132 GEOLOGY 



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 PL VIII, and in section by the third and lower lines in Fig. 89. 

 Topographic old age sometimes has a different expression; this 

 is shown in Fig. 98, where most of the surface has been brought 

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

 in area, but have abrupt slopes. This phase of old-age topog- 



Fig. 98. A peneplain near Camp Douglass, Wis. (Atwood.) 



raphy is usually the result of the 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 VIII) 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, and whether the inter- 

 vening ridges are high or low, depends on the original height of 

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

 the nearer it is to the sea, the greater the relief developed by erosion. 



