THE GENESIS OF LAKE AGASSI Z 635 



before Lake Agassiz came into existence and its sediments are 

 buried beneath those of Lake Agassiz. 



It is at least certain that the waters of Lake Agassiz stood at 

 one time at about the present level of Lake of the Woods, and that 

 they later rose considerably higher. It seems probable also that 

 the lake which preceded Lake Agassiz was almost completely 

 drained, and that Red River valley was a land surface before the 

 latest advance of the ice brought Lake Agassiz into existence; for 

 the character of the deposits in Red River valley, which Upham 1 

 regarded as post-Lake Agassiz fluvial deposits, suggests rather 

 that they are lacustrine deposits and that they are unconformable 

 on the underlying sediments. 



Regarding these deposits Upham stated : 



Thus the occurrence of shells, rushes and sedges in these alluvial beds at 

 McCauleyville, Minnesota, 32 and 45 feet below the surface or about 7 and 20 

 feet below the level of Red River, of sheets of turf, many fragments of decaying 

 wood and a log a foot in diameter at Glyndon, Minnesota, 13 to 35 feet below 

 the surface, and numerous other observations of vegetation elsewhere along the 

 Red River valley in these beds, demonstrate that Lake Agassiz had been 

 drained away, and that the valley was a land surface subject to overflow by 

 the river at its stages of flood when these remains were deposited. 2 



He also stated: "The deposits have commonly greater thickness 

 and extent than the underlying silt of glacial Lake Agassiz." 



It is evident that a land surface existed in Red River valley 

 before these sediments were laid down ; but it seems probable that 

 the sediments are largely lacustrine in origin and not fluvial. 

 G. M. Dawson, in describing the section across Red River valley 

 near the international boundary stated that the valley is floored 

 with a fine silty deposit, a portion of the upper layers of which may 

 have been formed by the overflow of the river itself. He described 

 the typical deposit as of great thickness and consisting of fine 

 yellowish, marly, and arenaceous clay, holding considerable cal- 

 careous matter, and effervescing freely with an acid. 3 The great 

 extent and thickness and the high calcareous content of the clays 



1 Warren Upham, U.S. Geol. Survey, Monograph 25 (1895), p. 253. 



2 Ibid., p. 202. 



3 G. M. Dawson, Report on the Geology and Resources of the Region in the Vicinity 

 of the Forty-ninth Parallel, 1875, PP- 248-49. 



