At the start of the last ice age, 2.6 million years ago, a sheet of frozen water formed atop North America that kept expanding and thickening until it reached a maximum depth of roughly two miles.

At its southern edge, the vast body deposited tons of rocky debris — from sand and pebbles to boulders the size of school buses. Then, some 18,000 years ago, the planet began to warm and the gargantuan sheet of ice began to melt and retreat.

Today, the southernmost edge of that frozen expanse is marked by a line of rubble that extends across the northern United States for thousands of miles. The largest deposits form what geologists call a terminal moraine.

The intermittent ridge runs from Puget Sound to the Missouri River to Montauk Point on Long Island, forming the prominence that supports its old lighthouse. The ancient sheet of ice also left its mark on a very modern phenomenon: New York City.