THE BUFFALO DELTA. 291 



The area of tlie Buffalo delta extends 7 miles from north to south, with 

 a width of 2 to 3i miles Its average thickness is probably about 50 feet, 

 and its volume is therefore approximately one-sixth of a cubic mile. It 

 would make a very sightly hill if its inaterial were piled on the flat plain 

 of the Red River Valley, for it would cover a circle 2 miles in diameter and 

 rise to a peak about 900 feet high. Lying on the slope which rises east ffom 

 this valley, however, and being spread over a considerable area with com- 

 pai-atively little thickness, its mass does not especially command attention 

 until investigation reveals that it came almost wholly from drift that was 

 contained within the ice-sheet, being deposited here by the streams from 

 its melting. 



The existence of well-defined and conspicuous delta deposits having 

 the altitude of the Herman beach, where the Buff'alo and Sand Hill rivers 

 enter the east side of the area of Lake Agassiz, while no such deposits are 

 found where other streams of equal or larger size enter this area, as the 

 Red River, the Wild Rice, and the Red Lake River, seems explicable oidy 

 by the derivation of the gravel and sand forming these deltas mostly from 

 the englacial drift of the melting ice-sheet upon the adjacent area at the 

 east. Comparatively small tribute was brought into this glacial lake from 

 the erosion of the stream valleys after their areas became uncovered 

 from the ice, excepting where it received the very large rivers flowing from 

 other glacial lakes at the west. Here and there, because of irregularities 

 in the outline of the ice-sheet, by which the drainage of its surface was 

 poured down upon certain limited tracts and was discharged thence along 

 the courses of now existing streams, as the Buff'alo and Sand Hill rivers, 

 and because the retreat of the ice was now rapid and anon was interrupted 

 by halt or readvance, with the accumulation of moraines, much of the 

 material which had been inclosed within the basal part of the ice-mass 

 seems to have been washed away by its streams and carried into Lake 

 Agassiz to form deltas. 



When such glacial streams encountered no lake to receive their tribute, 

 and flowed far before reaching the sea, the gravel, sand, and fine silt or clay 

 which they brought were spread by the rivers along their courses as plains 

 of modified drift. In some instances, since the ice-sheet disappeared and 



