EROSION BY THE RIVEE WARREN. 223 



of drift tlii-oug-li which the river flowed, is the height of the Herman beach, 

 which was the shore of the glacial lake at an early stage and through a 

 long time ensuing. The somewhat higher Milnor beach appears to have 

 been due to the temporary barrier interposed at first by the delta gravel 

 and sand of the glacial Sheyenne River, spread wholly across the southern 

 end of the lake at its beginning. Over this barrier, to the west of the line 

 between White Rock and Wheaton, the River Warren flowed for a short 

 time with rapids, speedily cutting it down 20 or 25 feet to the bed of 

 the previously existing channel along the distance of 50 miles above the 

 present sites of Lakes Traverse and Big Stone. This channel, whose depth 

 determined the level of the Herman beach while the lake expanded with 

 the recession of the ice-sheet even to southern Manitoba, was, as I believe, 

 a vestige of a preglacial and possibly interglacial river course not wholly 

 filled by the drift deposits. Reasons for this belief ai-e sufficiently stated 

 in the memoir on the Minnesota Valley before referred to, and in the 

 description of certain remarkable chains of lakes in Martin County, Minn.^ 



Nearly all the changes in the relative heights of Lake Agassiz and the 

 basin that held it, by which the Herman beach became fourfold and even 

 sevenfold in proceeding northward, must be ascribed to epeirogenic uplifting 

 of the land, with only a very small element of change in the lowering of 

 the lake level by erosion of its outlet. The southern portion of this shore- 

 line, as far to the north as the latitude of Moorhead and Fargo, is marked 

 by a single beach ridge, very definite in form and course, but not massive 

 in comparison with the present beaches of the ocean or .of the great lakes 

 tributary to the St. Lawrence, the Nelson, and the Mackenzie. . While Lake 

 Agassiz was forming the Herman beach, erosion probably lowered the chan- 

 nel of the River Warren and the level of the lake 5 or 10 feet. During 

 the same time a much greater differential northward uplift, presently to be 

 considered, was in progress. 



From the level of the Herman beach to that of the Norcross beach 

 Lake Agassiz fell somewhat suddenly 15 or 20 feet. As this change of 

 level aff"ected the southern part of the lake, adjoining its mouth, it is evi- 

 dent that between the dates of these shore-lines the River Warren eroded 



' Geology of Minnesota, Vol. I, 1884, pp. 479^85. 



