Rachel Carson wrote that earth's molten minerals cooled and solidified into rock before any rain could fall to collect in oceans. The first sentence of Genesis picks up the natural history there, with "a mightly wind that swept over the surface of the waters." That was the beginning of the beach because the offspring of that made-in-heaven match between wind and water was ocean waves. Like sound waves, they are manifest energy passing through a medium. The ocean's surface absorbs it from passing wind, underwater avalanches, boat wakes and other forces.

Waves per se don't move water horizontally very much; the wave's energy makes the surface move up and down like a jump rope held at one end and jiggled at the other. (Particles of ocean water typically move in very small orbits, like corks bobbing in a bathtub.) It's the currents and tides that move the water in bulk.

A wave moves forward until something stops or changes its course. Waves from an Azores storm pass slower ones from a line squall off the coast. Waves from different sources and at different depths chase each other, meet head-on, cross at every angle. Where their peaks momentarily coincide, they loom larger on the surface. The idea of the ninth wave consistently being the largest and the ninth ninth wave being a monster is a myth. It's not that simple.

For various submarine reasons, waves organize themselves when approaching land - even circular islands - and always strike nearly head-on all along its edge. Try to imagine the contrary: waves striking one side of an island and moving away from the other. It doesn't happen; waves approach every beach nearly parallel.

Waves break when their height is more than one-seventh their length from crest to crest because the water's surface tension can't hold them together any longer. This occurs at sea when a strong wind blows over them for a long distance and they get too tall. They also break on the coast when the bottom of the wave is slowed down by the friction of the sloping sea floor but the top of the wave keeps going.

When waves strike something, like a geologically young cliff, they break up, split apart, bounce back. But the repeated force has its effects: cliffs crumble in time, dropping chunks into the sea. Inland mountains contribute material, too, as rain erodes them and rivers carry their washed remains to the sea in a process that has been going on since the first rains fell. All this rocky debris works on itself in the ocean, grinding and crushing. But it doesn't turn to powder because of the water. Depending on rock density, it becomes particles of a minimum size that stop shrinking because a film of water surrounds each one, cushioning it from the others. Ergo, sand - particles measuring between .05 and 2 millimeters in diameter.

To simplify the ancient natural history, the first offshore sand was carried to the nascent beach and left there by a wave that slowed down as it crossed the continental shelf. That sand inhibited the next wave a little more, causing it to drop more sand and so on. But when storm waves came ashore and rushed madly back to sea, sand was carried away because of the faster moving water's higher "competence," as the geologists call it.

A natural beach normally may become very broad. It remains in place because even storm waves waste much of their energy when they break and run up its gentle slope and return seaward without much ado. If a storm wave washes over the low natural dune line, the marsh behind gains some sand, shell and nutrients. The dune may recover. A steep barrier dune, held in place by cultivated plants or fences, takes much more of a big storm wave's force right on the chin and is breached more often. Furthermore, a steep dune line can contribute to a steeper underwater profile, which in turn creates steeper, stronger waves which have higher competence to erode the beach.

Man has learned to manipulate these forces to a degree. The beach householder's popular method of preserving his precious 100-foot strand is to build a groin, a low wooden wall extending the property line out to sea.

The water's prevailing "littoral drift" running parallel to every beach will drop sand above the barrier. This is because the current loses speed, competence and sand where it slows down, changing direction to get around the obstacle. But one man's groin is his neighbor's littoral loss. Where the current straightens out again it picks up speed - and sand.

This, on a larger scale, is what happened at the north end of Assateague Island. Forty years ago a hurricane carved an inlet where none had been before, right below present day Ocean City. Convenient for local fisherman, the cut was secured by a breakwater which kept the north-south littoral drift from filling it in again. This current, which is less than half a knot, dropped sand on the north side of the jetty as it slowed down to get around the obstacle. Over the decades it broadened the beach at the south end of Ocean City. But where the current returned to its normal course and speed, it picked up new sand, stealing it from Assateague's north end.

In time new inlets may be carved among the Delmarva islands, by the kind of event that historian David Stick says is responsible for many of the Outer Banks inlets (which are really outlets). It happens this way: a hurricane moves up the coast. As the tide rises winds from the east blow abnormal amounts of water into the bays behind the barrier islands. As soon as the hurricane's eye passes the winds suddenly blow from the west and the water breaks through the islands wherever it can.

The upshot is that waves constantly make, unmake and remake beaches and barrier islands with every neap tide and hurricane. The inevitably temporary result depends on purely local conditions. Early maps show that the Outer Banks had a major cape at their northern corner four centuries ago. The corner is now a gentle curve. And while the beach to the east of Hatteras Lighthouse has receded, the point to the south is growing. The beach recedes at North Ocean City and grows further south because of the same current that reshapes Assateague.