COLERIDGE'S Wedding-Guest has a quaint line in similes:
I fear thee, ancient Mariner!
I fear thy skinny hand!
And thou art long, and lank, and brown,
As is the ribb'd sea-sand.
But you can get his drift. He was alluding to the ripples on the sandy sea shore so often evident when the tide is low.
Ridges and troughs in the seaside sand can be caused in two ways, by the wind or by the water. Those of the first kind are found in the dry, loose accumulation well above the high water-line, and occur because of the wind's habit of picking up grains of sand and transporting them for some distance until they fall to the ground again due to the force of gravity.
Now an area of loose sand is never entirely smooth. Here and there, inevitably, will be indentations of some kind one of which, for the purposes of this exercise, we will imagine as a little valley at right angles to the wind.
Airborne grains tending to fall in this vicinity will not land on the upwind side of the valley, but will rain down on the opposite face; in so doing they tend to push those grains already there up the slope, and create a small hill on the downwind lip of the existing trough.
But the wind still blows. Sand grains on the newly created hill are plucked from the crest, and being broadly uniform in size and weight, are deposited a common distance downwind, thereby creating yet another little ridge.
And so it goes; as the process continues, a state of equilibrium is reached for a given sweep of wind, whereby a series of ripples or ribs will have been created across the entire surface of the sand.
Ripples below the high tide mark are of different origin, having formed underwater and been left there by the ebbing tide. Water flowing in and out from the beach by the action of waves contains little swirls or eddies, that might be visualised as a kind of mobile version of the hair-curlers used to generate a perm.
Each little eddy scoops up a tiny consignment of sand on its formation, and all the eddies die after a more or less common interval of time, abandoning their little hoards of sand on the sea bed in a regular pattern which reflects the characteristics of the turbulent flow of the water at the time. When the tide has ebbed, we are left with the familiar ripples on the shore.