SURFACE GEOLOGY. 75 



the great glacier moving from the north encountered here a high ridge, 

 which, though altogether the result of erosion, seems to have had an 

 anterior existence ; since the ice rose up its northern side, planed all the 

 slope, and curved round and embraced its irregularities as though it fol- 

 lowed rather than fashioned the topography. In the excavation of the 

 Lake Erie basin the glacier by which it was formed moved in the line 

 of its major axis from Buffalo to the islands. In the immediate basin of 

 the Lake the rocks are all planed, scratched, and sometimes deeply fur- 

 rowed in this direction; while on the plateau between Lake Ontario 

 and Lake Erie the bearing of the marks is nearly north and south. 

 That the depths of the basin were not excavated by the glacier which 

 produced these last named grooves is certain, from the fact that the east 

 and west grooves prevail almost exclusively on the islands and on the 

 immediate shore of the Lake ; the north-south furrows being very rarely 

 visible, and where the two systems are seen together, the east and west 

 grooves seem to be the most recent. 



The central and eastern portions of the bed of Lake Erie were once 

 occupied by quite soft rocks — Hamilton, Genesee, Portage and Chemung, 

 and Waverly. Of these, more than a thousand feet in thickness were 

 removed; and this portion of the basin was cut, to what depth we do not 

 know, as it is much silted up, but certainly much deeper than elsewhere. 

 When, however, the glacier which excavated the basin reached the Cin- 

 cinnati arch it encountered a massive barrier of hard rock, which offered 

 an obstinate resistance to its erosive action, and caused it to rise more 

 than 300 feet above its eastern level. This barrier has been deeply 

 scored, and the islands of the Lake have been wrought out of the solid 

 beds of the Corniferous and Helderberg limestones. As I have -men- 

 tioned elsewhere, previous to this time the basin of Lake Erie was tra- 

 versed by a deep river channel, into which the profound gorges of Grand 

 river, the Cuyahoga, etc., lead. Doubtless this river valley guided the 

 excavation of the Lake Erie basin, as it did that of Lake Ontario. It 

 traversed the area ©f the latter Lake nearly east and west, and connected 

 with the Hudson through the Mohawk gap. 



After leaving the basin of the present Lake, the Erie glacier was 

 deflected toward the south, and apparently flowed down the course of 

 the Wabash. The following table gives the bearings of the furrows made 

 by the Lake Erie glacier at different points : 



