Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/29

16 much opportunity for widely distributing the matter in mechanical suspension brought out from the rivers, a preponderance of some deposits occurring along the lines of the ebb tide. We can readily understand different areas partaking of characteristic deposits corresponding with the kinds of detritus borne down given rivers, so that though contemporaneous they may differ materially. At moderate depths the action of the tidal current would certainly tend to mingle them considerably, as it were shading them off into each other, so far, at least, as such currents extend: but it should be recollected that a tidal current flowing at three miles per hour would only cause any substance freely suspended in it to move backwards and forwards a distance of eighteen miles, where the ebb and flood are each six hours.

As the tide rises it flows up the estuaries, checking that immediate discharge of detritus that would have been otherwise effected; and on shallow coasts, such as the mouths of estuaries usually present, the breakers acting on a wide area also tend to force much of the detritus back. In such localities blown sands are necessarily abundant, the onshore wind, particularly during heavy gales, drying the higher parts of the sands, the soonest and longest exposed to its influence, larger belts being also often only covered by water during spring tides, and exposed to drying influences during neap tides, and thus the sand is borne inland beyond the reach of the breakers.

We may regard the detritus around tidal coasts as brought under a great smoothing process, the mass of waters moving backwards and forwards, and the action of waves catching up mud or sand as the depths permit, and assisting in the general distribution over an uniform surface. The strongest tidal currents will hollow out minor depressions, where they can be felt, and therefore in the situations where the tidal wave is so impeded in one place as to be thrown with increased force on another. These situations are commonly off headlands, or amid the complication of banks at the mouths of rivers; when considered, however, with reference to sections having the same scale for height and distance, they are commonly very insignificant, and their importance, however great as respects navigation, very slight as regards geology.

This movement around lands washed by tidal seas would produce different effects according to depths; and where these were considerable, accumulations would be produced similar to those in deep tideless seas. The depths at which the friction may be sufficiently considerable, with a given velocity of surface water, to produce the ridges and furrows, commonly termed "ripple marks," is still matter of inquiry, and, considering its geological bearings, one of much interest, particularly when we compare the physical evidences of deposits having been effected in