Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/25

12 drainage waters of the land, when these are not sufficiently powerful to contend with its piling influence, and it heaps up huge beaches, breaking upon the barriers which it has itself raised; but where the coast is fairly exposed, rising well above the sea level, no soft rock can stand before this power, and all such are ground away; many a patch of soft rocks above the action of the breakers attesting their once farther range seaward.

The rounded boulders and pebbles produced by this grinding action, many a fragment detached from the cliff above by the influence of ordinary atmospheric causes being added to them, are necessarily moved in the direction of the prevailing force acting upon them. This will not be seaward, because the great power of breakers is coastward, driving all that they can either gather up in mechanical suspension, or carry on by friction, towards the land,—the return action of the breaker, unless it should have a cliff to strike against, and thereby rebound with great force, possessing less power than its fall forwards. A fact abundantly proved by the lines of shingle beaches in front of low lands.

Though not borne outwards, if there be any force acting upon them in the direction of the coast, more one way than another, the shingles will necessarily travel in conformity with it; and hence, where the prevalent winds, and, consequently, the lines of waves, fall obliquely on a coast, the shingles will strive to find a passage, or travel in the direction of the impulse given them. This is found to be strictly true; for although breakers may generally appear to adjust themselves to the tortuous character of a coast line, there is always a slight oblique action from the main direction of the wind at the time, if it be sufficiently powerful, producing waves.

The travelling of the shingles, thus produced, is modified or arrested by several circumstances, which the following figure (Fig. 6) may serve to explain. Let G, C, B, A, F, represent a line of coast exposed to the prevalent winds W.W., producing lines of waves and of action represented by the dotted lines at right angles to their direction. The shingles would strive to travel on one side from A to F, and, on the other, from A to G. On the former they could readily, but gradually, travel along shore until they arrived at the river F, supposed to be of sufficient size to keep its embouchure clear for a free passage of its waters. Here the river would interrupt the further progress of the shingle, and a struggle would commence between the action of the river endeavouring to turn the shingle back, and the action of the prevalent breakers forcing it forward. The results necessarily depend upon the general power of each action. Usually the breakers gain, so as to produce a long line of shingles, turning the course of the river, until the latter obtains support from a hard cliff, as at E, and is no longer forced aside. In