THE PLEISTOCENE OR GLACIAL PERIOD 905 



Apart from such formations, there are others which seem to be 

 measurably or wholly independent of the ice. The wide-spread 

 gravels of the western plains have been referred to (p. 829), but 

 their deposition continued into the Pleistocene, and is indeed still 

 in progress. There are numerous tracts and belts of dunes where 

 conditions favor their development, as in central Nebraska, 1 and 

 some parts of Kansas. Dunes are of common occurrence locally 

 even east of the Great Plains, as about the head of Lake Michigan 

 and along its eastern shore. Even where dunes are wanting, wind- 

 blown sand and dust in small quantities are wide-spread. 



Outside the region affected by the ice-sheets, erosion rather 

 than deposition was the great feature of the Quaternary in the 

 interior. In the erosion, wind, running water, and ground-water 



have co-operated. 



In the West 



The Quaternary formations of the west belong to all the several 

 categories mentioned on p. 900, and in addition there is much 

 glacial drift left by mountain glaciers. Few of these various sorts 

 of deposits have received close study over any considerable area, 

 though something is known of all. The deposits of some of the 

 lakes at various points west of the Rocky Mountains, especially 

 those of the Great Basin, deserve special mention. 



Lacustrine deposits. The most considerable of the western 

 Pleistocene lakes was Lake Bonneville 2 (Fig. 596) of which Great 

 Salt Lake is the diminutive descendant. Its basin is believed to 

 have been due to deformation and faulting. Previous to the for- 

 mation of the lake, the basin is thought to have been arid, a con- 

 clusion based on the great alluvial cones and fans subsequently 

 covered by the lake. During the pre-lacustrine period of aridity, 

 such quantities of debris from the surrounding mountains were 

 brought into the basin as to bury the bases of the mountains to 

 depths of perhaps 2,000 feet, at a maximum. 



Later, climatic conditions became such as to bring a large lake 



1 Darton, 19th Ann. Kept., U. S. Geol. Surv., Pt. IV; and topographic 

 maps of Camp Clarke, Brown's Creek, and St. Paul sheets, and the folios of 

 the state, published by the U. S. Geol. Surv. 



2 Gilbert, Mono. I, U. S. Geol. Surv. 



