DENUDATION 103 



resistance of the harder rocks, eventually both hard and soft are 

 cut back by the sea at approximately the same rate, and the sea 

 advances steadily upon the land, cutting the cliffs back until, if 

 left alone to its work, it would mow down the whole of the land 

 and reduce it to a flat plane below its own surface. Such sub- 

 marine plains are to be found round most continents and islands, 

 and are called plains of marine denudation. 



The action of the sea in planing down the land before it is 

 in strong contrast to that of streams in lowering only their own 

 definite paths. The latter are engaged in producing relief of the 

 surface, the former in levelling down all outstanding features. 

 The ultimate effect of the sea must be inevitably to cut down all 

 land to a plain (Fig. 14), while the stream and its tributaries tend to 

 accentuate relief of the surface above sea level. It is less easy to 

 draw any distinction between the fragments denuded by running 

 water and those prepared by the waves. Both are rolled and 

 rounded, but the action of the sea produces greater perfection in 

 these characters than does that of flowing water. 



As in the study of a road and of a stream, the powerful effects 

 of the sea are not so much due to everyday work as to occasional 

 and infrequent heavy storms (Fig. 14). These phenomena, which 

 to human reckoning are uncommon and infrequent, are of the 

 class of irregularly recurring actions, when long periods of time 

 are in question, and the effect of their recurrence is a matter to 

 be seriously reckoned with. 



Wind is in some places an important transporting and eroding 

 agent. It acts chiefly on light small materials, such as sand 

 grains. These when drifted gradually become rounded and 

 polished, and they react on the rocks that they strike, etching 

 and undercutting them. Fig. 15 gives a characteristic example of 

 a projecting rocl: the softer layers of which have been cut into by 

 wind-drifted sand and dust. 



Denudation by the agents so far dealt with is a function of 

 time and energy. It is most active with heavy rain and winds, 

 with steep bare slopes, with rapidly moving waters, and in 

 jointed or soft rocks. It is perhaps less easy to show that at 

 all times and under all circumstances slow but certain denuda- 

 tion is being carried out, so long as the slopes of the country are 



