LAND WATERS STREAMS 133 



j 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 



1 than in the latter. 



General Characteristics of Topographies Developed by River Erosion 

 With the characteristics of river valleys and the methods by 

 which they grow clearly in mind, it is easy to say whether rivers 

 have been the chief agents in the development of a given topog- 

 raphy. River valleys are distinguished from other depressions 

 on land surfaces by their linear form, and, leaving out of consider- 

 ation the relatively insignificant inequalities in streams' channels, 

 by the fact that any point in the bottom is lower than any other 

 point farther up stream in the same valley, and higher than any 

 point farther down stream. The second point might be otherwise 

 stated by saying that every valley excavated by erosion leads to 

 a lower valley, to the sea, or to an island basin. Streams which 

 dry up, or otherwise disappear as they flow, constitute partial 

 exceptions. If, therefore, the depressions on a land surface are 

 linear, lead to other and deeper valleys, and finally to an inland 

 basin, or the sea, and if the elevations between these valleys are 

 such as might have been left by the excavation of the valleys, it 

 is generally clear that rain and rivers have been the chief factors 

 in the development of the topography. If, on the other hand, a 

 surface is characterized by topographic features which streams 

 cannot develop, such as enclosed depressions, or hills and ridges 

 whose arrangement is independent of drainage lines, other agents 

 besides rain and surface streams have been concerned in its devel- 

 opment. 



Special Features Resulting from Special Conditions of Erosion 



Many striking topographic and scenic features result from 

 rain and river erosion. Some of them depend primarily on the 

 conditions of erosion, such as climate, altitude, etc., while others 

 depend largely on the structure and resistance of the rock. 



