1 8 8 2 .] The Loess of North America. 543 



was borne downward to the still portions below, and here the bur- 

 den of the waters was cast as a blanket over their bottoms. The 

 continued recession of the glaciers by dissolution kept the supply 

 of water constant, or often swelled it to floods, thus permitting 

 extensive denudations over large areas. " Below the falls of the 

 Missouri the channel makes its way through the soft yielding 

 clays and sands of the Cretaceous beds for about 250 miles " 

 (Hayden). These beds extend nearly to the mouth of the Milk 

 river, and then begin the Tertiary lignite formations, which are 

 also here composed of sands, marls and clays. For another 250 

 miles the Missouri flows through rocks of this age, and then the 

 Cretaceous slates again appear, which, according to Hayden, ex- 

 tend nearly to Council Bluffs, a distance of over 500 miles in a 

 straight line. Now the amount of denudation possible to be ac- 

 complished by so large a stream, and in some of the most rapid 

 portions of its ancient and present course, cannot be estimated, 

 and especially is it needless to hazard any conjecture when we 

 consider the character of the formations it traversed for a distance 

 much exceeding 1000 miles ! It is entirely adequate to form de- 

 posits many times greater than the whole extent of the loess in 

 America. Into the ancient Lake Missouri, then, was poured the 

 drainage of a vast area, and over a tenth of Iowa and a fourth of 

 Nebraska the sediment it contained was deposited. 



To another cause than the depression of the continent 1 must 

 the great extent of the Missouri valley loess be attributed, and 

 that cause was the narrowing of the valley south of St Joseph, 

 acting as a barrier to the waters which then were spread out into 

 lake-like proportions. With the gradual disintegration of its 

 barrier and the recession of the sea consequent on the reelevation 

 of the land, the lake was gradually drained, leaving its former 

 silt-covered bed as the surface soil— a waste of mud and lakelets 

 in place of the former broad expanse of water. The changes in 

 level and aspect keep pace each with the other. Soon the desert 

 waste abounds with a rich vegetation, the soil is clothed with 

 giass and flowers, and the forests begin anew to grow. But the 

 rivers w °rc not idle. A period of erosion began — not yet ended 



