DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 233 



Another class of terraces having most interesting con- 

 nection with the Glacial period is found in the arid basins 

 west of the Eocky Mountains. Over wide areas in Utah 

 and Nevada the evaporation now just balances the pre- 

 cipitation, and all the streams disappear in shallow bodies 

 of salt water of moderate dimensions, of which Great 

 Salt Lake in Utah, and Mono, Pyramid, and North 

 Carson Lakes in Nevada, are the most familiar examples. 

 These occupy the lowest sinks of enclosed basins of great 

 depth. 



But there is abundant evidence that in consequence of 

 the increased precipitation and diminished evaporation of 

 the Glacial period one of these basins was filled to the brim 

 and the other to a depth of several hundred feet. These 

 former enlargements have been named after the first ex- 

 plorers of the region, Captains Lahontan and Bonneville, 

 and are shown on the accompanying sketch map by the 

 shading surrounding the existing lakes. 



Lake Lahontan has been carefully studied by Mr. I. C. 

 Eussell, and has been found to extend from the boundary 

 of Oregon to latitude 38° 30' south, a distance of two hun- 

 dred and sixty miles. The Central Pacific Eailroad runs 

 through its dried-up bed from Golconda to Wadsworth, 

 a distance of one hundred and sixty-five miles. The ter- 

 races of the former lake are distinctly traceable at a height 

 of 700 feet above the present level of Ldke Mono. 



Lake Bonneville, whose present representative is Great 

 Salt Lake, is the subject of a recent monograph by Mr. G. 

 K. Gilbert, from which it appears that this ancient body 

 of water occupied 19,750 square miles— an area about ten 

 times that of the present lake. At the time of its maxi- 

 mum extension its depth was 1,000 feet, while Great Salt 

 Lake ranges only from fifteen to fifty feet in depth. 



The pass through which the discharge finally took 

 place is at Eed Eock, on the Utah and Northern Eailroad. 

 at the head of Cache Valley on the south and the lower 



