656 



HISTORICAL GEOLOGY 



or less at the southern end, and those of Maine which are 200 or more 

 feet in height, show both the amount of sinking in that epoch and the 

 differential uplift since then. 



Other Pleistocene Lakes 



Lake Agassiz. — The Red River valley of Manitoba, North 

 Dakota, and Minnesota, so remarkable for its fertility as well as for 

 its flatness, is the result of a glacial dam which prevented the usual 

 drainage through the Red River to the north, and produced a great 

 lake (Lake Agassiz) which discharged by way of the Minnesota River 

 into the Mississippi. On its bottom the silts carried in by the streams 

 were deposited, making a surface as flat perhaps as any on earth. 

 Upon the retreat of the ice, drainage to the north was permitted, and 

 the lake disappeared. At its greatest extent this lake had a larger 

 area than that of the present Great Lakes combined. 



Lake Bascom. — In rugged regions the ice sheet formed many 

 temporary lakes, a rather remarkable example of which is to be found 

 in northwestern Massachusetts, where a lake (Lake Bascom) first 

 stood at an elevation of 1100 feet above the sea, the level of the lake 

 being determined by the pass (col) through which the water was dis- 

 charged. As the ice retreated, lower passes, approximately 1000, 



900, 700, and 600 

 feet above the sea, 

 were found ; and 

 the lake was finally 

 drained when the 

 present outlet — 

 the Hoosic River 

 — was uncovered. 

 Great Basin 

 Lakes. — In some 

 semiarid regions 

 not covered by the 

 ice sheet the cli- 

 mate of the Pleis- 

 tocene seems to 

 have been moister 

 than at present 

 and previous to 



I [G. 572. — M;i|) showing the position of Lake Bonneville 

 on the east and Lake Lahontan on the west. The relative 

 lize of Great Sail Lake is indicated. 



