262 THE ICE AGE IF FORTH AMERICA. 



are often completely disintegrated to a great depth, some- 

 times amounting to scores of feet. What seem like beds of 

 gravel (and which can be handled with a shovel) often prove 

 to be horizontal strata of gneiss from which the cementing 

 material has been removed by the slow action of percolating 

 acids brought down by the rains. North of the glacial 

 boundary it is very rare to find any such extensive evidence 

 of disintegrating agencies. Since the ice passed over this 

 region there has not been sufficient time for subaerial agen- 

 cies to produce any marked disintegrating effect. 



Now, it is with much plausibility contended that the 

 action of the ice has been limited chiefly to the transporta- 

 tion of this disintegrated material, and that it has had little 

 effect as an eroding agency. The strong point in this repre- 

 sentation is, that there is little more loose soil over the 

 margin of the glaciated region than would result from the 

 simple transportation of the disintegrated material from the 

 northern and central portions of the glaciated region to the 

 marginal area. It certainly is clear, both from the necessities 

 of the case and from actual observation, that the area of 

 greatest erosion is nearest the center from which the ice 

 radiated, and that, as the amount of deposition increased 

 toward the margin, the erosion diminished. 



The advocates of the great erosive power of glacial ice 

 appeal, also, to the general appearance of the glaciated sur- 

 faces wherever exposed. The islands near Sandusky, in the 

 western part of Lake Erie, for example, present some of the 

 most marked indications of glacial erosion anywhere to be 

 found, and the facts there are justly appealed to by Professor 

 Newberry in support of the theory that the ice was a promi- 

 nent agent in the formation of the basins of the Great 

 Lakes. 



As this is so important a region for glacial study, I will 

 give somewhat in detail the result of my own recent obser- 

 vations. There are twelve or fifteen islands near the west- 

 ern end of Lake Erie, of which Kelly's, North Bass, Middle 

 Bass, South Bass, and Pelee are the principal, each having 



