How the Glacier Built the Great Arm of Sand

Cape Cod is one of the youngest pieces of land in America, and it is made almost entirely of sand and gravel dropped by a glacier. There is no ancient bedrock under your feet here, the way there is across most of New England. There is only the debris of ice.

During the last ice age, the great Laurentide Ice Sheet crept down over New England and reached its farthest edge, marked roughly by Nantucket and Martha's Vineyard, about twenty-three thousand years ago. As the climate warmed and the ice melted back, it left behind long ridges of bulldozed rubble called moraines, among them the Sandwich Moraine and the Buzzards Bay Moraine that form the Cape's spine. Beyond those ridges, meltwater streams spread out broad plains of sand and gravel called outwash.

The land itself is only about sixteen to twenty thousand years old. Even the Cape's ponds are glacial. When the ice retreated, huge blocks of it were buried in the sand and slowly melted, leaving round depressions that filled with clear, cold water.

Those are the kettle ponds. And the Cape is still changing. The Atlantic is eroding the outer shore, carrying that sand north, which is why the tip at Provincetown is actually a growing spit built from land washed away farther south.

This whole flexed arm of sand, curling out into the sea, is a temporary sculpture. Given enough thousands of years, the ocean will take it back.