i 7 8 



PHYSICAL GEOLOGY 



mode of their formation, but there is abundant evidence that they 

 were developed beneath the margin of the ice and were built up by 

 the addition of successive layers of till. 



Stratified Drift. — An estimate 1 has been made, based upon a 

 study of the Malaspina Glacier of Alaska (p. 168), that at certain 

 stages of the withdrawal of the great ice sheets the Mississippi River 

 had a volume sixty times greater than at present. When one con- 

 siders the number of streams flowing on, under, and in front of the 

 ice, whose combined volumes made the greater rivers of the time, 

 we can understand the abundance of stratified deposits, such as 

 kames, eskers, deltas (laid down in temporary lakes), outwash plains, 

 and like deposits, so common in the glaciated portions of North 

 America. 



Outwash Plains. — The streams which flowed from the fronts of 

 the continental ice sheets were usually heavily charged with silt, 



Fig. 163. — Outwash plain. New York. (Photo. H. L. Fairchild.) 



sand, and gravel, which they obtained from the ground and terminal 

 moraines (p. 175), so that they were able to aggrade their beds, often 

 leaving thick deposits. If they flowed through well-defined valleys, 

 the loads were deposited in the valley bottoms and formed valley 

 trains. If a number of streams issued from the ice front on a 

 plain, or in a shallow valley, they gradually raised its level as they 

 deposited their surplus loads. In this way the streams quickly built 

 their beds above the level of the surrounding regions and were in 

 consequence forced to shift their positions to lower places, forming 

 braided streams (p. 86). When valley trains grew to such an extent 

 tii.it they overlapped, an outwash plain resulted (Fig. 163), much as 



1 O. D. von Engeln. 



