During the previous ice age, which began 2.6 million years ago, a gargantuan sheet of ice formed over North America, whose southern edge extended as far as what’s now New York, ending in a sharp cliff.
Over Manhattan, this layer was more than 2,000 feet thick, taller than One World Trade. Then, some 18,000 years ago, when the planet began to thaw, it began to melt and retreat.
But the rocky junk that it’d brought along stayed behind, forming a line of rubble in the shape of an intermittent ridge called “moraine,” which runs all the way from Puget Sound to Montauk Point on Long Island, forming the promontory on which the old lighthouse stands.
In the five boroughs, it’s marked by a series of hillocks, which, at its maximum height, is about 200 feet. Many neighborhoods take their names from these elevations as well as its leafy embellishments: Cobble Hill, Park Slope, Bay Ridge, Cypress Hills, etc.
The ridge once connected Staten Island and Brooklyn. Some 13,000 years ago, floodwaters from melting glaciers upstate, rushed down the valley of the Hudson River, smashing it, creating the Narrows.