Page:The Journal of geology (IA journalofgeology11893univ).pdf/413

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 debris 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