•aughet.] length of THE LOESS AGE. 253 



the Niobrara, and the Republican covered their respective flood-plains in 

 the same way. In the smaller streams of the State, those that origin- 

 ated within or near the Lacustrine deposits, such as the Elkhorn, Loup, 

 Bow, Blue, and the ISTemahas, we see the same general form of flood- 

 plain as on the larger rivers, and no doubt their entire bottoms were 

 also covered with water during this period. Hayden, in his first reports, 

 has already expressed the same opinion as to the original size of these 

 rivers. Only a few geologists will dissent from this view. The gradu- 

 ally melting glaciers, which had been accumulating for so many ages at 

 the sources of these great rivers, the vast floods of water caused by the 

 necessarily moist climate and heavy rains, the i)resent forms and mate- 

 rials of the river-bottoms, are some of the causes which in my opinion 

 would operate to produce such vast volumes of water. 



The changes of level were not all upward during this age. The ter- 

 races along the Missouri, Platte, and Republican indicate that there 

 were long periods when this portion of the continent was stationary. 

 Once, at least, the movement was downward. Along the bluff's 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 mingled with organic matter. 

 It is, in fact, an old bed, where vegetation must have flourished for a long 

 period. It can be traced from Orleans upward in places for 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 accumulation of Loess materials on top of it. I have 

 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 perhaps be made 

 by comparison with the present deposits of the Missouri. The great 

 lakes of the Loess age extended, with few interruptions, 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 Mis- 

 souri spread over such a vast area, the thickness of the deposit would 

 be less than one-sixteenth of an inch. Probably the yearly accumula- 

 tions of sediment during 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 receive 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 

 agencies which removed so much of their materials had done their 

 work. Now, in places these sediments are even yet 200 or more feet in 

 thickness, so that it would be safe to estimate the average thickness of 

 the original deposit at 100 feet, A yearly increase of one-sixteenth of 

 an inch in thickness, would at this rate have required 19,200 years to 

 form these deposits. This I consider a low estimate for the length of 

 the Loess age. 



