Imagine yourself - or perhaps, if the day is fine, you are already there - strolling along the seashore, where boundless and bare, the lone and level sands stretch far away.
You will notice that while the sands may well be lone (if by that young Shelley meant unoccupied), they are unlikely to be level, and if the tide is out the surface of the sand below high water mark is likely to be corrugated by a regular pattern of ripples.
The ripples are formed underwater and have been left there by the ebbing tide. Water flowing in and out from the beach, by the action of the 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 seabed 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.
Higher up the beach, beyond the immediate influence of the tides, the sand is not entirely smooth. Here the dry, loose accumulation exhibits undulations that are different in appearance from the ripples further down.
These arise from the wind's habit of picking up grains of sand and transporting them for some distance until they fall to the ground again by the force of gravity. Initially, the surface of even the most undisturbed area of sand will have 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 sheltered side of this valley, but will rain down on the opposite face; in so doing they tend to push those grains already on the slope, and create a small hill on the downwind lip of the existing trough.
The process continues. Sand grains on the newly created hill are plucked from the crest by the wind, and being broadly uniform in size and weight, are deposited a common distance downwind, thereby creating yet another little ridge.
As the process continues, a state of equilibrium is reached for a given sweep of wind, whereby a series of ripples will have been created across the entire surface of the sand.