New England; similar glacial formations make up Long Island in New York and Block Island in Rhode Island. Together, these formations are known as the Outer Lands, or more obscurely as the "Isles of Stirling". Geologically speaking, Cape Cod is quite young, having been 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 per 1,000 years, but then the rate declined. On Cape Cod, sea level rose roughly per millennium between 6,000 and 2,000 years ago. After that, it continued to rise at about 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 through a process known as longshore drift. Those reworked sediments that moved north went to the tip of Cape Cod. The entire town of Provincetown, at the extreme tip of the Cape, is a spit consisting largely of deposited marine sediment that was eroded and transported from farther south along the shore. Those sediments that instead 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 in just the last 6,000 years. This process continues today. Due to their exposure to the open ocean, the Cape and islands are subject to considerable coastal erosion. Geologists