308 



PAST CLIMATES AND CLIMAXES. 



taries are extended in every direction, but chiefly in the line of elevation, and 

 (2) one of lateral planation, in which the divides between the streams are worn 

 down. Such a distinction is artificial in that both processes are simultaneous, 

 but elongation is characteristic of the earlier phase and planation of the later. 

 The change of dominance from elongation to planation may be used to mark 

 the change from a land-form predominantly climax in the nature of its vege- 

 tation to one predominantly successional. 

 The essential processes of a cycle of erosion may best be shown by the fol- 

 lowing series of figures (Cham- 

 berlin and SaUsbury 1906: 1: 

 80-82). An area or region 

 recently elevated will have 

 gullies formed at its lower 

 edge by the erosive action of 

 run-off, and these gullies will 

 grow into ravines and valleys. 

 The gully stage persists at the 

 head of each ravine and valley 

 imtil the divide is reached. 

 In other words, the gullies 

 are the growing-points of a 

 drainage system which persist 

 just as long as growth is pos- 

 sible. If the rise of the land 

 be imiform and the surface of 

 uniform hardness, the initial 

 gullies will elongate into par- 

 allel valleys. These conditions 

 rarely obtain, however, and 

 the main valleys soon begin to 

 turn and twist, and to develop 

 tributaries. The latter, Uke 

 the main stream, elongate and 

 branch in turn by gxilly for- 

 mation. This continues in- 

 definitely until a river system 

 resembles a greatly branched 

 Fig. 25.— Development of a base-level of erosion in tree, in which the river forms 

 a region with parallel valleys. After Chamber- the trunk, its chief tribu- 

 hn and Salisbury. taries the main branches, 



the brooks and ravines the twigs, and the gullies the almost innunierable 

 growing-points. The original plains between the main valleys or their chief 

 tributaries are invaded by the growth of new valleys, and are finally dissected 

 into ridges. The latter are then broken up by the penetration of gullies along 

 their flanks, and a range of hills results. During this phase, the main valley 

 and its branches have been elongating by the growth of terminal guUies, and 

 branching as a consequence of the formation of lateral gullies. 



When the whole plain has been dissected into hills, the elongation of valleys 

 is greatly reduced or ceases altogether, though gullies and short ravines con- 



