6i6 



THE PLEISTOCENE PERIOD 



reached the plains outside the mountains, 

 to have been buried beneath ice as now. 



Antarctica is assumed 



The Criteria of Glaciation 



The area of North America which was overspread by ice is 

 covered by a mantle of clay, sand, and bowlders which, taken to- 

 gether, constitute the drift. The various lines of evidence which 

 have led to the general acceptance of the glacial theory have to 

 do with (i) the drift, (2) the surface of the rock which underlies 



Fig. 506. "Pilot Rock," a glacial bowlder near Coulee City, Wash. (Garrey.) 



it, and (3) the relations of the drift to the bed. Some of the prin- 

 cipal considerations are the following: 1 



i. Constitution. One of the distinctive characteristics of the 

 drift is its heterogeneity, both physical and lithological. It is made 

 up at one extreme, of huge bowlders (Fig. 506), and at the other of 

 fine earthy matter. Between these extremes there are materials of 

 all sizes, and the proportions of coarse and fine are subject to great 

 variations. Coarse materials are, on the whole, most abundant in 

 regions of rough topography, where the underlying and neighboring 

 formations in the direction from which the drift came are resistant; 



1 Jour. Geol., Vol. II, pp. 708-724 and 807-835, and Vol. Ill, pp. 70-97. 



