366 HISTORICAL GEOLOGY 



as due to subsidence (or submergence) below sea level and 

 deposition of sediments, followed by elevation (or emergence) 

 and erosion. The fossil evidence regarding non-marine or marine 

 origin of the deposits is far from conclusive. 



Western United States. — Quaternary deposits, representing 

 many types of origin, are known in the west. 



Volcanic deposits of this age in the west are not always clearly 

 separable from those of the Tertiary, but it is certain that pretty 

 vigorous vulcanism continued into the Quaternary. Such volcanic 

 deposits, including lava-flows, cinders, and volcanic ashes, are 

 known in New Mexico, Utah, Idaho, and all the states farther 

 west as well as in Alaska. Mount Shasta shows lava flows of post- 

 Glacial or recent age, while small lava fields and cinder cones in 

 northern and southern California and in northern Arizona must be 

 of late Quaternary age because they are so unaffected by weather- 

 ing and erosion. As already mentioned in the discussion of 

 Tertiary vulcanism, a cinder cone and small lava field have cer- 

 tainly been built up within the last 200 years, while Lassen Peak 

 in northern California is now (1916) actually in eruption. 



At the bases of mountains throughout the arid and semi-arid 

 regions of the west, great accumulations of talus and alluvial 

 materials took place. Some of the alluvial cones or fans have a 

 thickness of fully a thousand feet. Extensive flood-plain deposits 

 are found in many places. 



Recent studies have shown that the wind has been, and is, a 

 very important agent of erosion and deposition, particularly in 

 the arid western regions. Deep and extensive wind-blown deposits 

 are still forming in many of the intermontane basins. 



During part of the Quaternary, at least, the Great Basin region 

 had a moister climate than at present, because lakes were much 

 more numerous and larger than now (Fig. 227) . One of the largest 

 of these was Lake Bonneville, which represented a greatly enlarged 

 stage of the Great Salt Lake. Lake Bonneville was of fresh water; 

 covered 19,000 square miles; and had a maximum depth of 1000 

 feet. Its remnant, the present heavy brine of the Great Salt Lake, 

 covers about 2000 square miles and has a maximum depth of 

 only about 50 feet. The outlet of Lake Bonneville was northward 

 into Snake River. The former existence of this great body of water 

 is positively proved by the perfectly preserved beaches, wave-cut 

 terraces, deltas, etc. Another very large body of water, called 



