The relations of these masses to well defined shore lines have never been determined. If it should be found that they are above all former shores, it is evident that they must have been carried by some other agency than the one mentioned.

A chemical examination of the clays, or of their contained water, may indicate whether or not the basin was formerly in direct communication with the ocean. Analyses of the clays of the Champlain valley and of the similar clays in the Ontario and Erie basins might indicate whether or not they were deposited under similar conditions.

4. _Shore records._ Beaches and terraces have been studied at many localities about the borders of the present lakes, sometimes at a distance of more than twenty miles from their margins and at various elevations up to several hundred feet above their surfaces. In some instances these ancient shore records have been followed continuously for scores of miles. The tracing and mapping of individual beaches is one of the most important parts of the study here outlined, and is already well advanced. Confusion has unfortunately arisen, however, for the reason that topographic features, due to shore action, have, in some instances, been confounded with somewhat similar features due to other causes. Moraines and gravel ridges, formed by glacial streams, have been mistaken for beach ridges, and terraces of various origin have not been clearly discriminated.

In order not to be led astray by topographic forms that simulate shore phenomena, the student should examine the shores of existing lakes and learn what records are there being made. In the study of topography, “the present is the key to the past,” just as definitely as in any other branch of geology. The topography of lake shores has already received attention from one skilled in reading geological history in the relief of the land[52] and the study of existing shores in the light of what has already been done in that direction should enable even the beginner to avoid falling into serious error in interpreting ancient records of the same nature.

To be able to discriminate clearly between shore features and somewhat similar glacial phenomena, it is necessary to become familiar also with the topography of glacial deposits. Fortunately in this study also a guide is at hand[53] which, in connection with field observations, should soon train the eye to discriminate the shapes assumed by moraines and the deposits of glacial streams from all other topographic forms.

In examining the records of former lakes it will soon be observed that, in many instances, where the highest of a series of ancient beaches is obscure and indefinite, the topographic expression above and below a certain horizon, and also the character of the surface material, whether of the nature of lacustral clays and sands or of glacial débris, residual clay, etc., above and below the same level, are significant, and enable one to map the outline of a former water body with considerable accuracy.

In tracing ancient beaches and terraces, their forms and internal structure need to be recorded, so that the fact of their being true shore records may be made plain to others. The elevations of various well-defined points throughout the extent of an ancient shore should be carefully measured, for, as will be noticed below, although originally horizontal, they have, in many instances, been elevated or depressed, owing to broad general movements of the earth’s crust. The continuous tracing of individual shore lines for as great a distance as possible is highly desirable, especially in a wooded country, in order to be positive as to which ridge or terrace measurements of elevation relate, and also for the purpose of observing the nature of the changes that occur when a shore line gives place to other records. For example: some of the ancient beach ridges about the west end of Lake Erie have been found to be continuations of moraines. In other instances shore ridges have been reported to end indefinitely and to be replaced at the same general horizon by glacial records of various character. The correct interpretation of phenomena of this nature is especially important.