A Glacial Base
The Wisconsin Stage Glacier reached into southern New England 25,000 years ago and melted away more than 17,000 years ago, leaving behind moraine and outwash deposits that became the base of Cape Cod, Martha’s Vineyard, Nantucket, and the Elizabeth Islands (see illustrations at right). But this was only the latest in a series of Pleistocene glaciers that covered Cape Cod during the past 1.5 million years. Even the deposits of these earlier glaciers may rest on a much older and more stable land form, a wide, seaward-sloping surface called a coastal plain or, if submerged, a continental shelf. The plain and shelf are underlain by sediments. The shelf was exposed during the ice ages by a concomitant drop in sea level of some 400-500 feet, but today it lies drowned under rising waters. Even more deeply buried, some 500-800 feet beneath the surface, lies the ancient granite bedrock that is so characteristic of most other New England areas. There is evidence that the post-glacial landscape looked very much like arctic tundra. Deep in the bottom of Truro’s Great Pond, twigs of arctic willow have been retrieved with 11,000-year-old scale insects still attached to the bark. Caribou, arctic fox, and perhaps musk ox, roamed lichen-covered plains where white-tailed deer and red fox live today. Gradually a boreal forest of spruce and fir took root, much like the forests that now cover northern Canada.
Outwash Plains and Kettles
As the Buzzards Bay, Cape Cod Bay, and, perhaps, the South Channel glacial lobes waned, they left behind moraines (above right). Meltwater streams carried sand and gravel that then formed outwash plains beyond the ice (above and below). Sometimes these materials buried large blocks of ice. In time, the ice blocks melted, leaving steep-sided depressions called kettles that became freshwater ponds if they were deep enough to intersect the water table. Some of these ponds became connected to the sea and turned into salt ponds, such as the one near the Seashore visitor center in Eastham. Over the years peat has filled in many of the ponds.