|Ice Age: New York|
|New York City marks the southernmost boundary of an ice sheet that mantled much of North America. The Laurentide ice sheet advanced and retreated over a period of 60,000 years. The last advance reached maximum 22,000 years ago, extending from southeastern Alberta across what are now the Great Lakes to the east coast. Two lobes surged down the Connecticut and Hudson river valleys, sculpting the land and depositing the moraines and outwash that would become Long Island. Sea water was locked up in ice, lowering the sea level by 350 feet compared to today; from Cape Cod southward, the coastal plain jutted out 50 to 100 miles further than the present-day shoreline, almost to the edge of the continental shelf. The Hudson River sluiced through a deep gorge that cut through the plain and emptied into the Atlantic.
Glaciers literally created Long Island, and carved out the landscape we know today as the New York City region. Moraines, lakes and ponds, kettle holes, peat bogs, meltwater streams and valleys – all are relics of glacial topography. Between the receding ice front and terminal moraines, glacial meltwater formed huge lakes. By 12,000 years ago, the rising sea breached the morainal dams at several points, including the Verrazano Narrows. Hudson Lake released its waters to flow into the present-day channel of the Hudson River – a channel that had been established 120 million years earlier, when the continents split apart and the Palisades were exposed. The channel carved by the glacial Hudson River was drowned and is now an underwater canyon known to fisherman for its rich abundance of deep-water fish like tuna. As the sea continued to encroach, reaching its present level by a mere 6,000 years ago, the archipelago of New York City emerged.
The end of the last ice age was a time of extreme and rapid climatic change. Global warming accelerated 10,000 years ago, triggering rapid changes in plant and animal life. The changeover from spruce to pine forest in our region was possibly witnessed by Paleo-Indian hunters in a single lifetime. As pines thickly colonized the region, tundra disappeared – and with it the mammals that grazed on it. In our region, mastodon bones have been unearthed in peat deposits of the Harlem River and 22 feet below ground at Broadway and Dyckman Street in upper Manhattan; mastodon and mammoth teeth have been trawled by fishermen from undersea banks off the Atlantic coast, indicating these creatures roamed an outwash plain of mixed tundra (favored by mammoths) and black spruce swamps (browsed by mastodons).
Where to see glacial terrain
One of the best places to experience the glacial terrain of New York City is Long Island. The necks and incised bays that form Long Island’s North Shore were sculpted by the receding glacier into morainal hills and meltwater drainage valleys. Long Island Sound is itself a drowned valley carved out by earlier glaciers, and filled in its deeper portions with several proglacial lakes that formed between the melting ice front and terminal moraine – until breached by the rising sea. As the ice front retreated and stopped, retreated and stopped, it left behind a series of recessional moraines, including the Sands Point, Oyster Bay, Northport, Stony Brook and Mt. Sinai moraines. Moraines were cut through by meltwater channels such as the Connectquot and Nissequogue rivers that were originally drainage channels for the melting glacier. Meltwater sluiced sediments into lake deltas, and piled up conical hills of sand and gravel called kames. The Ronkonkoma Moraine is a kame moraine. Where blocks of ice were stranded by the receding glacier and sediments deposited around them, boggy depressions or kettle ponds were left when the ice melted. Success Lake and Lake Ronkokoma are kettle lakes. On the South Shore, a remnant of a glacial meltwater channel is Valley Stream, which winds southward from New Hyde Park through Franklin Square and into Valley Stream Park, flowing underneath Sunrise Highway and emerging as a tidal stream that joins the Hook Creek estuary. Moraine ridges can be walked at Forest Park, Queens, where the highest elevation is 180 feet, and at Prospect Park, Brooklyn, where the elevation rises to 188 feet.
Several good exposures of glacial till on western Long Island’s North Shore are open to the public. The beach bluffs at Garvies Point Museum and Preserve reveal a clay-and-sand Cretaceous layer the glacier had excavated from the valley of Long Island Sound, overlain by a thick layer of pebbly outwash, capped by reddish till. The 126-foot-high bluffs of Caumsett State Park at Lloyds Point offer beautiful exposures of pink-and-white sands and red clays of Cretaceous age. The low eroding cliffs of Target Rock National Wildlife Refuge expose Cretaceous sands beneath ascending layers of reddish brown till, yellowish outwash, brownish till, and loess (windblown deposits); in the higher bluffs one can see rippled layers of clay and fine sand deposited at the bottom of a lake that had formed between the receding glacier and its moraine.
North Shore beaches were once littered with erratics – boulders ferried by the glacier and left behind as calling cards. Unfortunately, many of these ice age relics were removed when residents wanted a nice sand beach. But there are several places preserved from development and privatization, such as Target Rock National Refuge and Garvies Point Preserve, where they may be found. At Garvie’s Point, one can see crystalline basement rocks of gneiss the glacier excavated from the Adirondacks and basalt boulders quarried from the Palisades. Smaller erratics include rounded Cretaceous-age concretions known as “Indian paint pots,” presumably because the red clay enclosed in the iron oxide crusts was used by the early Americans for pigment. As the sea continues to encroach and erode the bluffs, more glacial debris is washed out.
Ice Age New York Reading List
Sirkin, Les. Western Long Island Geology. Watch Hill, Rhode Island: Book and Tackle Shop, 1996.
|Pleistocene glaciation. Map: USGS|
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