PLEISTOCENE EPOCH

The Pleistocene epoch is often informally called the Ice Age, a two-million-year-long interval that features four main intervals of global cold and glacial ice advance in North America with intervening warm (or interglacial) times. For the last 10,000 years, Earth has been in an interglacial interval. The sculpting of the state’s landscape by continental glaciations during this epoch complicated research on the older rocks by burying much of the state under late and postglacial deposits. All of the state, with exceptions of the Salamanca reentrant in the Allegany State Park area, was covered by the last advance of continental glaciers. The Salamanca reentrant was a notch in the roughly east-west-trending south margin of the continental ice cap. It was bounded by two ice tongues that did not overrun the relatively high Allegany State Park region. Scours and scratched made by ice-transported rock fragments on the highest Adirondack peaks record the thickness of the ice cap. Depression of the northern Lake Champlain and St. Lawrence Valley by about 540 feet (165m) under the weight of the ice further demonstrates its thickness. With so much water trapped in ice on land, sea levels were about 330 feet (100m) lower. The remains of many of the first humans and animals to return to the New York State region are probably submerged along the old Ice Age shoreline well south of Long Island.

Most of the major highlands, rivers, and lowlands of New York State existed prior to the Ice Age and were only modified by glaciation and with the melting of the ice cap. The Finger Lakes were river valleys that flowed south but were scoured out to great depths by glacial ice and then had their north ends dammed with sediment derived from the melting of the ice sheet. The Mohawk, and Hudson are old rivers that also existed before glaciaiton. The soft Triassic and Jurassic rocks of the lower Hudson below Peekskill were deeply eroded almost 1,000 feet (300 m) below sea level by a tongue of glacial ice. This wide area of the Hudson is the modern Tappan Zee. With the melting of the ice sheets, huge amounts of water from the Great Lakes drained through the Niagara River and began erosion of Niagara Gorge and Niagara Falls. This meltwater filled Lake Iroquois (the ancestor of Lake Ontario), which overflowed near Rome (Oneida Co.) into the Mohawk River. This enormous amount of water cut the deep water gaps, such as the Noses, in the Mohawk Valley and formed Cohoes Falls on the Albany-Saratoga county line at the confluence with the Hudson River.

Further downstream, the raging meltwaters of the Hudson River deeply eroded the softer Upper Ordovician sandstones and shales just east of the hard rocks of the Taconic overthrust and laid down a thick blanket of sediment in the Tappan Zee. With low sea level just at the end of the Ice Age, the sand- and mud-charged waters of the Hudson River cut a deep valley across the exposed continental shelf. The Hudson River discharged sediment into the deep waters of the Atlantic through the Hudson canyon on the continental slope. With the final melting of the ice sheets, sea level rose to fill the Tappan Zee and the old river lowland that is now Long Island Sound.

 

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