Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/66

40 as also the stream out of Wexford Harbour, which is ebbing for some hours after the tide is flowing in the Bay, must more or less prevent the offshoots from the up-channel current increasing progressively in strength. Consequently the results on the Cahore beach are not so regular or so marked as on the Chesil beach; but notwithstanding all these adverse circumstances, the tendency on the Cahore beach is for the largest fragments to be carried furthest and thrown up highest.

Another point to be considered is the great size of Lyme Bay compared with that of Wexford Bay. Lyme Bay, as just mentioned, is open, and the Chesil beach exposed to the full force not only of the tidal current but also of the most prevailing and effective winds. The current also is progressively increasing in velocity as it goes eastward, and therefore carries large fragments with it, to be driven on shore and accumulated in a high and massive beach when the current has its maximum power during excessive winds. As the currents branching in shore from the main up-channel stream are continually increasing in velocity from west to east, each carries different-sized fragments, which, when stranded, cause the shore-accumulation to have its peculiar arrangement.

The site of the Cahore lagoon, prior to its being a lagoon, and when the sea was at least 30 feet lower than at present, was a peat-bog, the land then stretching considerably to seaward, as the bog is found off shore at the four-fathom line. Since then there have been different oscillations in the level of the sea, during one of which a gravel and shingle beach, which eventually grew into an Æolian drift bank, was formed, joining Cahore with the land to the south, and enclosing to the west of it a lagoon. As on this portion of the coast the more prevailing on-shore winds are from the north-eastward, they often, while they last, pile up large quantities of sand in Pollduff, the small bay on the north of Cahore. These accumulations, however, are carried away in an incredibly short time if the tidal current going northward is augmented by one or two heavy gales from the south-east. On account of this periodical filling-up of Pollduff, the drainage of the Cahore lagoon was at first accomplished by a canal cut through the middle of the Æolian-drift ridge. This canal, however, was always filling up by the tidal and wind driftages, and eventually, about twenty years ago, was abandoned; now, and for seme years past, all traces of it are obliterated. At present the tract is drained by works that open into Pollduff.

From the facts now stated, it seems reasonable to conclude that the Chesil and Cahore beaches are due to nearly the same natural causes,