ANCIENT GLACIERS. 119 



while ten miles back from its front there has been ten 

 times as much moving ice actually engaged in erosion, 

 and in the extreme north several hundred times as much 

 ice, Thus it is evident that we do not need to resort to 

 two glacial periods to account for the relatively small 

 amount of erosion exhibited over the southern portion of 

 our glaciated area. 



At the same time, it should be said that the indica- 

 tions of active glacial erosion near the margin are by no 

 means few or small. In Lawrence County, Pennsylvania, 

 on the very margin of the glaciated area, Mr. Max Fo- 

 shay * has discovered very extensive glacial grooves, indicat- 

 ing much vigour of ice-action even beyond the more exten- 

 sive glacial deposits which Professor Lewis and myself had 

 fixed upon as the terminal moraine. In Highland and 

 Butler Counties, Ohio, and in southwestern Indiana and 

 southern Illinois, near the glacial margin, glacial grooves 

 and striae are as clear and distinct in many cases as can 

 anywhere be found ; while upon the surface of the lime- 

 stone rocks within the limits of the city of St. Louis, 

 where the glacial covering is thin, and where disintegrat- 

 ing agencies had had special opportunities to work, I 

 found very clear evidences of a powerful ice-movement, 

 which had planed and scratched the rock surface ; and 

 at Du Quoin, Illinois, as already related, the fragments 

 thrown up from the surface of the rock, fifty or sixty feet 

 below the top of the soil, were most beautifully planed 

 and striated. It should be observed, also, that this whole 

 area is so deeply covered with debris that the extent of 

 glacial erosion underneath is pretty generally hid from 

 view. 



4. The uniformity of the distribution of the glacial 

 deposits over the southern portion of the glaciated area in 

 the Mississippi Valley is partly an illusion, due to the 



* Bulletin of the Geological Society, vol. ii. pp. 457-464. 



