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.