Geology
Cape Cod is part of a continuous archipelagic region consisting of a thin line of islands stretching toward New York, historically known by naturalists as the Outer Lands. This continuity is because the islands and Cape are all terminal glacial moraines laid down some 16,000 to 20,000 years ago.
Most of Cape Cod's geological history involves the advance and retreat of the Laurentide ice sheet in the late Pleistocene geological era and the subsequent changes in sea level. Using radiocarbon dating techniques, researchers have determined that around 23,000 years ago, the ice sheet reached its maximum southward advance over North America, and then started to retreat. Many kettle ponds – clear, cold lakes – were formed and remain on Cape Cod as a result of the receding glacier. By about 18,000 years ago, the ice sheet had retreated past Cape Cod. By roughly 15,000 years ago, it had retreated past southern New England. When so much of Earth's water was locked up in massive ice sheets, the sea level was lower. Truro's bayside beaches used to be a petrified forest, before it became a beach.
As the ice began to melt, the sea began to rise. Initially, sea level rose quickly, about 15 metres (49 ft) per 1,000 years, but then the rate declined. On Cape Cod, sea level rose roughly 3 metres (9.8 ft) per millennium between 6,000 and 2,000 years ago. After that, it continued to rise at about 1 metre (3.3 ft) per millennium. By 6,000 years ago, the sea level was high enough to start eroding the glacial deposits that the vanished continental ice sheet had left on Cape Cod. The water transported the eroded deposits north and south along the outer Cape's shoreline. Those reworked sediments that moved north went to the tip of Cape Cod.
Provincetown Spit, at the northern end of the Cape, consists largely of marine deposits, transported from farther up the shore. Sediments that moved south created the islands and shoals of Monomoy. So while other parts of the Cape have dwindled from the action of the waves, these parts of the Cape have grown through the deposition of sediment.
This process continues today. Due to their position jutting out into the Atlantic Ocean, the Cape and islands are subject to massive coastal erosion. Geologists say that, due to erosion, the Cape will be completely submerged by the sea in thousands of years. This erosion causes the washout of beaches and the destruction of the barrier islands; for example, the ocean broke through the barrier island at Chatham during Hurricane Bob in 1991, allowing waves and storm surges to hit the coast with no obstruction. Consequently, the sediment and sand from the beaches is being washed away and deposited elsewhere. While this destroys land in some places, it creates land elsewhere, most noticeably in marshes where sediment is deposited by waters running through them.
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