242 i'HE GLACIAL LAKE AGASSIZ. 



direct product of the ice-sheet, very slightly changed by its deposition in 

 the lake, and not covered by any aqueous sediments. 



Only where tributaries entered this lake and brought, Ijesides the ordi- 

 nary alluvium of their erosion, a much larger volume of moditied drift from 

 the melting ice-sheet, were any important lacustrine sediments laid down; 

 and these appear in the form of extensive deltas of gravel and sand, with 

 fine silt spread over adjoining parts of the lake bottom. Other inflowing 

 streams, though in several instances important rivers, as the Red River 

 itself above Fergus Falls, the Wild Rice River of Minnesota, and the Red 

 Lake River, formed no noteworthy delta accumulations, which, however, 

 could not have failed to be conspicuously developed if the lake had long 

 remained at any of the levels of its many successive shore-lines. The sedi- 

 ments iu Lakes Bonneville and Lahontan were evidenth' derived in large 

 part from wave erosion of their shores; but in the case of Lake Agassiz, 

 though its shores are the easily eroded di-ift, no appreciable lacustrine beds 

 were supplied from this source. The duration of Lake Agassiz was very 

 short in comparison with either of the Pleistocene humid epochs of the 

 Great Basin and Cordilleran mountain belt. 



Brevity of time required for the formation of terminal moraines. — The 

 shortness of the existence of Lake Agassiz seems, at first thought, to pre- 

 sent a difficulty in the brevity of the time which would be allowed, if the 

 glacial lake endured only a thousand years or less, for the accumulation of 

 the moraines described in the preceding chapter. By the probable ratios 

 of time deduced from the extent of the upper HeiTuan beach and the 

 sequence of all the lower and later beaches, we see that the formation of 

 even the great moraines of the Leaf Hills and of the south side of Devils 

 Lake could have occupied only a small fraction of the whole duration of 

 Lake Agassiz, perhaps not more than fifty or even twenty-five years for 

 amassino- these morainic hills 100 to 350 feet high on a belt 3 to 5 miles 

 wide! For the Dovre, Fergus Falls, Leaf Hills, and Itasca moraines were 

 apparently formed before the completion of the highest one iu the series of 

 four principal beaches whicli unite in the Herman beach along tlie southern 

 75 miles of Lake Agassiz. But this may be easily accepted wlien we recall 

 the rapidity of motion of the thick and wide glaciers of Greenland and 



