THE ECONOMIC ASPECTS OF SOIL EROSION 
331 
rarely, if ever, do the great waves of the wide ocean attain tlie 
continental shores. The decay of the wave is due to the application 
of its energy to the erosion work Avhich it has done on the sea door. 
* The loss is shown not so much by the decrease in the height of 
the surge as in the shortening of its width and the slowing of its 
motion. A good share of its height is preserved in a peculiar 
manner: as the undulation comes over the shallowing floor of 
the sea the hindrance to its ongoing is proportionate to the dimi- 
nution of the depth. The result is that the front of the wave, 
being in the least depth, is held back to a greater degree than 
the rear which is in deeper water. The two sides of the wave 
are thus crowded together, so that the crest of the arch is rela- 
tively uplifted. For all this, however, the wave when it over- 
turns — that is, when the top, or part least held back by the 
friction on the bottom, shoots over the base and falls in the re- 
current cataract of the surf — probably never exceeds twenty feet 
in height and the energ}’^ left in the surging water may be reck- 
oned at less than one-tenth of that which is held by the greater 
waves of the o|)en sea. 
When the wave delivers its finishing stroke in the surf line 
and its splash front, the modes in which its energy is applied 
suddenly become changed. The falling mass of water strikes a 
powerful blow, which, coming upon firm-set rock or sand, has 
but little effect; but Avhen, as is often the case, the beach is 
covered Avith loose stones, these fragments are driven about in a 
violent manner and strike heaA'y bloAvs. When the Avave over- 
turns, the mass of Avater SAveeps u]) the slope of the strand, urging 
before it all the rock fragments Avhich it can driA-^e onward. If 
the upper edge of the beach is bordered by cliffs, as is generally 
the case along rock-bound shores, the SAvash and secondary 
Avaves Avhich gather inside the tumble of the surf send the 
boulders Avith each stroke to batter the base of the bluff 
Although the Avaves have in all cases lost a large ]>art of their 
energy before they are able to do this Avork of battering the shore 
cliffs they are still, Avhen armed Avith rock fragments, comj)ctcnt 
to accomi»lish a great deal of erosion. Whenever the cliff is 
cfmijiosed of ordinary hard rock, the battering at its base cuts a 
recess, causing the cliff to overhang. In time the Aveight of the 
mass Avhich is thus unsupported brings it in ruins to the beach, 
Avhere the fragments are ground into santl or mud by the action 
of the AvaAX'S and rcmoA'cd to the deep sea or the distant reaches 
of the shore. 
