THE DATE OF THE GLACIAL PERIOD. 591 



In both cases, doubtless, the wide troughs were preserved 

 by the presence of lingering masses of the melting ice sheet. 



3. The theory of a general depression of the glaciated 

 area with reference to the sea-level may apply to a certain 

 portion of a single period as well as to one of two distinct 

 periods. We may suppose a low slope of a surface and the 

 consequent imperfect drainage and slow-moving waters dur- 

 ing the maximum extent of a single glacial epoch as well 

 as during the first of two epochs. The theory that the 

 weight and attraction of the ice were tangible factors in pro- 

 ducing the relative depression of land which characterized a 

 portion of the Ice age would lead us to expect the greatest 

 depression during the period of maximum extension. When 

 the ice-front had retreated from Carbondale, 111., to Mad- 

 ison, Wis., the intervening area had been relieved from an 

 enormous amount of pressure. 



4. With reference to the comparative absence of glacial 

 striae and of planing and grooving over the southern area, it 

 should be noted, first, that fresh exposures of rock in that 

 region are very infrequent, owing to the great depth of till 

 and loess; and, secondly, that upon any theory the gla- 

 cial grooving and striatum would necessarily grow fainter as 

 the boundary was approached, because the movement of ice 

 over that portion was so much less than over the central and 

 northern portions ; and, thirdly, the absence of planation 

 is not relatively so great as is sometimes represented. The 

 grooves and striae in Highland and Butler counties, Ohio, 

 very near the margin, and in southwestern Indiana and 

 southern Illinois, still nearer the margin, are as clear and 

 distinct as can anywhere be found. Also, upon the surface 

 of the limestone rocks, within the limits of the city of St. 

 Louis, where the glacial covering was thin, and disintegrat- 

 ing agencies had had special opportunities to work, I found 

 very clear evidences of a powerful ice-movement ; and at 

 Du Quoin, 111., only forty or fifty miles back from the ex- 

 treme limit of glaciation, I was greatly impressed with the 

 extent to which the surface rock had been planed, by ex- 



