92 GEOLOGY. 



maturity has been reached. Their presence is evidence that the 

 region where they occur has not yet been thoroughly dissected by 

 erosion hues, and therefore has not reached maturity. Still other 

 marks of topographic youth, such as rapids, falls, etc., as well as marks 

 of topographic maturity and old age, will be mentioned in the follow- 

 ing pages. 



GENERAL CHARACTERISTICS OF TOPOGRAPHIES DEVELOPED 



BY RIVER EROSION. 



With the characteristics of river valleys and the methods b}^ wkich 

 they grow clearly in mind it is easy to say whether rivers have been 

 the chief agents in the development of a given topography. River 

 valleys are distinguished from other depressions on land surfaces by 

 their hnear form and, leaving out of consideration the relatively in- 

 significant inequalities in a stream's channel, by the fact that any 

 point in the bottom of a river valley is lower than any other point 

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

 farther down the stream. The second point might be otherwise stated 

 by saying that every valley excavated by erosion leads to a lower 

 valley, or to the sea, or an inland 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 fea- 

 tures 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 con- 

 cerned in its development. 



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 



