QUATERNARY AGE. 293 



upward movement of this portion of the continent, amounting 

 to perhaps one or two feet to the century. 



The terraces made during this epoch occupy various heights 

 above the flood-plains. The one next to the rivers in the interior 

 ranges from three to six feet above the lowest bottom. The next 

 is from twelve to twenty-five feet above the first, and a third at 

 varying heights above the last. Often terraces intermediate be- 

 tween these are detected. They vary so much in height that the 

 system ascertained to exist at one place is no guide for the next 

 river. This variation, no doubt, is partly caused by one or two or 

 more corresponding terraces being removed by subsequent erosion. 

 They are the memorials of the rivers' former stay for an indefinite 

 time at that level. It is possible that this Terrace Epoch was as long 

 as the Loess period, but of this there is no certainty, as it partakes 

 in part of the character of a lost interval of geological history. 



Alluvium. — Next to the Loess deposits, in an economical point of 

 view, the Alluvium formations are the most important. The val- 

 leys and flood -plains of the rivers and smaller streams, where these 

 deposits are found, are a prominent feature of the surface geology 

 of the State. All the rivers of the interior, such as the Platte, the 

 Republican, the Niobrara, the Elkhorn, the Blues, the Nemahas, 

 and their tributaries, have broad bottoms in the center or on one side 

 of which the streams have their beds. The width of these bottoms 

 seem to be dependent on the character of the underlying rock-forma- 

 tion. Where this is soft or yielding, the bottoms are broad, but 

 where it is hard and compact they contract. This is, no doubt, one 

 reason why the bottoms on the middle or upper courses of some of 

 the rivers are wider than farther down.* These broad bottoms, as 

 we have already seen, represent the ancient river-beds toward the 

 close of the Loess age. It required many ages to drain the mighty 

 ancient lake-bed; and when the present rivers were first outlined, 

 the greater part of it was yet a vast swamp or bog. But, gradually, 

 as the continent rose to a higher level, the rivers cut deeper and 

 deeper, filling the whole flood-plain from bluff to bluff. Not until 

 the drainage of this region was completed and the continent had 

 reached nearly to its present level, was the volume of water so 

 much diminished that the rivers contracted their currents and cut 

 new beds somewhere through the present bottoms. The terraces, 

 which are so numerous along many of the river-bottoms, indicates 



♦See ou this subject Hayden's Report for 1S7>. 



