THE DATE OF THE GLACIAL PERIOD. 609 



Lake Lahontan, as that of which Great Salt Lake is the rem- 

 nant was named Lake Bonneville, after the first explorers of 

 the region. These basins have now no outlet to the sea. 

 That of Lake Lahontan never had any; but, if the relative 

 levels were the same at former times as now, Lake Bonne- 

 ville at its greatest extent poured through Snake River into 

 the Columbia. 



During the year 1890 Mr. Gilbert published the first volume 

 of his monograph upon Lake Bonneville — the ancient enlarge- 

 ment of Great Salt Lake, Utah — to which reference has just 

 been made above. Mr. Gilbert estimates that at its maxi- 

 mum stage the area of this lake was 19,750 square miles — that 

 is, about ten times the present size of Great Salt Lake — and 

 that its maximum depth was one thousand and fifty feet, as 

 compared with about forty feet at present. The climatic 

 changes indicated by the studies of this ancient lake cor- 

 respond closely with those indicated b3 r Mr. Russell's study 

 of Lake Lahontan as detailed on page 607. Early in post- 

 tertiary times there was a great rise in these lakes, though 

 not sufficient by ninety feet to reach the passage through 

 the Port Neuf River into the Snake. This first rise was fol- 

 lowed by a long epoch of desiccation, during which it is prob- 

 able the lake entirely disappeared. This inter-lacustrine epoch 

 was a long one, as is indicated by the extent of the gravel 

 deposits which were then laid down. After this there was a 

 second rise, in which the water attained the height of the pass- 

 age from the Cache Valley to the Port Neuf, and then rapidly 

 " cut a channel three hundred and seventy-five feet deep in the 

 alluvium to a sill of limestone." At this level (about six hun- 

 dred feet above the Great Salt Lake) the water was held for a 

 long time, forming what is known as the Provost shore-line. 

 During the period of the Provost shore-line, glaciers descended 

 from the Wahsatch Mountains, and left their moraines near 

 the margin of the lake. 



