Supposing C to be another point of land advancing into deep water, the shingles obtained from the coast between M and C would be there arrested in their progress, as at B, in their course barring the mouth of a shallow sub-aqueous valley, considered to exist at E, the river waters falling into which were of insufficient volume to counteract the formation of a bar of shingles coming down from the direction of M. In such a condition of things, as in the Looe Pool in Cornwall, the bar would separate a fresh-water lake from the sea, the excess of the river waters percolating through the shingles seaward.[1]
A variety of modifications, not here noticed, will readily be conceived, and an examination of any extended line of coast will furnish abundant examples of them. The general result is an unequally distributed fringe of pebbles adjoining the coast, behind which there may be patches of mud and sand, sometimes containing the remains of estuary, at others of fresh water animals. Under very favourable conditions of position and of winds, causing the advance of pebbles, a mass of shingles may be collected, as at Dungeness, forming a continuous and nearly level bed, of considerable area.
Though, as regards the tideless seas and lakes, the prevalent winds also cause the breakers to drive shingle in the manner here described, the amount of pebbles produced is, as before noticed, smaller, and, from the generally inferior power of the breakers, less force is employed to propel them onwards. Nevertheless, they do often accumulate from this cause in considerable abundance, and effects of much geological importance are produced. The rise and fall of tide, however, combined with the generally greater power of the breakers, gives a driving force in the one case far more considerable than can be obtained in the other.
Sand is necessarily subjected to the same influences as the pebbles, and nothing is so well known as the removal of a mass of sandy beach from one place to another, as different winds, causing breakers to fall in one direction and then in another, act on a coast The sand being somewhat readily caught up, however, in mechanical suspension after
- ↑ In such situations it sometimes happens that a great flood of waters from the interior will force a passage through the bar, so as to allow the sea water to flow in and out with the tides for a time, consequently driving away the fresh-water creatures which can escape, and introducing marine animals. When, from the blocking up of the passage, which generally happens during a gale of wind» the fresh-water lake is restored, all animals which cannot sustain the change are killed, so that deposits taking place during this time upon such areas may contain singular mixtures of fresh-water and marine remains. In the Looe Pool, which is let out by artificial means occasionally (see Report on the Geology of Cornwall, Devon, &c., p. 447), common sand shrimps (Crangon vulgaris) are numerous in the fresh-water lake, forming no small portion of the food of its celebrated trout. In the lake behind Slapton Sands, Devon, flounders are shut in with the trout.
