(ill „,:,„■.] GREAT SALT LAKE AND LAKE BONNEVILLE. 875 



In Pleistocene time the lakes of the Great basin were larger, and 

 many perennial lakes were formed in valleys whose floors are now 

 saline and desert. Greal Salt lake was expanded so as to coalesce 

 With the lakes of contiguous basins, producing a body of water 10,750 

 square miles (51,000 sq. km.) in extent, which has been named Lake 

 Bonneville This lake was twice formed and twice dried away, each 

 time depositing over the plain a sheet of calcareous elay with fresh- 

 water fossils. The two sheets of elay are separated by an unconfor- 

 mity, the first having been eroded before the second was laid down. 

 In some places, moreover, a wedge of alluvial gravel intervenes be- 

 tween the two clays, showing that the lacustrine epochs were separa- 

 ted by an arid interval, during which alluvial deposition took place, as 

 at the present lame. 



The highest water stage was attained during the second lacustrine 

 epoch, and is recorded in a conspicuous series of sea cliffs, terraces. 

 and beaches known as the Bonneville shore line. This shore line has 

 a general altitude of 1,000 feet (-S00 m.) above Great Salt lake, or 

 5,200 feet (1,580 m.) above the ocean, but its height varies from place 

 to place, ranging in the vicinity of Great Salt lake from 960 feet 

 (290 im) to 1,060 feet (320 m.) above the modern water surface. As 

 all parts of the shore line-were produced at the same time, their pres- 

 ent differences in altitude indicate a warping of the earth's crust since 

 the period of their formation. 



At this stage the lake overflowed the northern rim of its basin, and 

 the channel of onttlow was eroded to a depth of nearly 400 feet (120 

 m.), when the cutting was arrested by a ledge of limestone and the 

 water was held for a long period at one level, giving the waves time to 

 sculpture a second series of terraces, etc.. constituting the PrOVO shore 

 line. At many heights between the Bonneville shore line and the Great 

 Salt Lake shore line, the waves of the oscillating water have left their 

 traces, so that the number of fossil shore lines is very great: but the 

 Provo shore line is distinguished from all of these by the magnitude 

 of its features. Its terraces are broader, its cliffs are higher, its spits 

 are greater, and with it are associated great delta terraces built by 

 tributary creeks and rivers during the Provo epoch. 



During the Ion-- period for which the lake maintained an outlet its 

 water must have been completely freshened, so that the salt contained 

 in the modern lake has all been accumulated in recent times. A com- 

 parison of the high salinity of the modern lake With the approximate 



purity of its tributaries enables one to realize the antiquity of the 



PrOVO epoch, and yet in this arid climate the vestiges of wave action 

 have been preserved almost unscathed. 



