LAKE MINNESOTA. 265 



with a width of 40 miles iu Blue Earth and Faribault counties, attaining 

 an area of more than 3,000 square miles. The continued glacial recession 

 afterward opened lower outlets eastwai'd to the Cannon River, and at the 

 time of the Waconia moraine had uncovered the lower part of the Minne- 

 sota Valley, permitting the lake to be wholly drained northeastward to the 

 Mississippi.^ Its existence was thus ended previous to the beginning of 

 Lake Agassiz, which dates from the next ensuing Dovre moraine. 



The modified drift from the retreating ice on the upper Minnesota basin 

 was deposited along the lower half of this valley, filling it with stratified 

 graved, sand, and clay to a depth 75 to 150 feet above the present river 

 from New Ulm to its mouth, which shows that at least this portion of the 

 valley had nearly its present form at the time of final recession of the ice- 

 sheet. It seems also probable that the upper part of the channel above 

 New Ulm, occupied Ijy the River Warren at the time of the Herman 

 beaches, was already a distinctly marked topographic feature when the 

 ice retreated, so that the first outflow from Lake Agassiz took its course 

 at a level some 50 feet below the general surface adjoining Lakes Traverse 

 and Big Stone and Browns Valley.^ As long as streams poured into this 

 valley directly from the melting ice-sheet, its modified drift, gathered from 

 the ice in which it had been held, continued to increase in depth; but 

 when the ice had retreated beyond the limits of the Minnesota liasin, the 

 water discharged here from Lake Agassiz brought no modified drift, and was 

 consequently a most powerful eroding agent. By this River Warren the 

 valley drift, so recently deposited, was mostly swept away, and the channel 

 was excavated to a depth much lower than the present river.' But since 

 Lake Agassiz began to outflow northeastward, the Minnesota Valley and 

 that of the Mississippi below, carrying only a small fraction of their former 

 volume of water, have become considerably filled by alluvial gravel, sand, 

 clay, and silt, which have been brought in by tributaries, being spread for 

 the most part somewhat evenly along these valleys by their floods.' 



1 Geology of Minnesot.a, Vol. I, pp. 460, 622, 642. 



2 Compare with Geology of Miunosota, Vol. I, pp. 479-485, describing the chains of lakes in 

 Martin County, Miun., which are apparently due to preglacial or interghicial watercourses that were 

 not wholly filled with drift. Several such chains of lakes are also found in the vicinity of Eckelson, 

 N. Dak. (Chapter IV, p. 144). 



'"The Minnesota Valley in the Ice age," Proc. A. A. A. S., Vol. XXXII. for 1883. pp. 213-231; also 

 in Am. .Jour. Sci. (3). Vol. XXVII, Jan. and Feb., 1884. 



