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 ivers on the East Coast once gently flowed into the Atlantic, their mouths adjusted to a sea level that had been stable for millions of years. But during the Pleistocene Epoch, 1.6 million to 10,000 years ago, 16 million cubic miles of water were locked on land within glaciers, and oceans were 400 feet lower than present levels. The Atlantic shoreline lay 60 miles east of New York City rather than lapping at the feet of Coney Island as it does today. Sea level was so low that rivers were forced to cut deep valleys as they approached the ocean. When the ice finally melted, sea level shot up by almost half an inch a year. Squint at a map of the modern East Coast and you’ll see the ghost of those now-drowned valleys in the outlines of Chesapeake and Delaware bays.
The northeastern seaboard was totally transformed both by the arrival and the departure of the Pleistocene ice ages. Nothing, absolutely nothing, can haul heavy loads like a glacier. Glaciers pluck mountains apart, scrape valleys open, and transport entire landscapes rock by rock to new locations tens or even hundreds of miles away. Back in the Pleistocene, ice sheets spread out from northern Canada, extending from southern Alaska to coastal New England. Other sheets were centered over England and affected Europe. At the peak of the most recent glacial stage called the Wisconsinan, 17 million square miles of the world’s surface lay buried beneath ice.
Glaciers overrode portions of New England, including Martha’s Vineyard and Nantucket Island, and then migrated as far south as Long Island. En route, the glaciers scooped up sand and gravel that would eventually be deposited as a vast gentle slope at the farthest extent of the ice. Bedrock was covered locally to depths of 600 feet, its surface pocked here and there with isolated blocks of ice. With time, the blocks melted and left behind depressions called kettles that now dot Cape Cod.
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