This is not a level, stable, protected kind of beach. It is steep, full of long shoulders and curves, and fluctuates in outline not only as a result of storms but with each tide and even with every wave, making new bays, curves, shallow hills, and hollows; but the beach is an interbalanced system. All its materials come from offshore or the erosion of the cliffs. Wave action removes the cliff material, and currents moving parallel to the shore take it both north and south: there being a neutral point around Cahoon’s Hollow, halfway between Highland Light and South Wellfleet, although its location is dependent on the angle at which the waves come in along the shore. Half the cliff material moves north to build up the hood at Provincetown, and half moves south to be deposited along the sandpits from Nauset to Monomoy.
Then, after the ice had come and gone, swallowing the forests in its passage, the warming climate produced a southerly assortment of plants on the land surface. In vivid contrast to the trees of the peat, there now came sycamores, chestnuts, magnolias, sour gums, cedars and poplars, threading their roots through the till and the remains of their buried predecessors. Such southern species were to survive only as long as the interstadial period lasted, and were themselves to fall with the next ice advance.
Meanwhile, streams developed channels in the tree-shaded landscape. They carved down through the till and carried large quantities of it to the sea. The wave-beaten shores of the mainland also yielded their loose glacial debris. As the sea ate into the land’s soft margins, it sorted the materials of the drift. Large glacial boulders, too heavy for the water to move far, remained near the shore. Most of the friable peat quickly became dust. The rest of the debris was caught up in the turbulent eddies and currents of the shore zone. Drawn into the deeper water, it was segregated, by varying weights, into zones of gravel, sand, silt, and clay. The tiny size of pollen grains linked their fate to that of the equally small grains of clay, and together these drifted to the quiet water where they settled down. A thick bed of clay formed.
--- from These Fragile Outposts:A Geological Look at Cape Cod, Marthas Vineyard, and Nantucket by Barbara Blau Chamberlin.