108 LAKES OF NORTH AMERICA. ^ 



until they found an outlet and escaped through a channel leading north- 

 ward from Cache valley, in Utah and Idaho, to Snake river and thence to 

 the Columbia. The outflowing stream at its source crossed incoherent 

 alluvial deposits and rapidly cut down a channel of discharge to a depth 

 of 370 feet, thus lowering the lake by tiiat amount. During this episode 

 in its history the lake was fresh, but at later stages, when its surface fell 

 below the level of the bottom of the channel of discharge, it became 

 saline. The water supply of Lake Lahontan Avas less abundant and it 

 never rose so as to find an outlet. Its waters were perhaps brackish 

 during its higher stages, and became saline and alkaline as concentration 

 progressed. 



Each of these lakes had two high-water stages, separated by a time of 

 low water and probably of complete desiccation. The second high-water 

 stage in eacli instance was the more marked of the two. These fluctua- 

 tions are indicated in the foUoAving diagram of the rise and fall of Lake 

 Lahontan. 



Each lake spread out two sheets of fine, evenly-laminated clays, sepa- 



FiG. 8.— Diagram showing the Kise assv Fall of Lake Lahont.i^'. 



rated, at least about their borders, by deposits of coarse gravel and sand 

 washed in from the adjacent slopes during the inter-lacustral time of low 

 water. 



There are many reasons for concluding that the two high-water stages 

 recorded by beach lines and by sedimentary deposits in the basins of lakes 

 Bonneville and Lahontan, correspond in time witli two of the periods of 

 glaciation recorded in the Laurentian basin. Two periods of marked 

 advance separated by a time of retreat, are also indicated by the glacial 

 records in the cailons of the Sierra Nevada. 



The waters of both Bonneville and Lahontan underw.ent many minor 

 fluctuations of level as is the rule Avith all enclosed lakes. The terraces, 

 embankments, deltas, etc., constructed about the shores of Lake Bonne- 

 ville are on a grander scale than in the basin of its companion lake, for 



