The Parallel Drift-Hills of Wester?i New York. 201 



borings for briiie, and in driving piles for tlie canal aqueduct, 

 to the depth of a hundred feet or more. Our estimate of the 

 extent of glacial erosion in the vicinity of these lakes is, how- 

 ever, scarcely begun when we have sounded their depths, for 

 more than a thousand feet of rock were removed before the 

 present level of their waters was reached. 



Attention has already been invited to the fact, that the U]3per 

 Helderberg escarpment bends several miles south in approach- 

 ing Cayuga and Seneca lakes, and also to the indentation of the 

 Ontario shore oi:)posite the locality. Now this indentation of 

 the Ontario shore is where the Medina sandstone is found at its 

 lowest level. Taking these facts, together with that of the 

 maximum of glacial erosion being found along the line where the 

 exposed rocks are seen at their lowest levels, have we not an 

 indication of the causes which influenced the ice-flow in this 

 region ? 



Glaciers, like water, at first follow the lines of lowest level. 

 In the original topography of this region, previous to the ice 

 period, there was a valley here — not a deep one, it is true, but 

 deep enough to influence an ice-current. Evidence that this 

 valley was not confined to the immediate shore of Ontario, is 

 not wanting. Several miles south of Geneva, the outlet of 

 Crooked Lake, in its easterly course to Seneca Lake, exhibits a 

 fine section of the Portage group, Genesee slate, and Hamilton 

 shales, all dipping to the east at a comparatively high angle. 



I think Ave may safely assume that the pre-glacial drainage of 

 this valley contributed not a little to fit it for the great ice- 

 current which Avas to come. Indeed, it is generally conceded 

 by geologists that the ancient excavation in Avhich lies Onondaga 

 Lake, is probably a buried pre-glacial river-channel; and some 

 even suppose that this river drained Lake Ontai'io in a south- 

 easterly direction— a supposition Avhich is highly improbable. 

 It is much more reasonable to assume that the pre-glacial drain- 

 age of this region was not far different from its present; and 

 that the channel of the river where now lies Onondaga Lake, was 

 not only deepened, but subsequently filled up by the glacial 

 action, even as appears to have been the case with the north end 

 of Cayuga Lake, to which allusion has already been made. 



However this may be, it appears evident that when the ice 



