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 lakes would 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.