Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/24

Rh the sea and estuaries, which may be termed funnel-shaped, such, for instance, as the Bristol Channel, and the Severn at its termination, there are great sweeping powers of the waters, arising from the velocity produced by the mode in which the tidal wave is caused to act, so that the upper portions of such estuaries are kept constantly turbid by the finer detritus held in mechanical suspension. Instead, therefore, of such finer detritus being discharged readily seaward, as in tideless seas, it is deposited in the sheltered parts of these estuaries, where there is little movement in the water, filling up such places to the level that the sediment-bearing waters can cover. Consequently, in estuaries we often find the shores muddy and the central parts sandy, the friction and motion of the water preventing any mud accumulations in the latter.

The action of the tides has a tendency to flatten out the sedimentary matter, either arising from a deposit of the detritus held in mechanical suspension, or borne into tidal influences by the friction of moving water on yielding sandy or other bottoms. This action also would produce, by friction, those ridges and furrows so commonly seen beneath running waters, sandy tracts left dry at low tides, and upon blown sands. It will be readily understood that, upon a given locality, from the greater volume and velocity of the water during spring tides, such ridges and furrows could be then produced which could not be formed at the neap tides, from the absence of the adequate volume and velocity of moving water above these sea bottoms at that time.

While in tideless seas and great lakes the destructive action of the breakers keeps a horizontal line, varying but little, the detritus re- moved from the coast so acted upon travelling outwards but a slight distance into the still water adjoining, and the waves, from the movement of the water, may keep the finer detritus mechanically suspended along shore, there is greater destruction of the land on tidal coasts, and the tides become carriers and sweepers of the sand and mud thus brought within their influence.

On a tidal coast, as a whole, the breakers are heavier than on a tideless shore, from the more open and exposed character of the one than the other. Ground-swells, the heavy breakers from which are so destructive, are necessarily more common on the one than the other, these being little else than the waves produced, during heavy gales in the open oceans, rolling on the coasts, when such waves have originally been of sufficient magnitude to reach them. The friction and roar of the surf, as it is termed, is well known on some oceanic coasts to be ceaseless.

A slight study of any well-exposed oceanic coast is sufficient to show the effects of the degrading force of breakers. No doubt, here and there, it blocks up the mouths of valleys, damming up the free flow of the