Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/28

Rh the waves and, consequently, the breakers have attained a fair size, it is not unfrequently removed from exposed shores during gales of wind, and carried with the current of tide outwards, or thrown down in sheltered situations. More moderate weather with minor breakers usually heaves this sand back again, so that the general character of a coast remains as a whole the same. The steep slopes of shingle beaches often pass down beneath the sands, covering up their base, the sands heaped up or removed according to circumstances.

The piling action of breakers acting in shallow water on the one hand, and the checked motion of detritus-bearing rivers in flood, where they meet the sea, on the other, produce those accumulations of sand, across many tidal rivers, so well known as bars. A tidal river exercises its chief scouring action, forcing a passage and cutting through these bars by friction on the ebb tide, when the whole of the river waters, ponded back by the rise of the flood tide, pass out with the sea waters that have entered during the flood, the usual flow of the river still continuing to add to this mass of water flowing out seaward. The height of a bar is therefore an adjustment to these conditions; and any alteration in the volume of the combined sea and river waters passing out would produce its corresponding effect on the bar, the greatest effect being when a freshet or flood in the river is combined with a high spring tide, the volume of water and the velocity of its passage out being then the most considerable. If, therefore, an estuary or tidal river is filling up inside the bar, as the volume of sea water which can enter is decreased thereby, so is the power to cut down the bar diminished. As the bar rises, the power of the sand borne down by the river to escape becomes lessened, so that a large muddy and sandy accumulation succeeds to the area once occupied by tidal waters. The mouths of the river may indeed advance, but the same results will follow, and the detritus borne down by the river will accumulate. It is well known that large advances are in this way made in tidal seas, the current of tide making a more general distribution than on tideless seas, the great deposit still, however, moving onward and gradually filling up the sea before it.

In the discharge of estuary waters seaward, the waters of the ebb are necessarily carried in the direction in which that tide sets along any given shore. The rush of the river waters ceasing, the matter held in mechanical suspension will be deposited according to its volume and weight. The finer particles may be carried for a long distance, particularly when gales of wind keep the sea, into which the estuary waters have been discharged, in violent agitation. The same waters, charged with whatever may fall into them, are carried backwards and forwards by the tides; so that, looking at the average chances of deposit, there is