LAND FORMS, THEIR DEFORMATION AND FORMATION 



39 



characterized by clear definition of the origin of ele- 

 vations and often by sharp, V-shaped valleys and by 

 somevk'hat sharp mountain crests. In the stage of ma- 

 turity, mountain slopes are often gentler and their 

 crests are rounded or dissected. The valleys among 

 the gently sloping mountains often are more rounded 

 and U-shaped. In the final stage of erosion the 

 landscape becomes a flattened peneplain with scat- 

 tered, low, and much-rounded hills. 



In an area of slow land formation, the same stages 

 and general tendencies prevail; however, the char- 

 acteristic landscapes are obscured. Youth is almost 

 absent; it is very difficult to find either sharp ridges or 

 V-shaped valleys. Maturity is like that in areas of 

 rapid land formation, but the hills are much lower 

 and almost immediately reach the relief just preced- 

 ing old age. The landscape of old age is the same as 

 that produced by rapid constructive processes. 



The erosion cycle, modified by rapid or slow 

 processes of uplift and mountain building, is 

 basically the same in humid and arid climates (Figure 

 4.2). Modifications due to climatic differences are 

 fairly obvious. In moist climates, the streams are 

 joined and there is more erosion than deposition be- 

 cause sediments are generally transported elsewhere 

 by the streams. Elevations are rapidly removed and 

 the final stage, formation of a peneplain, is ar- 

 rived at sooner. Only the early stage of the cycle 



HUMID 



ARID 



OLD AGE 



Figure 4.2 The erosion cycle; some contrasts between humid and 

 arid regions. 



contains the clear features of constructive processes. 

 In maturity, the mountain types lose much of their 

 bold outline; erosion produces sharp cuts in the 

 constructional relief. Although this cycle terminates 

 in peneplaination as a result of erosion by streams, 

 the stages of landscape and streams do not tend to 

 coincide. For example, although young streams may 

 occur in young landscapes, the independence of 

 stream and erosion cycles is more likely. 



In arid climates, the modified landscape results 

 from streams not being able to remove erosional 

 debris. Also, the streams are independent of one 

 another and are often temporary, each often flowing 

 into a basin and forming a shallow, often seasonal 

 lake. These independent streams are probably the 

 main factor of erosion; however, in arid climates 

 wind is of great importance. Wind scours the land- 

 scape and transports materials within the area and 

 to other areas. Youth lasts relatively longer than in 

 humid climates, and the outlines of constructive 

 processes last into late youth. During this early 

 stage, the land forms emphasize the erosion of 

 higher elevations and deposition of the products at 

 lower elevations within the area. This process con- 

 tinues into and through maturity. In maturity, the 

 streams join into a united network and the landscape 

 assumes the general appearance of maturity in a 

 moist locality. Finally, after more erosion and de- 

 position within the area, the landscape becomes a 

 peneplain of old age. 



A complete erosion cycle, including the final stage 

 in the form of a peneplain, is no more than a very 

 strong hypothesis because erosion might never pro- 

 ceed to completion before new constructive proc- 

 esses occur. The discussion of a complete erosion cy- 

 cle is a device for understanding, rather than a de- 

 scription of, what may have happened in any one 

 area, where destruction and construction of the land 

 may work simultaneously. 



A peneplain is mostly a surface of gently rolling 

 plains, often a gentle slope from sea level to higher, 

 interior elevations. However, these land forms are 

 far from being monotonous terrains with no relief. 

 It is true that their general landscape is rolling, 

 consisting mostly of poorly defined divides between 

 the erosion and flood plains of old streams, but there 

 are often isolated elevations. These elevations are 

 composed of exceptionally resistant rock, either 

 bedrock not yet weathered or rocks more durable 

 than most in the area. 



