CRITERIA OF GLACIATION 617 



fine materials are most abundant where the underlying formations 

 and neighboring formations in the direction from which the drift 

 camr. arc \\vak. The fine part of the drift is made up largely of the 

 sanu- mall-rials as the gravel and bowlders, but of these materials in 

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

 as a rule, mixed without trace of assortment or arrangement. The 

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

 formalioii ovi-r which the ice which reached that locality had passed; 

 but the larger part of the drift of any place is from formations near 



^ 



Fig. 507. A large bowlder in northwestern Illinois. (Carman.) 



at hand. Over large areas it is probable that 75% of the drift 

 was not moved 50 miles. 1 No agent except glacial ice makes de- 

 posits with these characteristics. 



2. Bowlders of the drift. Many of the bowlders and smaller 

 stones of unstratified drift have smooth surfaces, but they are not 

 generally rounded. Many are subangular, and the wear which 

 they have suffered was effected by planing and bruising, rather than 

 by rolling (Figs. 147 and 508). Some of these planed, subangular 

 bowlders and stones are distinctly marked with one or more series of 

 lines or stria on one or more of their faces. The lines of each series 

 are parallel, but those of different sets may cross at any angle. By 

 no means all the stones of the drift show striae. They are rarely 

 seen on those which have lain long at the surface, and they are more 

 common on the less resistant sorts of rock, such as limestone. No 



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



