88 GEOLOGY, 



shown, in Fig. 71, where the reUef is moderate, and in Figs. 72 and 73, 

 where it is great. Figs. 72 and 73 illustrate clearly the universal 

 tendency of rivers in regions of notable relief to develop new flats 

 well below the old surface of the region. At the same time that 

 these low-lying flats are developing, tributary drainage is dissecting 

 and roughening the upper surfaces. This process is well shown in 

 Fig. 73. In both Figs. 72 and 73 the summits of the mountains on 



J^IG 71 — Mature erosion topography in a region of slight reUef , Towa. (Calvin.) 



Fig. 72. — Mature erosion in a mountain region. From mouth of Gray Copper Gulch, 

 Silverton, Colo., quadrangle. (Cross, U. S. Geol. Surv.) 



either side of the valleys appear to have had about the same elevation. 

 The new flat is therefore developed at the expense of the old flat. As 

 will be seen in the sequel, the first fiat which a stream develops along 

 its course is usually somewhat above base-level. It is a graded flat. 



