THE PLEISTOCENE OR GLACIAL PERIOD 851 



are most abundant where the underlying formations and neigh- 

 boring formations in the direction from which the drift came are 

 weak. The fine part of the drift is made up chiefly of the same 

 materials as the gravel and bowlders, but of these materials in a 

 fine state of subdivision. The coarse and the fine materials are 

 often mixed without trace of assortment or arrangement. 



The drift of any locality is likely to contain rock material from 

 every formation over which the ice which reached that locality 

 had passed; but the larger part of the drift of any place is of ma- 



Fig. 563. A large bowlder in northwestern Illinois. (Carman.) 



terials from formations near at hand. Probably 75% of the 

 material of the drift was not moved 50 miles. 1 No agent except 

 glacial ice can impress these precise features on the deposits which 

 it makes. 



2. Peculiarities of the bowlders, etc., of the drift. The bowl- 

 ders and smaller stones of unstratified drift possess significant 

 features. Many of them have smooth surfaces, but they are not 

 generally rounded. They are often subangular, and the wear which 

 they have suffered has been effected obviously by planing and 

 bruising, rather than by rolling (Figs. 205 and 564). Some of these 

 planed, subangular bowlders and stones are distinctly marked with 



1 The Local Origin of the Drift, Jour. Geol., Vol. VIII, p. 426.. 



