302 LAND SCULPTURE 



The topography of any region is, as we have seen, the resultant 

 of the very complex interaction of many different kinds of factors, 

 and is subject to continual change according to definite laws. 

 Let us suppose, in the first instance, a region newly upheaved 

 from beneath the sea into dry land. The topography of such an 

 area will be constructional, due entirely to the processes of dias- 

 trophism and accumulation, and characterized by the absence of 

 a highly developed system of drainage by streams. The coastal 

 plain of the middle and southern Atlantic States is an example of 

 such topography but slightly modified. 



Next, the processes of denudation begin their work upon the 

 region. The sea attacks the coast-line by cutting it back in one 

 place and building out in another, until a condition of equilibrium 

 is attained. Rivers are established, adjusting themselves to the 

 structure of the underlying rocks, and cutting deep, trench-like 

 valleys, while the atmospheric agencies widen out the valleys, 

 slowly wearing down and washing away the sides and tops of the 

 hills. This is the stage in which we find the greatest degree and 

 variety of relief, and it may be called the stage of maturity, as 

 contrasted with the first, which is a stage of youth. The continu- 

 ance of the degrading operations will, if uninterrupted, eventually 

 wear down the region to a nearly plane surface, through which 

 sluggish streams meander, the featureless condition of old age. 

 When the process is complete, the country is said to be base- 

 levelled. 



The term age as applied to topographical features does not 

 mean the length of time required for their formation, but merely 

 the stage of development which they have attained. The length 

 of time required to reach a given stage of such development will 

 vary greatly in different regions, in accordance with climatic con- 

 ditions, the resistance of the rocks, their altitude above sea-level, 

 and similar factors. An area of resistant rocks in an arid climate 

 will be hardly at all affected in the time that a mass of soft rocks 

 exposed to a heavy rainfall will be cut down to base-level. 



It seldom, if ever, happens that the topographical development 

 of a region proceeds uninterruptedly through the stages of youth, 



