the small waves raised on the surface of the water, by the passage of a slight breeze, are called Ripple; and a series of marks, very similar in appearance, which are sometimes seen at low water on the flat part of a sea-beach formed of fine sand, are called ripple-marks. Such marks occur in various strata of stone, and at various depths below the solid surface of the globe, and are regarded as evidence of their having been formed beneath the sea. Similar appearances occur when a strong wind drives over the face of a sandy plain, and are frequently seen upon the surface of snow.
It appears that two fluids of different specific gravity, the lighter passing over the surface of the former, always concur in the formation of ripple. It seems also [252/253] that the lines of ripple-mark are at right angles to the direction of the current which forms them.
If a fluid like air pass over the surface of perfectly quiescent water, in a plain absolutely parallel, it will have no effect; but if it impinge on the surface of the water with the slightest inclination, it will raise a small wave, which will be propagated by undulations to great distances. If the direction of the wind is very nearly parallel to the surface, this first wave, being raised above the general surface, will protect that part of the water immediately beyond it from the full effect of the wind, which will therefore again impinge upon the water at a little distance: and, this impact concurring with the undulation, will tend to produce another small wave, and thus, new waves will be produced. But the under surface of the air itself will also during this process assume the form of waves, and so, on the slightest deviation at any one point from absolute parallelism in the two fluids, their whole surfaces will become covered with ripples.
If one of the fluids be water, and the lower fluid be fine sand, partially suspended in water, these marks do not disappear when the cause ceases to act, as they do when formed by movement of air over the surface of water; but they remain and form the ridges or ripple which we observe when the tide has receded from a flat, sandy shore. [253/254]
If, after the formation of ripple-marks at the bottom of a shallow sea, some adjacent river or some current deposit upon them the mud which it holds in suspension, then the first marks will be preserved, and new ripple marks may appear above them. Such is the origin of those marks we observe in various sand-stones, from the most recent down to those of the coal measures.
Dr. Fitton informs me, that the sand hills on the south of Etapes (in France) consist of ripple-marks produced by the wind on a very large scale. They are crescent-shaped hillocks, many of which are more than a hundred feet high. The height is greatest in the middle of the crescents, declining towards the points; and the slope on the inner side of the crescent, which is remote from the prevailing direction of the winds, is much more rapid than that on which it strikes.
Mr. Lyell has observed and described this mode of formation of ripple on the dunes of sand near Calais; remarking, that in that case there is an actual lateral transfer; the grains of sand being carried by the wind up the less inclined slope of the ripple, and falling over the steep scarp. I have observed the same fact at Swansea.
A similar explanation seems to present itself as the origin of that form of clouds familiarly known as " a mackarel sky" — a wave-like appearance, which [254/255] probably arises from the passage of a current of air above or below a thin stratum of clouds. The air, being of nearly the same specific gravity as that of the cloud it acts upon, would produce ripple of larger size than would otherwise occur.
The surface of the sun presents to very good telescopes a certain mottled appearance, which is not exactly ripple, and which it is difficult to convey by description. It may, however, be suggested, that wherever such appearances occur, whether in planetary or in stellar bodies, or in the minuter precincts of the dye-house and the engine-boiler, they indicate the fitness of an inquiry, whether there are not two currents of fluid or semi-fluid matter, one moving with a different velocity over the other, the direction of the motion being at right angles to the lines of waves. [255/256]
14 December 2008