The surface of our planet is constantly affected by 

 two antagonistic forces, one destructive and the other 

 constructive. Erosion, produced mainly by weather- 

 ing, underground water, streams, glaciers, waves, and 

 the activities of organisms, tears down or reduces the 

 landscape. Uplift and mountain building, primarily 

 the products of volcanoes, folds, and faults, build up 

 the land. Together, these antagonistic forces produce 

 the features of the earth's crust and appear to fol- 

 low a sequence of events known as a geomnrphic 

 cycle (Figure 4.1). 



V ulcanism ig-cONSTRUCTIVE ' PHASE 'S:::: '"i*'°' 

 d last ro p h I sm .:.:............................................■.......■.■...•....:•:•: v^ 



[DESTRUCTIVE PHASE; 

 ::::: (Erosion Cycle) jjjjji 



old age :::::::::::::::::::::::::::::::::::::::: youth 



EROSION 



EROSION CYCLE 



The erosion cycle, the destructive phase of a geo- 

 morphic cycle, involves the history of a land mass 

 from the time of its uplift to its final flattening to a 

 jieneplatn (Figure 4.1). The greatest amount of 

 erosion normally is produced by streams; however, 

 in ce.'^tain situations other factors ma/ be very ef- 

 fective. Wind is important in arid regions, and waves 

 are most important along coasts. Erosion by under- 

 ground water produces the prominent features of 

 limestone landscapes, and coral deposits form the 

 reefs that are so characteristic of tropical coastal 

 seas. Weathering influences most landscapes, but 



maturity 



Figure 4.1 The geomorphic cycle, showing relationships of included 

 cycles, phases, and stages. 



usually leaves less dramatic evidence of its eroding 

 action. For convenience, any erosion cycle is said to 

 start from an initial, mostly constructional stage, 

 and is further subdivided into youth, maturity, and 

 old age. 



Two main conditions affect the erosion cycle in a 

 particular locality: amount of moisture, usually from 

 precipitation, and amount of uplift or other con- 

 structive process. In a region in which land forma- 

 tion is rapid, the erosion cycle stages of youth, ma- 

 turity, and old age are quite distinctive. Youth is 



38 



