264 GEOLOGY. 



of this period of glaciation in Ohio. Here, however, it is clear. 

 It has also been observed in northeastern Iowa by W. J. McGee.* 

 I attribute the absence of this Forest Bed in many sections of Ne- 

 braska to the second advance of the ice sheet in these regions. It 

 probably failed to advance so far south in Ohio and other sections 

 of the Mississippi Valley. When this ice sheet commenced its re- 

 treat, another period of depression came on, whefi the land was 

 again flooded, and a lake of fresh water again occupied the plains. 

 This body of water for ages abutted against the ice sheet on the 

 north, from which it received icebergs that floated over its waters. 

 In these waters the materials left by the retreating glaciers were 

 remodified in their upper portion, and new matter was brought 

 down by torrents and icebergs. When the ice sheet retreated from 

 the shores of this lake or interior sea, finer sediment* began to be 

 laid down. Fine sand took the place of gravels and boulders, and 

 as the waters contracted in volume the calcareous matter held in 

 suspension began to be precipitated. There is no evidence that the 

 lake was entirely dessicated previous to the beginning of the Loess 

 period. It was only reduced to smaller dimensions. W^hen at last 

 central and eastern Iowa became dry land, and the ice sheet had 

 retreated to the upper Missouri and the Yellowstone, the Loess 

 materials began to be laid down on the floor of the old lake bed. 

 So important, however, are these Loess materials in historic and 

 economic geology that they will be discussed in a separate chapter. 



*See American Journal of Science and Arts, Vol. 15, page 339. 



