278 GEOLOGY. 



Probably the huge terminal morraines helped to confine the water 

 and produce the great lakes of the time. Now it can easily be 

 seen that a certain stage would be reached in the re-elevation of the 

 land when the surface conditions would be precisely such as is 

 claimed for the great lakes of the Loess period. Confirmatory of 

 this induction is the fact that the Loess valleys running proxi- 

 mately east and west through Nebraska have almost universally 

 long gentle slopes on the north side and steeper bluffs on the south.. 

 As the continent rose towards the north slowly and gently, the- 

 streams retired gradually towards the south side of the valleys and: 

 produced this peculiar configuration. It is true that here the Loess- 

 in southeast Nebraska is over 3,000 feet below the highest point 011 

 the west line of the State. At other points the difference of level 

 in the Loess of Nebraska is over 3,500 feet. But this is more 

 than paralleled by the remnants of the old Pliocene lake of the 

 plains, where the present difference of level between its eastern and 

 western shore is over 7,000 feet. No geologist, however, doubts^ 

 that in Pliocene times it occupied about the same plane. The 

 change in level, therefore, on the theory that the Loess was formed 

 in a lake, since the close of that period, is only about half as great 

 as that which occurred since the close of Pliocene times. 



The assumed fact that fresh water shells are absent and land' 

 shells abundant in the Loess, is also depended on by Richthofen to- 

 prove his theory. However it may be in China, here fresh water 

 shells are quite abundant at some horizons. The species of land 

 and fresh water shells that I have thus far identified from the Loess 

 of Nebraska are appended to the end of this chapter. It will be 

 seen that large numbers of them are fresh water shells. They are 

 not found merely near existing fresh water streams, as has been 

 suggested — they are equally abundant on the divides wherever 

 there are well shafts to bring them to light. It is an interesting 

 analogous fact that in the eddies and in the sand bars and silted up 

 hollows of the Missouri, at the present time, about the same rela- 

 tive proportion of land and fresh water shells are found as in the 

 Loess. For example, four miles below Dakota City, on a sand 

 bar, I have on several occasions examined the exposed silt after 

 flood time for shells. In 1871 I here obtained of existing kinds 

 brought down by the river, thirty-five species of land and twenty 

 of fresh water shell*. Three years afterwards, at the same point, I 

 obtained five less of the former and six of the latter. The Mis- 



