458 GEOLOGY. 



The maximum stand of the water is recorded in various topographic 

 forms characteristic of shores. The outflow of the lake cut down 

 the outlet 375 feet, and at this new and lower level, distinct shore 

 marks were developed. Later, evaporation from the lake again became 

 more considerable than precipitation and inflow, and the lake gradu- 

 ally shrank to the present dimensions of Great Salt Lake. At its 

 maximum, Lake Bonneville was more than 1000 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 (its average less than 20 feet) and its 

 area but about ■£$ that of its ancestor. 



Fig. 538. — Ancient deltas of Logan River, at Logan, Utah. (Gilbert, U. S. Geol. Surv.) 



As the lake dried up, its waters became separated into numerous 

 basins, corresponding to the lowest parts of the Bonneville bottom. 

 Some of these basins, besides that of Great Salt Lake, contain, or have 

 recently contained, lakes. Others have playas in their lowest parts, 

 where water gathers after every rain, but does not persist. Great 

 Salt Lake is apparently doomed to still further decrease by the diver- 

 sion of water from the feeding streams for purposes of irrigation. 



Terraces, deltas, and embankments of other sorts were developed 

 about the shores of Lake Bonneville wherever the appropriate con- 

 ditions existed (Figs. 537-539), and the aridity of the climate since 

 the lake sank below them, has allowed them to remain with little modi- 

 fication by erosion. As the lake dried up, deposits of salts were made, 

 among which sodium chloride and sodium sulphate are most abundant. 

 Gypsum crystals are plentiful, and in places they have been heaped 

 up into dunes. Great Salt Lake is estimated to contain 400,000,000 

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



