Pages 20-21: This satellite view clearly shows the flexed arm shape of the Cape.
Overwashes of sand become readily apparent from the air over North Beach off Chatham.
A Sliver of Sand
From the air the great curled arm of Cape Cod looks like a mere film of sand, a whimsical momentary shape floating on the vast ocean around it. Its flimsy fabric appears torn and rent by hundreds of holes, large and small, and dozens of slits at the edges, where the water shows through. So sheer and vulnerable does it appear that it seems as if the slightest push might sink it beneath the sea.
This somewhat fanciful impression nonetheless contains several grains of geologic truth, for water, in its various guises, permeates the Cape’s past, present, and future. Created by the frozen water of vast glaciers, shaped today by the water of tides, waves, currents, and storm surges, this prominent hook of land is destined to succumb at last to the steadily rising waters of the sea—all in the merest flick of geologic time.
Some 75,000 years ago, when the Earth’s climate entered a cooling period, the most recent of the vast continental ice sheets, known as the Laurentide, began to form across eastern Canada. As it spread and thickened, much of the oceans’ waters became trapped in its mass, lowering sea levels by several hundred feet. The visible bulk of Cape Cod is primarily the work of the Wisconsin Stage Glacier, a towering wall of ice 10,000 feet thick that moved south over New England some 25,000 years ago. Grinding forward in rounded fronts, or lobes, the ice sheared off the tops of mountains, gouged huge valleys through granite hills, and plowed up tons upon tons of rocks, material, and debris from what had been the floor of the sea.
Advancing, hesitating, and advancing again, the glacier moved as far south as Long Island, Block Island, Martha’s Vineyard, and Nantucket. Then, as the climate warmed, the front edges of the glacier began to melt; the ice sheet retreated north of the present outline of Cape Cod, dropping rocks and large chunks of ice as it went. Then, as cooler weather returned, it paused, and, as the flow of ice exceeded the rate of melting, the glacial lobes once more advanced. This time they pushed enormous amounts of drift material across the face of the Upper and Lower Cape, creating the Elizabeth Islands in Buzzards Bay and the rocky, morainal hills on the western and northern sides of the Upper Cape.
When the glacier melted, it left behind huge rocks, known as erratics, scattered on the landscape.