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 toexamine 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 formedby 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 theresult 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 drainagesystem. 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 theancient 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. Thetopography of lake shores has already received attention from one skilled in reading geological history in the relief of the land52and 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 hand53which, 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 thatoccur 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.
Accurate measurements of the vertical intervals between well defined beaches at many localities would enable one to identify special horizons, providing orographic movements were not in progress during the time the series was forming. This method has recently been successfully applied on the north shore of Lake Superior, where the character of the country does not admit of the tracing of individual terraces for considerable distances.
The deltas of tributary streams should also be revealed in the topography of the basin of an ancient water body. Changes in the character of lacustral sediments near where rivers emptied are also to be looked for. Sand dunes are frequently an important accompaniment of existing shores, and their association, perhaps, in a modified form, with ancient beaches is to be expected.
5.Fossils.Thus far only a few fossils have been found in the stratified clays and sands or in the ancient beaches of the Laurentian basin. Such observations as have been made in this connection indicate an absence of the remains of marine life and the presence, in a few instances, of fresh-water shells in all of the basin west of the eastern border of the basin of Lake Ontario. To the eastward of Lake Ontario, however, in the St. Lawrence and Champlain valleys, marine fossils are common in deposits supposed to be contemporaneous with the stratified clays to the west.
A careful search in the clays and beaches left by the former water bodies might be rewarded by important discoveries. In this examination microscopical organisms should not be neglected. If after a detailed examination no fossils are discovered,this negative evidence would have its value, as it would indicate that the physical conditions were not favorable to life, and an explanation for this fact might be found. It is scarcely necessary to mention that care should be taken not to mistake the shells occurring in modern swamp deposits associated with the ancient beaches for true lacustral fossils.
About the borders of the present lakes and sometimes even below the level of the lowest of the ancient beaches the remains of the mastodon, elephant, giant beaver, elk, bison, deer, etc., have been found. The recency of the existence of such of these animals as are extinct may thus be established, as well as the former distribution of those still living in other regions.
Evidence of the existence of man has been reported from one of the old lake ridges in New York, and it is important that this interesting discovery should be sustained by evidence from other localities. Stone implements especially should be looked for in undisturbed lacustral clays, and in the gravels of the ancient beaches.
The remains of forests have been stated to occur in the lacustral clays adjacent to the south shore of Lake Erie. It is desirable to know the extent of these deposits and how continuous they are; also the character of the plant remains they contain, and whether they have been disturbed from the position in which they grew. Some of the questions that may be asked in this connection are: Was the basin drained and forest covered before the vegetable remains were buried, or were the plants floated to their present position, or did they grow on moraines covering the stagnant border of the retreating glacier and become involved and buried in morainal material as the ice melted?
6.Life in the present lakes.The fauna of the present lakes has a bearing on their past history, for the reason that in the deeper parts of lakes Superior and Michigan crustaceans and fishes have been found which are believed to be identical with marine forms. These may be considered as “living fossils,” and are thought by some to indicate that the lakes in which they occur were formerly in direct communication with the ocean. Ifthe occurrence of living marine species in the present lakes is found to be widely at variance with the history of the basin as determined from physical evidence, an inquiry should be made in reference to the manner in which the species discovered might migrate.
7.Changes in elevation.One of the most difficult problems in connection with the history of an inland region is the determination of changes of level. By leveling along an ancient beach, post-lacustral changes in the relative elevations of various points may be readily ascertained. Pre-lacustral changes, however, by which ancient valleys have been obstructed, are much more difficult of direct observation, but might appear from the study of the hard-rock topography, as has already been suggested. This branch of the investigation, however, should more properly begin at the coast and be extended inland.
8.Former outlets.Several localities where the waters of the Laurentian basin have overflowed during former high-water stages have been pointed out, but some confusion has arisen in this connection, for the reason that the channels formed by streams issuing from the margin of the ice during the closing stages of the Glacial epoch have, in some instances, been mistaken for evidence of former lake outlets. The old outlets which seem to have been well determined are situated at different levels, and show that the entire basin could not have been occupied by a single great water-body, unless, as has been supposed by some, it was in direct communication with the sea. This hypothesis will be considered below. It has sometimes been assumed that all of the basin below the level of some ancient outlet was once flooded, so as to form a great lake in all of the basin now situated at a lower level; but, in making such generalizations, the possibility of places in the rim of the basin being at a lower level than the outlet discovered, thus necessitating a special explanation, such as the partial occupation of the basin by glacial ice, or changes in elevation of such a character as to raise the locality of former overflow or to depress other regions, have to be considered.
Former outlets should bear a definite relation to neighboring shore lines and to sedimentary deposits. The channels leading from former points of discharge merit examination, as here again changes of level may perhaps be detected in the gradients of stream terraces.
Most of the ancient outlets thus far recognized lead southward, but as previously mentioned, a former channel of discharge north of Lake Superior has recently been reported. If this observation is confirmed, it will have an important bearing on questions relating to changes of level and to the position of the ice front during the later stages in the retreat of the glaciers.
