NON-GLACIAL FORMATIONS 657 



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

 than (U-positioii was the great feature of the Quaternary in the in- 

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

 have co-operated. 



The West. The Quaternary formations of the west belong to 

 all the several categories mentioned on p. 652, 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 l of which Great Salt Lake is 

 the diminutive descendant. Its basin is believed to have been due 

 to deformation and faulting. Previous to the formation of the lake, 

 the basin is thought to have been arid. During the period of 

 aridity, such quantities of debris came down from the surrounding 

 mountains as to bury their bases to depths of perhaps 2,000 feet at 

 a maximum. 



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

 into existence, but after a time it appears to have dried up, probably 

 because of another change of climate. Still later, the lake was 

 restored, and its water rose higher than before, and found an outlet 

 northward. In the course of time, evaporation from the lake 

 atiain became greater than precipitation and inflow, and the lake 

 gradually shrank until it became Great Salt Lake. At its maximum, 

 Lake Bonneville was more than 1,000 feet deep, and had an area 

 of more than 19,000 square miles; the maximum depth of Great Salt 

 Lake is less than 50 feet (average less than 20), and its area but about 

 one-tenth that of its ancestor. 



Terraces, deltas, and embankments of other sorts were developed 

 about the shores of Lake Bonneville wherever the appropriate con- 

 ditions existed (Figs. 202 and 539), and because of the aridity of 

 the climate since the lake sank below them, they have been modified 

 but little by erosion. As the lake dried up, deposits of salts were 

 made, among which sodium chloride and sodium sulphate are most 

 abundant. Great Salt Lake is estimated to contain 400,000,000 

 tons of common salt, and 30,000,000 tons of sodium sulphate. 



Igneous eruptions (Fig. 538) have taken place in the basin at 



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



