150 PHYSIOGRAPHY 



basin up to this level, for shore lines and deltas are found about its 

 border at many points. When the lake stood at this level, it had 

 an outlet to the northward over to Snake River. In the sands 

 and muds deposited at this high stage of the lake, there are shells 

 of fresh-water snails, and remains of other fresh-water animals. 

 These show that the water was then fresh. This great lake, though 

 now extinct, has been named Lake Bonneville (Fig. 151). When it 

 was largest, it covered an area of about 17,000 square miles. 



After the lake had stood at this level for a time, the climate of 

 this region seems to have become so dry that evaporation from the 

 surface of the lake was more than the inflow of streams and the 

 rainfall on the lake, and so the level of the lake became lower. As 

 the water evaporated, the mineral matter which it held in solution 

 was left behind. Salt was one of these substances, and as more 

 and more water evaporated, the saltness of that which remained 

 increased. This condition of things went on until the former great 

 lake was reduced to the relatively small Great Salt Lake of the 

 present, with an area of about 2,000 square miles, and an average 

 depth of only about 15 feet. Its waters are now saturated with 

 salt, and much salt has been deposited from them. 



Other lakes in the region east of the Sierra Mountains have 

 undergone the same history. The shore terraces, deltas, etc., of 

 these former lakes are still distinct. The time since the lakes ex- 

 isted has therefore not been long enough for the erosion of this arid 

 region to destroy or even to greatly obscure them. 



Salt lakes, and the sites of former salt lakes, furnish much of 

 the salt of commerce. Great Salt Lake has been estimated to con- 

 tain 400,000,000 tons of common salt, besides large quantities of 

 other mineral matter. Utah produced more than 400,000 barrels 

 of salt in 1902, and 242,678 (value $169,833) in 1908. 



The Origin of Lake Basins 



Lake basins have come into existence in many different ways. 

 Most of them are the result of gradational processes, but some are 

 made in other ways. 



By gradation. Rivers, waves, and glaciers produce lake basins, 

 and some of these agents produce them in several different ways. 



