23± 



MAX AND TIIE GLACIAL PERIOD. 



part of Marsh Creek Valley on the north. During the 

 long period preceding and accompanying the gradual rise 

 of water in the Utah basin to the level of the highest ter- 



FlG. 



61.— Map of the Quaternary Lakes. Bonneville and Lahontan (after Gilbert 

 and Russell). 



race, Marsh Creek (the upper portion of which comes from 

 the mountains on the east and turns at right angles) had 

 been at work depositing a delta of loose material in the 

 col which separates the two valleys. This deposit rested 

 upon a stratum of limestone at the bottom of the pass, 

 and covered it with sand, clay, and gravel to a depth of 

 375 feet. Thus, when the water was approaching its up- 

 per level, the only barrier to prevent its escape was this 

 unstable accumulation of loose material upon top of the 

 rock. It would have required, therefore, no prophet's 

 eye to predict that the way was preparing for a tremen- 

 dous debacle. 



The critical point at length was reached. After remain- 

 ing nearly at the elevation of the pass for a considerable 

 period, during which the 1,000-foot shore-line was formed, 



