210 THE GLACIAL LAKE AGASSIZ. 



may well astonish us in what it implies conceruiiig the rajjidity of the re- 

 cession of the ice-sheet, and the brevity, geologically speaking, of the stages 

 of pause or readvance when its moraines were accumulated. 



Between the times of accumulation of the successive terminal moraines, 

 the ablation of the ice surface and the retreat of its border caused the por- 

 tion of the drift which had been inclosed within the ice-sheet to be rapidly 

 deposited on the land, partly as till and partly as stratified gravel, sand, and 

 clay, brought by the streams that were produced by the glacial melting. 

 Thus while the series of Herman beaches was being formed not only were 

 several large moraines amassed, but also much englacial till was spread 

 over the country between the moraines, and glacial rivers deposited a broad 

 belt of modified drift that stretches from central Minnesota to Red Lake 

 and the Lake of the Woods, and continues northward in Manitoba, as de- 

 scribed in pages 181-183. The most southeastern part of this prolonged 

 tract of plentiful modified drift, in the vicinity of St. Paul and Minneapolis 

 and northwestward to St. Cloud, belongs to a time previous to Lake Agas- 

 siz; the portion of these stratified beds between St. Cloud and Lake Itasca 

 represents the time of formation of the highest Herman beach; and the 

 deposition of their northern half, continuing from the headwaters of the 

 Mississippi to the southwest part of the Lake of the Woods and to the Birds 

 Hill group of eskers, was contemporaneous with the lower Herman shores 

 of Lake Agassiz. Toward this belt great areas of the ice-sheet sloped con- 

 vergingly during its maximum extension, and in the early part of its time of 

 recession rivers flowed thither from the ice-lobes on the northeast and north- 

 west until this glacial lake began to exist and to grow northward, occupying 

 the Red River Valley. 



STAGES OF GROWTH SHOWN BY MORAINES. 



The retreat of the ice between the Waconia and Dovre moraines (pp. 

 142, 147) began to uncover the southern end of the bed of Lake Agas- 

 siz, into which the inflowing glacial Sheyenne River, even at that early 

 stage, brought much gravel and sand. Tins first delta deposit of the glacial 

 lake is spread along its southwestern margin from near Taylor Lake to the 

 bluff", in the northeast corner of South Dakota, that overlooks the valley of 



