Cape Cod, a sandy peninsula formed mostly during the Ice Age, sticks out into the Atlantic, looking from above like a bodybuilder's flexed arm. Cape Cod is particularly interesting, geologically speaking, because it was formed by glaciers very recently in terms of geologic time. The geologic history of Cape Cod mostly involves the last advance and retreat of glacial ice in southern New England and the rise in sea level that followed the melting of the ice. These events occurred within the last 25,000 years. Sometime between 18,000 and 23,000 years ago, the Wisconsin ice sheet (large glacier that completely covers the terrain) that had been moving southeast across all of New England reached its maximum advance. About 18,000 years ago, the glacial ice started to recede rapidly northward by melting; within about 4,000 years, the ice sheet front had retreated to just north of Boston.
|How leachate from a solid waste disposal site contaminates a well. From Environmental Impact of Ground Water Use on Cape Cod. (Strahler, 1972).
As it retreated, the glacier deposited rock debris, called drift. On Cape Cod, drift overlies a surface of much older rock. This older rock is buried by drift 200-600 feet thick. Most of the drift on Cape Cod has been fashioned into either moraines or outwash plains. Both features mark the various positions of the front of the glacier as it moved. Moraines are ridges of rock debris formed by moving ice. In a moraine, rock fragments carried by the ice are piled up along the ice front. Moraines may also form when the ice front advances and bulldozes the sand and gravel of an outwash plain into a ridge. The moraines on Cape Cod were formed by a combination of these processes. Outwash plains make up most of the landscape of Cape Cod. They were built by meltwater streams flowing from the glacier margin that deposited sand and gravel to form a broad, flat, porous plain.
Cape Cod's landscape is defined by the glacier's deposition of loose material. These porous, sandy soils are highly absorbent. Such soils have a profound effect on the quality of underground water. Sandy soils make the underground water supply vulnerable to contamination - toxic substances on the surface can travel through the soil quickly and can move great distances underground.