back wash (A, fig. 1) of each wave will carry a quantity of sand and the like with it, which at the junction (B) with the incoming wave will be met, tossed about, and carried back again; but if the tide is ebbing, scarcely any particles will be carried by the back wash, and the junction (B) with the incoming wave will scarcely have even a particle of sand in it. In the first case, a bucketful taken at the junction would be sand and water, while in the second it would be nearly pure water.
Fig. 1.—Diagram of a Wind-wave breaking on a shelving beach.
Third Point.—"Currents carry the smallest fragments furthest." Tidal currents are different from other currents. They break on the shore in successive waves, whether these be augmented by wind-waves or not, drive the fragments obliquely up the beach, the larger ones to remain or only to be brought back a little way, while the smaller ones are caught in the "back wash," and carried back again till they meet the succeeding incoming wave. Thus the larger fragments have a tendency to be driven higher, and further up, and along a beach than the smaller ones. Also, if the progress of a beach is stopped by a groyne, whether natural or artificial, the larger pebbles accumulate behind it, and form a shingle beach.
Chesil and Cahore beaches are remarkable on account of their being very similarly circumstanced in several particulars; this will be evident from the following table of comparison. All the statements in regard to the Cahore beach are given from personal examination; but for those relating to the Chesil beach we have to rely on the notes and publications of other observers. Some of the points of comparison are partly speculative, but agreeable with the known results of certain natural laws.
A Comparison of the Conditions at the Chesil Beach, Dorsetshire, and at the Cahore Shingle Beach, co. Wexford.
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Chesil Beach. 1. The beach is situated on the north side of the English Channel, where there is a smaller "rise" of the tide than on the south side (coast of France). |
Cahore Shingle Beach. 1. The beach is situated on the west side of the Irish Sea, where there is a smaller "rise" of the tide than on the east side (coast of England). |
