XQ2 1 THE -,BOOK OF NATURE STUDY 



But at the sea-side special attention can be given to marine 

 denudation, because all stages from the rock of the cliffs to the 

 rounded shingle can be easily observed. The cliffs can easily 

 be seen to be undergoing both chemical and mechanical dis- 

 integration, especially the latter. The usual agents are at work, 

 frost and gravitation send down broken fragments to lower levels ; 

 rain and carbonic acid remove cement ; the rain mixes with softer 

 material making mud, which can flow or slip downwards ; and 

 springs carry out material or undermine rocks and cause land- 

 slips. But the waves have their own ways of disintegrating, partly 

 by hurling shingle, stones, timber, or at times ice, at the cliffs, 

 and partly by driving air into the crevices with a force often 

 amounting to thousands of pounds per square foot. The com- 

 pressed air pounded up in this way expands on the retreat of the 

 waves and is like a charge of explosive inside the rock, causing it 

 to burst outwards into fragments (Fig. 12). 



The transporting effect of the sea is due to its wind-waves, 

 especially during storms, its tides, tidal currents, and ordinary 

 currents. Wind-waves are continually picking up and dashing 

 down all rocks that they can lift. The friction and impact reduce 

 the size of these rocks, round their corners and angles, reduce 

 them eventually to pebbles and shingle, and make sand and mud 

 out of the chips broken from them in the process. All stages of 

 the work can usually be observed at the sea-side. The rise and 

 fall of the tide extends the range of the sea's action. The coarser 

 denuded material is left between tide-marks until it is reduced 

 to a fine enough condition to be transported by currents, when 

 the sand is spread out down to and below low tide, and the finer 

 mud swept some distance out from shore. 



It will be observed that it is the harder rocks of the shore from 

 which pebbles are made, the softer being broken down to sand and 

 mud and carried away. Further observation will show, if the land 

 is made of several different kinds of rock, that the contour of the 

 coast also follows the law of resistance, the hard rocks jut out as 

 capes and headlands, the softer are recessed into bays and gulfs. 

 In other words, the hard rocks project beyond the average contour 

 of the country into the sea, just as they project above its average 

 relief into the air as hills (Fig. 13). But in spite of the greater 



