This paper examines the geological history of Long Island, New York, tracing the island's formation from its pre-Mesozoic bedrock and Cretaceous sediment deposits through the dramatic transformations caused by the Wisconsin stage of the Pleistocene glaciation. It discusses the two terminal moraines — the Ronkonkoma and the Harbor Hill — that mark the southernmost extent of glacial advance, the kettle lakes left behind as the ice retreated, and the coastal changes produced by rising sea levels after the last Ice Age. The paper concludes by noting that tidal and current action continues to reshape Long Island's shoreline today.
Long Island runs along the eastern coast of the United States, from New York City in the southwest to the northeast, parallel to the state of Connecticut. At its longest and widest points, it is almost 120 miles long and just under 25 miles wide, and contains two boroughs of New York City — Queens and Brooklyn — as well as two larger counties, Nassau and Suffolk, which are largely suburban. The island is surrounded by the Atlantic Ocean on the southeast and Long Island Sound to the northwest, but is connected to Manhattan by a number of bridges and tunnels.
Long Island is believed to consist of a layer of bedrock made up of "metamorphic and igneous rocks of the pre-Mesozoic age" (Lewis, 1987) and marked the southernmost point of glaciation during the last Ice Age. Because of the effects of glaciation during that period, the geology of Long Island was profoundly transformed. It is therefore illuminating to examine the geology of Long Island before, during, and after the most recent period of glaciation.
Long Island rests on bedrock made up of metamorphic and igneous rocks, but has been covered by deposits of sands and clays from the Cretaceous period, approximately 70 million years ago ("Geology of Long Island"). This was the geological condition of Long Island prior to the glaciers moving into the area during the Wisconsin stage of the Pleistocene Epoch. The Wisconsin stage "started in Canada about 85,000 years ago, reached Connecticut about 26,000 years ago, and began to wane on Long Island about 21,000 years ago" ("Geologic History").
The ice sheet extended approximately halfway down Long Island, and its southernmost point is marked by the accumulation of rocks and other debris carried by the glacier — a feature known as a terminal moraine.
There are two terminal moraines on Long Island: the Ronkonkoma and the Harbor Hill. Each marks the southernmost point of glaciation during two distinct sub-stages of the Wisconsin glaciation. These moraines currently represent the highest points of elevation on Long Island, standing as lasting physical evidence of the glacier's reach.
"Kettle lakes formed from melting glacial ice chunks"
"Meltwater and wave action reshape Long Island shores"
Originally formed from pre-Mesozoic rock sediments deposited during the Cretaceous period, Long Island underwent significant changes as a result of the most recent Ice Age. The Wisconsin stage of the Pleistocene left glaciers across the middle of Long Island, forming the two moraines which currently exist. As the glaciers receded, chunks of melting ice left behind formed a number of kettle lakes, while rising sea levels inundated the shoreline and caused dramatic changes to the shape of Long Island. Because of its location along the Atlantic coast, tides and currents continue to shape and reshape the geology of Long Island to this day.
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