490 11 E. Gregory — Formation and Distribution of 



bays in which wave work is negligible), and the height of 

 cliffs at 50 feet (a generous estimate), there will be removed 

 from the coast each century 2*24 cubic miles of material. The 

 contrast between rates of marine and subaerial denudation is 

 well brought out if the lowest estimate for subaerial erosion is 

 compared with the generous estimate of cliff recession. It is 

 thus seen that rivers are approximately fifty times more effec- 

 tive than waves in paring down continents. 



The relative proportions of gravel in total marine and 

 fluviatile sediments is an attractive problem awaiting discus- 

 sion, but there is no reason to believe that at any given time 

 the relative amount along shore exceeds that in valleys on the 

 land, and the figures given above may be taken to indicate that 

 the conditions for the accumulations of gravel for conversion 

 into conglomerate are much less favorable within the coastal 

 belt than over continental areas. 



Summary Analysis of River Work. 



The net tendency of the work of subaerial agents of erosion 

 is to reduce the land surface to a featureless plain. While 

 many essential and conditional factors are involved in pene- 

 planation, for the purpose at hand subaerial erosion may be 

 considered' as consisting of weathering and river transportation, 

 the former supplying the waste, the latter distributing it. 

 Rivers, to be sure, furnish their quota of waste by direct cor- 

 rasion, but as shown by Westgate,* weathering is an important 

 factor in valley deepening as well as in valley widening. With 

 a constant climate, weathering in comparison with stream cor- 

 rasion progressively increases throughout a cycle, reaching its 

 maximum in old age. 



Since weathering must be relied upon to furnish the supply 

 of gravel, and since both chemical and mechanical phases of 

 weathering are conditioned by climate and rock composition 

 and structure, it follows that the maximum amount of coarse 

 materials is favored by resistant jointed rock in an arid region. 

 On the other hand, where readily decomposed rocks lie on flat 

 surfaces, and chemical activity is facilitated by climate, no 

 gravel may be formed at the surface. Under suitable con- 

 ditions residual gravel of siliceous materials may accumulate 

 and the regolith may consist of rounded and subangular pebbles 

 and bowlders to a depth of several feet. The origin of such 

 deposits is revealed by the gradation in amount of chemical 

 decomposition from surface to bed rock. 



Throughout the larger part of a cycle gravel removal is 

 believed to follow gravel formation without much delay and 

 the products of weathering are furnished to streams to be 

 * Journal of Geology, xv, p. 114-122, 19.07. 



