DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 237 



in the depression with the sand and gravel now occupy- 

 ing it. 



What other effects of this remarkable outburst may be 

 traced farther down in the Snake Kiver Valley I cannot 

 say, but it will be surprising if they do not come to light 

 and help to solve some of the many geological problems 

 yet awaiting us in this interesting region. 



It should have been said that during the formation of 

 the 625-foot, or so-called Provo shore-line, glaciers de- 

 scended from the canons on the west flank of the Wah- 

 satch Mountains, and left terminal moraines to mark 

 the coincidence of the Glacial period with that stage 

 of the enlargement of the lake. Evidences of a similar 

 coincidence are to be found on the high-level terraces 

 surrounding Lake Mono, to which glaciers formerly de- 

 scended from the western flanks of the Sierra Nevada. 



The ancient shore-lines surrounding Lakes Bonneville 

 and Lahontan bear evidence also of various other episodes 

 in the Glacial period. Evidently there were two periods 

 of marked increase in the size of the lakes, with an arid 

 period intervening. During the first rise the level of 

 Bonneville attained to within ninety feet of the second, and 

 numerous beaches were formed, and a large amount of 

 yellow clay deposited. Then it seems to have been wholly 

 evaporated, while its soluble mineral matter was precipi- 

 tated, and so mingled with silt that it did not readily re- 

 dissolve during the second great rise of water. Partly on 

 this account, and partly through the influence of the out- 

 let into the Snake Kiver, the lake was nearly fresh during 

 its second enlargement. 



European Facts. 



In Chapter VI it came in place to mention many of 

 the facts connected with the influence of the Glacial pe- 

 riod upon the drainage systems of Europe. We there dis- 

 cussed briefly the probable influence of the ice-obstructions 



