72 WORK OF RUNNING WATER 



topography is also shown in Fig. 60, which illustrates the universal 

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

 well below the former surface of the region. 



The same processes which have made young valleys mature will 

 in time work further changes. When the gradients of the valleys 

 have become low and their bottoms wide, and when the intervening 

 ridges and hills have become narrow and small, the drainage and 

 the drainage topography have reached old age. This is illustrated 

 by Fig. i, PI. VII, and in section by the third and lower lines in 

 Fig. 54. Topographic old age may have a different expression; this 



Fig. 61. A peneplain near Camp Douglass, Wis. (At wood.) 



is shown in Fig. 61, where most of the surface has been brought 

 low. The elevations which rise above the general plain are small 

 in area, but have steep slopes. This expression of old-age topography 

 is usually the result of unequal resistance of the rock degraded. 



The marks of old streams are as characteristic as those of young 

 ones. They have low gradients and are sluggish. Instead of 

 lowering their channels steadily, they cut them down in flood, and 

 fill them up when their currents are not swollen. They meander 

 widely in their flat-bottomed valleys (PL VII) and their erosion, 

 except in time of flood, is largely lateral. 



The preceding discussion, and the illustrations which accom- 

 pany it, give some idea of the topography which characterizes an 

 area in various stages of its erosion history. Whether the valleys 

 are deep or shallow in youth and maturity depends on the height of 

 the land and its distance from the sea. The higher the land, and 



