THE GEOLOGY OF NORTH DAKOTA 27 



were thus formed, the best developed of which commonly rise 10 to 

 20 feet above the adjoining surface on the side toward the former 

 lake. They are composed of interstratified gravel and sand, and 

 vary in width from 10 to 30 rods. 



These beaches afford evidence of the elevation of the land to the 

 north, since they are no longer horizontal, nor are they parallel, but 

 show a divergence among themselves. All have a gradual ascent 

 toward the north or northeast; the upper or Hermon beach, for 

 example, rises 175 feet between Lake Traverse, at the south end of 

 Lake Agassiz, and the international boundary. As these shore lines 

 also show a divergence among themselves it is evident that this 

 upward movement of the earth's crust began while Lake Agassiz 

 was in existence and was probably largely completed before the lake 

 was finally drained. 



Sand and gravel deltas, so extensive as to constitute notable 

 topographic features, were formed by the streams that flowed into 

 Lake Agassiz while it stood at its highest stage. Those in North 

 Dakota were formed by the Cheyenne, Pembina, and a Pleistocene 

 river no longer in existence. Much of the finer sediment contrib- 

 uted to the lake by the inflowing streams was carried by the waves 

 and currents and spread over the bottom of the lake as a fine silt. 

 It is this fine loam, mingled with decayed vegetation, which forms 

 the rich black soil of the Red River Valley, one of the great wheat 

 regions of the world. 



So recent is it geologically since the last ice sheet withdrew from 

 North Dakota, and since Lake Agassiz was drained, that the drift 

 surface and lake bed have been but sHghtly affected by erosion, and 

 are still much as they were left at the close of the glacial period. 



