DEFORMATION AND GRADATION. 309 



tinue to develop in some degree until the hill is worn away. This is effected 

 chiefly by lateral planation, by which the successive deflection of the current 

 undercuts the banks, and may finally wear away an intervening hill or divide 

 itself. While planation is present in some small degree at the beginning of 

 valley formation, it becomes characteristic only as the valleys deepen and 

 the major and minor streams begin to meander. The combined action of 

 elongation and planation brings the plain constantly nearer to sea-level. 

 Theoretically, they would ultimately reduce it to an absolute base-level, but 

 actually this occurs only for the lower portion of the system. Indeed, the 

 base-level of any river system is but an approximation, since upper portions 

 must always be higher than lower ones as long as a system exists. Simi- 

 larly, during the gradational periods of the geological past, a base-level was 

 reached only in the regions bordering the seas, before a new deformation 

 initiated another cycle of erosion. 



Belation to vegetation. — In its simplest terms, the cycle of erosion is a period 

 marked by continuous and complementary erosion and deposit. Its relation 

 to succession, then, combines the effects of these two processes upon vegeta- 

 tion. As has elsewhere been shown, the erosion of the land smrface must 

 necessarily destroy the vegetation upon it, and at the same time produce a 

 bare area for colonization. Deposition may also destroy vegetation, if it 

 takes place upon land, but as a rule it produces new areas by filling water- 

 bodies up to the level where plant pioneers can enter. For any uniform or 

 localized area of erosion or deposit, the action is clear and the successional 

 relations of the simplest. But they appear to become involved when a whole 

 region is considered. Here we find erosions and deposits of various ages going 

 on; erosion and deposit may exist side by side, or the material eroded in one 

 place may be deposited in a distant one. An area of recent deposit may be 

 itself eroded, or one of erosion may undergo deposition. Finally, the size 

 of the area of erosion and deposit may vary in the widest degree. It may 

 occupy hundreds of square miles, or it may suffice only to destroy or establish 

 a single individual plant. It is this fragmentary action of erosion and deposit 

 over areas of the most various size, but especially small ones, that complicates 

 the correlation between the cycle of erosion and succession. 



However, this complication is superficial rather than developmental. Since 

 there are only three great vegetation eras, it is clear that even a major cycle 

 of erosion must operate well within the time limits of a cUmax vegetation. 

 Furthermore, the smaller regional erosion cycles within each deformation 

 must have acted chiefly within the actual area of a climax formation. In 

 short, a cycle of erosion involves the gradual destruction of the climax forma- 

 tion. This destruction is always fragmentary, but in the outcome it is com- 

 plete, either for the whole area or a portion of it. At the same time that 

 erosion is destroying the chmax, deposit is making new areas for its renewed 

 development or reproduction. In the bad lands, which furnish by far the best 

 examples of a local erosion cycle, it is far from unusual to find that the grass- 

 land climax, which is being destroyed at the edge of ths erosion level, is simul- 

 taneously being again developed on the lower deposit-level of the wider valleys. 

 Between the two levels, the rapid erosion of the gullied slopes also regularly 

 destroys the developmental communities, and this is true, though to a smaller 

 degree, of the areas of deposit at the base of slopes. 



