258 GEOLOGY OF THE DENVEK BASIN. 



larger than the modern streams, as evidenced by the relative dimensions 

 of their beds. With unimportant exceptions the streams of the present 

 day have followed the same general courses as did their former gigantic 

 representatives. As a consequence of the humid climate that must have 

 prevailed, the erosion of the plains kept pace with the corrasion of the 

 streams, producing a surface of low relic!', of broad, shallow valleys and 

 slopes that rarely exceed an angle of 5 ' with the horizon. 



I.oKssi.vi. EPOCH, 



Following the period of erosion and waste, in consequence of some 

 cause not yet definitely determined, but which probably resulted in a reduc- 

 tion of the general slope of the region, and may have been accompanied by 

 some climatic changes, came a period of gradual increase of deposition over 

 ablation, during which the stream Weds were choked and filled with coarse 

 gravel and sand, succeeded by liner material, until at length the whole 

 region was covered with a varying but great depth of the finest silt. 



The deposits formed during this period of deposition — which, as before 

 indicated, lias been called the loessial epoch — and prior to the modern 

 erosion period, which has given the finishing touches to the surface sculp- 

 turing of the present day. may lie divided into — 



1. The river drift, including the fluvial loess. 



2. The loess proper, including the glacio-natant drift. 



3. The highland drift. 



As will he seen later, it is possible that the glacio-natant and highland 

 drift were formed almosl contemporaneously, and are phases of the same 

 general phenomenon, differing with local conditions of deposition. 



RIVER DRIFT. 



The wide drainage channels formed during the earlier erosion epoch 

 were Idled by alternations of coarse and line gravels, sand, and clay, with 

 cobbles and occasional bowlders up to 2 and ."> feet in diameter at the bot- 

 tom, which present the characteristics common to ordinary river drift, such 

 a- cross-bedding and abrupt changes vertically and laterally. The material 

 of which the drift is composed varies with that which constituted the floor of 

 the basin that the ancient stream drained. Thus the drift of the ancient 



