FLUVIAL DEPOSITS. 253 



glacial lake, excepting the part due to the capacity of the lake as a reser- 

 voir, was comparatively small. In the winter Lake Agassiz, becoming at 

 times mostly frozen over, like the present .great Laurentian lakes, would 

 be drawn down to a level several feet below its summer height, and the 

 great outflowing river, as it was during the warm weather, would become 

 reduced to a stream resembling' the lowest stage of the Nelson. 



FLiUVIAL, DEPOSITS IN THE RED RIVER VALLEY. 



When the bed of Lake Agassiz was gradually uncovered from the 

 water of the receding lake, some parts of its central plain, through which 

 the Red River flows, probably remained as broad, shallow basins of water, 

 which that river and its tributaries have since filled with their fine clayey 

 alluvium. The similar clayey silt brought into Lake Agassiz by its delta- 

 forming affluents, the Buffalo, Sand Hill, Sheyenne, Pembina, and Assini- 

 boine rivers, and others farther north, had been spread over large areas of 

 the lake-bed, but more extensive portions had a surface of till, with no such 

 lacustrine deposit. Over these formations much alluvium has been laid 

 down along all the avenues of drainage of the old lake-bed, and it has 

 filled depressions of the original surface, whether of lacustrine sediments 

 or of till, being distinguishable from the former only by its containing in 

 some places shells like those now living in the shalloAV lakes of .the country 

 adjoining the area of Lake Agassiz, remains of rushes and sedges and peaty 

 deposits, as of the present marshes of the Red River Valley, and occasional 

 branches and logs of wood, such as are floated down by streams in their 

 stages of flood. Thus the occurrence of shells, rushes, and sedges in these 

 alluvial beds at McCauleyville, Minn., 32 and 45 feet below the surface, or 

 about 7 and 20 feet below the level of the Red River, of sheets of turf, 

 many fragments of decaying wood, and a log a foot in diameter at Glyn- 

 don, Minn., 13 to 35 feet below the surface, and numerous other observa- 

 tions of vegetation elsewhere along the Red River Valley in these beds, 

 demonstrate that Lake Agassiz had been di-ained away, and that the valley 

 was a land surface subject to ovei-flow by the river at its stages of flood. 



