QUATERNARY AGE. 



255 



flooding that followed immediately the first retreat of the glaciers. 

 In Nebraska, however, they must have been formed at first be- 

 neath a glacial mass and then modified only in part by the floods 

 that followed immediately on their retreat. The reason of this 

 opinion is that here I have frequently found the boulders imbedded 

 n the blue clay lying lengthwise of the path of the glacier, and 

 striated like the rock beneath as already stated. If this clay had 

 been deposited from water this peculiar distribution and position of 

 the boulders would have been impossible. 



The following analysis indicates the character of this blue clay. 

 For purposes of comparison, I give also Wormly's analysis of the 

 blue clay of Ohio :* 



The character, ac, will be observed, of these clays, though so 

 widely separated, closely resemble each other. It should, however, 

 be remembered that other specimens are widely different some 

 having more silicic acid, alkalies, iron or alumina. 



Above the blue clay, in a few places, a whitish clay occurs. I 

 have not ascertained what relation it sustains to the blue clay, or 

 what its chemical composition is. 



Above these clays or till beds of boulder clay occur that occa- 

 sionally exhibit true marks of stratification. Following this is or- 

 dinary drift material, which lies directly on the country rocks, where 

 the blue, white and boulder clays are absent, as often occurs, espe- 

 cially in north Nebraska. This drift material is the most widely dif- 

 fused geological deposit in the State, though in vertical thickness it is 

 much less than others. Sometimes, in a few townships of some coun- 

 ties, it constitutes the surface soil, but generally it is buried beneath 

 later deposits. In rare instances it seems to have been removed 

 from the uplands by denudation, before the Loess was formed. 



l. I. of Xewberry's Geological Survey of Ohio, page 177. 