9.Probable effects of a retreating ice sheet on drainage.The generally accepted conclusion that glaciers advanced southward and occupied the Laurentian basin during the Glacial epoch and retreated northward toward the close of that epoch, is sustained by a vast body of evidence. As the ice sheet withdrew it left a superficial deposit frequently one or two hundred feet thick over nearly all of the region it abandoned, and pre-glacial drainage lines were obstructed and mostly obliterated. As long as the slope in front of the ice was southward, the drainage from it found ready means of escape, but when the slope was northward towards the ice front, the drainage was obstructed and lakes were formed.
We have good reasons for believing that the topography of the Laurentian region was essentially the same at the close of the Glacial epoch as it is now, but the broader question of continental elevation is less definite. The inequalities of the surface being essentially as we now find them, it would follow that the first lake formed when the ice retreated to the north of the divide running through central Ohio and central New York, would be small and dependent on minor features in the relief of the land, and would discharge southward. As the ice retreated, the lakes would expand and become united one with another and the larger lakes thus formed would still find outlet across the southern rim of the basin. As the glaciers continued to retreat lower and lower, passes would become free of ice and the lakeswould be drained at lower levels, old beaches would be abandoned, the lakes would contract, and finally separate lakes would be formed in the lowest depression in the basins of the more ancient water bodies. The shape of the retreating ice front would be determined by topographic conditions and would in turn determine the northern outline of the lakes along its margin. This in brief is one hypothesis that has been proposed to explain the varied history recorded by the shore records, sediments, etc., within the basin.
10.Communication with the sea.Another hypothesis which assumes to account for some of the facts observed, is that the continent was depressed at the close of the Glacial epoch sufficiently to allow the sea to have access to the Laurentian basin. This hypothesis is coupled with others which do not recognize a period of Pleistocene glaciation, but, as already suggested, this is a matter that is considered by the great body of American geologists as not being any longer open to profitable discussion.
In the study here outlined the question whether the water bodies which formerly occupied the Laurentian basin were lakes or arms of the sea, should not be difficult of direct and positive determination. If fossils can be found within the basin, they might yield definite testimony, but even if they are absent or if their evidence is inconclusive, topography can be appealed to with the expectation of receiving a conclusive decision.
If the Laurentian basin was occupied by an arm of the sea during various stages in the Pleistocene elevation, then the records of such a submergence should occur both within and without the depression, and direct connection between the two should be expected. If the waters within the basin were capable of making such well-defined shore records as are now found, we are justified in assuming that the true ocean beach on the outer slopes of the basin would be still more conspicuous. Again, the waters within the basin deposited a sheet of sediment, certainly not less than one hundred feet thick; to be sure the conditions for rapid accumulation were there present, but if the ocean covered the adjacent land it should have left similar deposits.This is abundantly proven in the St. Lawrence and Champlain valleys, where clays containing marine fossils occur up to a certain horizon and record a Pleistocene invasion of these depressions by the sea. If the adjacent Ontario basin was occupied by the sea about the same time that the Champlain valley received its filling of clays containing marine fossils, there is every reason to believe that the deposits and their contained fossils in each basin would have been essentially the same.
One of the best known of the ancient shore lines about Lake Ontario has an average elevation of approximately 500 feet above the sea. If the sea had access to the basin at the time this breech was formed, then at corresponding horizon without the basin especially, to the south and southeast, where the full force of the Atlantic’s waves would have been felt, there should be still more prominent beaches.
Many well-defined shore lines in the Laurentian basin are much higher than the one just referred to, and if these were also formed during various stages of submergence, as has been claimed, it is evident that ocean beaches and ocean sediments of Pleistocene age should be looked for over nearly the whole of the eastern part of the United States. The student may easily answer this question for himself, and thus perhaps make a contribution to the subject here treated.
In the investigation here outlined, the work of previous observers should not be ignored, and every plausible hypothesis that has been advanced to account for the facts observed should be carefully tested. In writing these pages I have not quoted the writing of others, for the reason that a discussion of evidence has not been the aim in view, and also because the writings examined are so numerous that justice could not be done them in the space at command. That the literature relating to the subject is voluminous is indicated by the fact that an annotated bibliography of the Pleistocene history of the Laurentian basin, now in preparation, already contains over 200 entries of individual papers.
Israel C. Russell.
FOOTNOTES52The Topographic Features of Lake Shores, by G. K. Gilbert, in Fifth Ann. Rep. U. S. Geological Survey 1883–4.53Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch, by T. C. Chamberlin, in Third Ann. Rep. U. S. Geological Survey, 1881–2.
52The Topographic Features of Lake Shores, by G. K. Gilbert, in Fifth Ann. Rep. U. S. Geological Survey 1883–4.
52The Topographic Features of Lake Shores, by G. K. Gilbert, in Fifth Ann. Rep. U. S. Geological Survey 1883–4.
53Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch, by T. C. Chamberlin, in Third Ann. Rep. U. S. Geological Survey, 1881–2.
53Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch, by T. C. Chamberlin, in Third Ann. Rep. U. S. Geological Survey, 1881–2.