THE WORK OF RUNNING WATER. 91 



become relatively more important than before, and the stream has so 

 far lost velocity as the result of diminished gradient as to be unable 

 to carry away all the detritus washed down from the sides. As a result 

 of deposition at the bases of the side slopes, a concave curve has been 

 developed. Up the valley from the point where such a section as 

 is represented by 2-2 occurs, the valley may still have a section similar 

 to that represented by 1-1. 



Still later stages of development are represented by the cross-sections 

 3-3 and 4-4. Not only has the valley become larger, but the stream 

 has deposited detritus (not shown in the figure) in the bottom of its 

 valley, developing an alluvial flat. On this flat the stream meanders, 

 and the valley may be widened by the under-cutting of the bluffs 

 wherever the stream in its wanderings reaches them (PI. VIII, near 

 St. Louis). A valley might possess the characteristics shown by the 

 cross-sections 3-3, 2-2, and 1-1 , Fig. 64, in its lower, middle, and upper 

 courses, respectively. 



The preceding discussion, and the illustrations which accompany 

 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, and the intervening ridges 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 ero- 

 sion. A plateau near the sea may become mountainous in the mature 

 stage of its erosion history, while a plain in the same situation would 

 only become hilly. A plateau in the heart of a continent would have 

 less relief in its maturity than one of equal elevation near the sea, 

 since the grade-plain in the former position is higher than in the latter. 

 Plates IV and IX show youthful topography where the relief is relatively 

 slight, and Plate X shows youthful topography where the relief is great. 

 Similarly, Plates V and VT show mature topography where the rehef 

 is great, and Fig. 1, Plate III, shows mature topography where the 

 relief is relatively slight. 



Topographic youth, topographic maturity, and topographic old age 

 are also indicated in other ways, and especially by the presence of 

 features which rivers tend to destroy. If, for example, the surface 

 of the land, well above the valley bottoms, is marked by numerous 

 ponds and marshes, it is clear that drainage has not yet progressed 

 beyond its early stages, for, unless the lakes be very deep, valleys work- 

 ing back into the land will find and drain them before topographic 



