GLACIAL EROSION AND TRANSPORTATION. 265 



the ice, has moved along in a previously formed mold, 

 scouring it out and polishing it, and somewhat enlarging it. 

 Thus it comes about that sometimes even the under surface 

 of a projecting rock has been scratched and polished by the 

 ice-movement. In case of the shallower grooves, two or 

 three inches or less in depth, the abrupt variations are numer- 

 ous, as if a pebble had encountered an obstacle to its direct 

 movement, and turned aside to move onward in a new line 

 of least resistance, the general course remaining substantially 

 the same. 



That there has been here considerable erosion by direct 

 action of the ice there can be no doubt ; but that it is less 

 than some suppose is perhaps shown from the cross-striae, 

 which indicate four distinct but successive movements. The 

 course of events was plainly as follows: 



1. When the advancing ice filled the valley of Lake Erie, 

 but had not surmounted the summit of the water-shed to the 

 south, and before the Illinois lobe of ice had extended across 

 Lake Erie's natural southwestern outlet, the ice-movement 

 in Lake Erie was in the line of the longest diameter of the 

 lake — i. e., southwest. 



2. During the height of the Glacial period, w T hen the ice 

 surmounted the water-shed between the lake and the Missis- 

 sippi basin, about one hundred miles to the south and from 

 fiv& to seven hundred feet higher, the movement of the 

 ice was very nearly north by south. There was then no 

 opportunity for the ice to escape from the southwestern end, 

 because of the larger general movement of the glacier across 

 its pathway. So powerful was this movement in a southerly 

 direction that deep north-and-south channels were eroded in 

 all the islands. 



3. But, upon the retreat of the ice to the water-shed, and 

 the withdrawal of the glacier, which crossed its way on the 

 southwest, the former westerly line of movement was at once 

 resumed. Each later movement has done much to obliterate 

 the marks of the earlier. Upon the eastern slopes this oblit- 

 eration is sometimes complete. But wherever there was a 



