PLEISTOCENE FORMATIONS. 41 



of this erosion may have been already accomplished prior to the Glacial 

 period ir is impossible to determine, but it is probable that the greater 



part of the erosion was due to the melting of the ice. The II Is of 



thai period carved out river channels mure or less coincident with those 

 of the presenl stream beds of the Platte, Clear Creek, etc, which were 

 subsequently filled as the reduced slope of the streams diminished their 

 corrading power, and when, in a later period of erosion, these streams cut 

 their presenl beds, which are of relatively diminutive size as compared 

 with those of the ancient streams, they varied somewhat in detail, and their 

 • •nurses, though generally conforming to ancient drainage lines, cut into the 

 old river gravels to a depth of 30 to 50 feet. Between these two periods 

 of erosion there was deposited over the whole area of the Denver Basin, 

 below a level of 5,800 to 6,000 feet, a fine silt or loess, which apparently 

 extended out over the plains of Kansas and Nebraska, and which gives to 

 this region its remarkable fertility wherever it is susceptible of irrigation. 

 In its physical structure and composition this loess is very similar to that 

 of the Mississippi and Missouri valleys, the principal difference being its 

 greater coarseness of grain. With the exception of the alluvium of the 

 river valleys and of modern sand dunes, it is the most recent deposit of 

 tlie region, and hence, being unconsolidated and readily disintegrable, it 

 lias been so very largely removed by modern erosion that its original extent 

 is not easily determined. Its thickness in certain portions reaches 200 to 

 .".do teet. hut in the valleys is generally under 50 feet 



The origin of this peculiar and economically important deposit in 

 different parts of the world is one of the problems in geology that has 

 been found most difficult of satisfactory determination. The great loess 

 deposits of China, the most important in the ge'ologically known world, 

 are now conceded liv all geologists who have examined them to lie of 

 subaerial origin. On the other hand, the geologists who have made the most 



recent and detailed studies of the loess of the Mississippi and Missouri \ alle\ s 

 consider it to have been formed 1>\ the tine silt produced by the grinding 

 <>f the great northern ice-sheet. The great. difficulty involved in the latter 

 theory is to account for the existence of a body of comparatively tranquil 

 water in which such finely divided material could have been deposited. 



