QUATERNARY AGE. 283 



that there were long periods when this portion of the continent was 

 stationary. Several times the movement was downward. Along 

 the bluffs in the Republican Valley, at a depth varying from ten to- 

 thirty feet from the top, there is a line or streak of the Loess min- 

 gled with organic matter. It is, in fact, an old bed, where vegeta- 

 tion must have flourished for a long period. It can be traced from 

 Orleans upward in places f$r seventy-five miles. It indicates that 

 after this bed had, as dry land, sustained a growth of vegetation,, 

 an oscillation of level depressed it sufficiently to receive a great ac- 

 cumulation of Loess materials on top of it. Other oscillations of 

 this character occurred previously to and subsequent to this main 

 halt. These have already been discussed. I have also found traces 

 of this movement in many other portions of the State. 



LENGTH OF THE LOESS AGE. 



The bases for speculation concerning the length of the Loess 

 age are of course uncertain, yet an approximate estimate may per- 

 haps be made by comparison with the present deposits of the Mis- 

 souri. The great lakes of the Loess age extended, with a few in- 

 terruptions, almost to the Gulf, and some of them covered an area 

 of at least 75,000 square miles. Now, were all the sediment which 

 is at present brought down the Missouri spread over such a vast 

 area, the thickness of the deposit would be less than one sixteenth 

 of an inch. Probably the yearly accumulations of sediment dur- 

 ing the Loess age amounted to that much, owing to the then 

 greater volume of the Missouri and the aids to erosion from the 

 greater prevalence of ice near its sources. In many places along 

 the Missouri there are small lakes, formed from the old river-bed, 

 where there has been a cut-off. Even where these little lakes re- 

 ceive the overflow of the river each year, it often requires at least 

 a century to fill them up, even when aided by the sands which the 

 winds waft into them. I have attempted to measure the sediment 

 left by the river in these lakes, which are seldom half a mile in 

 breadth, and it rarely amounted to half an inch in a season. The 

 winds are a much more efficient agent for filling up small, narrow 

 lakes, but in Loess times, where there were such immense bodies of 

 fresh water, their effects could only have been appreciable along 

 the sandy shore-lines. The highest bluffs represent the original 

 level of the Loess deposits before the tremendous denuding agen- 

 cies which removed so much of their materials had done their work. 



