


4 a 
Parr II. Secor. ii. §7.] MARINE DENUDATION. 449 
natural and ordinary influence of the sea in wasting the land. 
Again, the influence. of movements of depression will probably be 
found to tend in an opposite direction. The lowering of the general 
level of the land will, as a rule, help to lessen the rainfall, and con- 
sequently the rate of subaerial denudation. At the same time it will 
aid the action of the waves by removing under their level the detritus 
produced by them and heaped up on the beach, and by thus bringing 
constantly within reach of the sea fresh portions of the Jand-surface. 
But even with these advantages in favour of marine denudation, the 
balance of power will probably, on the whole, remain always on the 
_ side of the subaerial agents. 
4. Marine Denudation—tis final result. 
The general result of the erosive action of the sea on the land 
is the production of a submarine plain. As the sea advances the 
sites of successive lines of beach pass under low-water mark. Where 
erosion is in full operation the littoral belt, as far down as wave- 
- action has influence, is ground down by moving detritus (Fig. 162). 
a ae | ae 
This result may often be instructively observed, on a small scale, 
upon rocky shores where sections like that in Fig. 165 occur. We 
can conceive that should no change of level between sea and land take 
place, the sea might slowly eat its way far into the land, and pro- 
duce a gently sloping yet apparently almost horizontal selvage of plain 
2 ; 
Fic. 168.—Rocks GROUND DOWN TO A PLAIN ON THE BEACH BY WAYE-ACTION. 
covered permanently by the waves. In such a submarine plain the 
influence of geological structure, and notably of the relative powers 
of resistance of different rocks, would make itself conspicuous, as 
“may be seen even on a small scale on any rocky beach (Fig. 168). 
The present promontories caused by the superior hardness of their 
component rocks would no doubt be represented by ridges on the 
‘subaqueous plateau, while the existing bays and creeks worn out of 
softer rocks would be marked by lines of valleys or hollows." 
This tendency to the formation of a submarine plain along the 
margin of the land deserves special attention by the student of 
denudation. The angle at which a mass of land descends to the 
sea-level serves roughly to indicate the depth of water near shore. 
A precipitous coast commonly rises out of deep water; a low coast 
is usually skirted with shallow water, the line of slope above sea-level 
1 Mr. Whitaker, in the excellent paper on subaerial denudation cited on p. 433, 
‘has pointed out the different results which are obtained by the subaerial forces from 
those of sea-action in the production of lines of cliff. 
2G 
