PLANATION BY RIVERS 77 



of reference is therefore the ocean level, and lakes above sealevel or seas 

 like the Caspian beneath sealevel are local baselevels which control the 

 plains formed bj^ the rivers emptying into them. 



As long as there is a steeper grade at any point on the land than is 

 just sufficient to allow the water to drain off some particles of land waste 

 will be carried toward the sea down these slopes, and though the process 

 will at last become very slow in operation, it must continue, if the land 

 and water do not change their respective levels, until the land is worn 

 down to a lowland, with the very gentlest of surface inequalities, but 

 little above the level of the sea, forming almost a plain or peneplain. 



When a region had been leveled by this method of planation there 

 would generally remain as witnesses of the process masses of rock rising 

 somewhat above the general level of the peneplain. These would con- 

 sist of more resistant rock than that which immediately surrounded 

 them, though the highest point may not necessarily be the hardest rock 

 in the whole region, and such masses would not lie close to large river 

 courses, nor be as numerous near the sea, which formed the baselevel, as 

 at some distance from it. Other things being equal, one would expect 

 to find the number of such masses of rock or monadnocks increasing in 

 numbers the farther one went from the sea. 



The stream courses on the surface of the subaerially planed area would 

 be largely on the weaker rocks, while if the cycle of down-cutting had 

 been made up of several divisions or epicj^cles, several periods of elevation 

 following one another with long periods of wearing down between each 

 uplift, this adjustment of stream to structures would be more perfect.^ 



The adjustment of drainage is tl)e best method known of distinguishing 

 this kind of planation from that performed by the sea. The lack of a 

 cover of marine sediments is negative evidence in the sam.e direction. 



WORK OF THE SEA 



The sea, aided by the atmospheric agencies, must also plane the land 

 to form a nearly level plain, the plain of marine denudation of Ramsay .f 

 This submarine platform is formed by the attack of the sea on the 

 land, and if the land remains stationary the sea will cut into it strip by 

 strip, depositing the greater portion of the detritus in a continental delta 

 to seaward of this platform, but also covering the portions first formed 

 with a thin cover of waste from the later abrasion. This cutting is lim- 

 ited -in depth by the power of the waves to abrade the bottom, the limit- 

 ing plane forming a local baselevel orvjave-baseX l.ying beneath the general 

 baselevel formed by the surface of the sea. 



* \V. M. Davis : Bull. Geol. Soc.Am., vol. 7, 1895, p. 395 ; London Geog. Jour., vol. v, 1895, p. 140. 

 t A. C. Ramsay : Denudation of South Wales. Mem. Geol. Sur. Great Britain, vol. i, 1840, p. 327 ; 

 Phys. Geol. and Geog. of Great Britain, 5tli ed., 1878, p. 497. 

 X Proc. Amer. Acad, of Arts and Sci., vol. xxxiv, 1898, p. 17C. 



XII— Bull. Geol. See. Am., Vol. 10, 1898 



) 



