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3 A t t h e G l a c i e r ’ s E d g e Following a winter snowstorm, I set out to walk the beach. A bitterly cold wind burns my face. My boots crunch through a knee-high crust of ice that looks like a miniature glacier, reminding me of a not-so-distant time when much of the New York region was buried under ice. Twenty-two thousand years ago, in the deep freeze of the last ice age, the shoreline was seventy miles distant, and the place where I now stand was part of a vast outwash plain at the glacier’s edge. A massive river of ice ground over the land, carved out valleys, and plucked boulders from bedrock to scatter like calling cards. Moving like a conveyor belt, the ice sheet ferried thousands of tons of debris, which melted out at the glacier’s margin to form towering moraines. Torrents of meltwater dissected the plain, depositing layers of sand and gravel as outwash. Now, as I stand on the outwash plain, waves crashing on the beach, I can almost hear the thundering crack of an ice lobe as the glacier gives way and surges forward, crushing forests in its path. The glacial episode is a recent event—a mere microsecond on the geological timescale—yet our human existence may ultimately prove less durable than the glacier’s scratches on a boulder. During the Wisconsin ice age, which lasted 140,000 years, successive ice sheets flowed southward from Canada into the United States. Several of these reached New York, as ice lobes surged down the Connecticut and Hudson river valleys. Between twenty thousand and thirteen thousand years ago, the most recent glacier, known as the Woodfordian, completely mantled Manhattan Island and the Upper New York Bay, and covered the northern tip of Staten Island and parts of Queens and Brooklyn in Long Island. Sea level was 350 feet lower than today, and from Cape Cod southward, the coastal plain jutted out fifty to one hundred miles farther than the present-day shoreline , almost to the edge of the continental shelf. Draining the waters of the Connecticut, Housatonic, Passaic, and Hackensack rivers, the Hudson cut a deep gorge through the coastal plain and emptied into the waters of the Atlantic.1 Only the most tenacious species of plants and animals could tolerate the harsh conditions of the glacial plain. Dwarf and shrub willows, birches, sedges, grasses, and tundra herbs provided forage for grazing herds of woolly mammoth , musk ox, and caribou. Mastodons, elk, and white-tailed deer browsed in patches of pine forests and black spruce swamps, while moose and giant beavers frequented glacial lakes and bogs. These herbivores were the prey of such carnivores as the timber wolf, dire wolf, bobcat, cougar, lynx, and giant short-faced bear—and at least by twelve thousand years ago, the omnivorous human hunter.2 We are able to reconstruct the story of ice age New York because of the painstaking work of geologists, paleoecologists, and other scientists who attempt to piece together the puzzle presented by grooves and scratches on exposed bedrock, the erratic wanderings of rocks that do not belong here, and sediments piled on sediments sometimes in neat layer-cake fashion, but more often mixed and swirled like a marble cake. How to account for the changes from layer to layer, and the forces that seemed to move mountains and carve up landscapes? In 1828, when geology was still a young science, physician and amateur geologist L. D. Gale conducted a street-by-street survey of Manhattan’s geological features before the island would be completely altered by anticipated development. His diary became an invaluable reference for subsequent New York City geological surveys. The island’s topography was one of rolling hills and valleys, many of which would be cut down and filled in to lay out streets and grade sites for building. At that time, the population was confined mostly to the west side up to Fourteenth Street, but the uptown streets had already been laid out. Beginning at Fourteenth Street, Gale walked up Tenth Avenue , which was then close to the river shoreline. He noted drifts of gravel and sand yielding to boulders of “greenstone, granite, and sandstone.” Between Thirty-second and Thirty-seventh streets he described a hill of “some forty to fifty feet in height,” and between Thirty-seventh and Forty-second streets a valley containing “many huge granite...

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