532 THE PLEISTOCENE EPOCH 



this river excavated its bed, it gradually lowered the water level 

 of Lake Agassiz. The drainage of the lake seems to have been 

 accomplished by the continued retreat of the Keewatin glacier 

 until it became separated from the Laurentide sheet, leaving the 

 way open around the northern edge of the latter to Hudson Bay. 



In the non-glaciated parts of the continent occur stratified 

 Pleistocene deposits, which it is very difficult to associate with the 

 events taking place in the glaciated area, for lack of any means 

 of direct comparison. On the Atlantic slope from New Jersey 

 southward a succession of Pleistocene gravels and sands constitutes 

 the Columbian formation, so called because of its typical develop- 

 ment in the District of Columbia. These deposits are estuarine 

 and marine deltas and shore sediments, partly submarine. Three 

 phases of the Columbian have been recognized, the fluvial, inter- 

 fluvial, and low-level. " The fluvial phase is found in its fullest 

 development along the leading waterways. It consists of a lower 

 horizon of coarse materials, pebbles, and boulders, generally pass- 

 ing upward into a brownish loam. The interfluvial phase is found 

 typically represented in the country which lies between the water- 

 ways, and is characterized by materials of local origin and pro- 

 duced largely by wave action. The low-level phase is developed 

 throughout the area farthest removed from the ancient shore-line. 

 The deposits consist of sands, clays, and loams. They indicate 

 a much less disturbed type of sedimentation than that found in 

 either the fluvial or interfluvial phase. They were scattered as 

 a coating of greater or less thickness over the eastern portion 

 of the district, and have since suffered but little from denudation. 

 The fossils are of recent species and indicate the marine origin of 

 the deposits." (Clark.) At present these deposits are from ioo 

 to 400 feet above sea-level and indicate a submergence and 

 reelevation of the middle Atlantic coast. 



Over the Great Plains from South Dakota to Texas the surface 

 formation is a fine, calcareous, sandy clay, which lies unconform- 

 ably upon the eroded surfaces of older strata, from the Blanco to 

 the Cretaceous. This formation may be called the Sheridan stage 

 (Equus Beds), from Sheridan County, Nebraska, where it is 



