THE PLEISTOCENE OR GLACIAL PERIOD 859 



extensive areas. From its distribution we know that the force or 

 forces which produced it were largely independent both of under- 

 lying rock formations and of topography. From its physical make 

 up we know that the agency or agencies which produced it must 

 have been able to carry and deposit, at one place and at one time, 

 materials as fine as the finest silt or mud, and bowlders many tons 

 in weight, while they were competent, under other circumstances, 

 to make deposits of much less extreme diversity. From its litho- 

 logical make up, and from the nature of the finer parts of the drift, 

 we know that the drift forces worked on different sorts of rock, 

 deriving materials from many; that they ground some of the ma- 

 terials into a fine earthy powder or "rock flour," commonly called 



Fig. 572. Small protuberances of rock showing the effect of ice wear. Gla- 

 cial knobs and trails. The projections consist of chert in limestone. 

 Near Darlington, Ind. (U. S. Geol. Surv.) 



clay; that they as a rule derived the larger part of the drift of any 

 locality from formations near at hand; and that the materials, even 

 large bowlders, were sometimes carried up to altitudes considerably 

 above their source. From the structure of the drift it is concluded 

 that the drift force or forces must have been capable of producing 

 deposits which were sometimes stratified and sometimes unstratified, 

 and that the deposition of these two phases of drift was sometimes 

 contemporaneous and sometimes successive, the number of alter- 

 nations being considerable in some places. From the striae on the 

 stones of the drift it is known that the production of the drift must 

 have involved the action of forces which, under some conditions, 



