EROSION TOPOGRAPHY 73 



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

 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 maturity than one of equal elevation near the 

 sea, since the grade-plain is higher in the former position than in 

 tin- latter. 



Characteristics of river-shaped topographies. With the char- 

 acteristics of river valleys 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 linear form, and, leaving out of consider- 

 ation the relatively insignificant inequalities in streams' channels, 

 l>y 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 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 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 development. 



Note. For laboratory work see p. 120. y 



ANALYSIS OF EROSION l 



Erosion is the term applied to all processes by which earthy 

 matter or rock is loosened or removed from one place to another. 

 It consists of several sub-processes, namely, weathering, transporta- 

 tion, forraxion, and corrosion. 



Weathering. Weathering is the term applied to nearly all those 



1 An excellent discussion of this subject is given by Gilbert in The Henry 

 Mountains, pp. 99 et seq., and more briefly in the Am. Jour. Sci., Vol. XII, p. 85, 

 et seq., 1876. 



