NYNew York Geography

The Adirondacks are not part of the Appalachians

Most maps shade New York's northern uplands the same colour as the ridges of Pennsylvania and Virginia, and the shading is misleading. The Adirondacks are a dome of anorthosite and related Grenville-age rock roughly a billion years old, lifted out of the same basement that forms the Canadian Shield and joined to it by the Frontenac Arch beneath the Thousand Islands. The Appalachians are folded and thrust sedimentary rock a few hundred million years younger. The two ranges share a latitude and nothing else.

The dome is also still rising, on the order of a millimetre or two a year, which is faster than erosion is taking it down. Mount Marcy, 5,344 feet, is the summit, and the Hudson River begins on its shoulder at Lake Tear of the Clouds, about 4,300 feet up. The mountains sit inside Adirondack Park, six million acres, a boundary that encloses more ground than Yellowstone, Yosemite, Grand Canyon, Glacier and Great Smoky Mountains combined. Roughly half of it is state Forest Preserve, which Article XIV of the state constitution has kept "forever wild" since 1894, and the rest is private land with working towns and farms on it, a mixed arrangement no national park uses.

A river that flows both ways

The Hudson is tidal for 153 miles, from the Battery all the way to the federal dam at Troy. Twice a day the flood tide pushes water upstream past Poughkeepsie, and in dry summers the salt front reaches Newburgh, sixty miles from the ocean. The Lenape name for it, Muhheakantuck, is usually translated as the river that flows both ways, which is a description rather than a metaphor.

The valley does not end at the harbour. The Hudson Canyon is a submarine gorge cut across the continental shelf when sea level stood some 400 feet lower during the last glaciation. It runs about 400 miles seaward, reaching depths of more than two miles, and it is the largest submarine canyon on the Atlantic coast of the United States.

Everything the ice sheet left behind

Long Island has no bedrock at the surface worth speaking of. It is a pile of debris: two parallel terminal moraines, the Ronkonkoma and the Harbor Hill, marking places where the retreating Laurentide ice sheet stalled, with outwash sand sloping south from them to the ocean. At 118 miles it is the longest island in the contiguous United States, and the glacial sand beneath it is the sole source aquifer for Nassau and Suffolk counties, which is why groundwater contamination there is a regional emergency rather than a local one.

Upstate the same ice deepened a set of old north-flowing river valleys into the Finger Lakes. Seneca is 618 feet deep and Cayuga 435, and both have bottoms well below sea level, meaning the ice cut holes deeper than the ocean surface. At Ithaca, Taughannock Falls drops 215 feet in a single plunge, taller than Niagara and the highest single-drop waterfall east of the Rockies.

Niagara itself pours over a Silurian dolostone cuesta, a hard cap over soft shale that undercuts and collapses. The falls began at Lewiston about 12,000 years ago and have retreated seven miles upstream since, carving the Niagara Gorge on the way. Diversion of water for hydropower has slowed the retreat to a fraction of its natural rate.

Five separate ways out of the state

New York's water leaves by an unusual number of exits. The Hudson runs directly to New York Harbor. The Great Lakes and St. Lawrence system carries away everything along Lakes Erie and Ontario and everything draining north from the Adirondacks. The Delaware rises in the Catskills, forms part of the Pennsylvania boundary, and reaches Delaware Bay. The Susquehanna starts at Otsego Lake in Cooperstown, loops through the Southern Tier, and ends in Chesapeake Bay. And in the far southwest, Cattaraugus County drains into the Allegheny, which becomes the Ohio and then the Mississippi, so one corner of New York empties into the Gulf of Mexico.

The Oblong, and a border with no land on it

The eastern boundary carries a scar from a 1731 settlement. Connecticut wanted to keep the towns of its southwestern panhandle, which lay west of where its line should have fallen, so it handed New York a compensating strip 1.81 miles wide running north from Long Island Sound, about 61,000 acres in total. The strip is called the Oblong, and the New York and Connecticut line still steps sideways where it starts and stops.

New York also shares a boundary with Rhode Island that touches no land at all, running through Block Island Sound by interstate compact. That gives the state six neighbours but only five you can drive to. In the harbour, the New Jersey line cuts through Ellis Island, and since a 1998 Supreme Court ruling the filled portion of the island, which is most of it, has legally been New Jersey.

Tug Hill, and the coldest night on record

Wind crossing the open water of Lake Ontario picks up moisture and then runs into the Tug Hill Plateau, which lifts about 1,800 feet just inland from the eastern shore. The lift squeezes the moisture out as snow, and Tug Hill averages more than 20 feet a year, the heaviest lake effect snowfall anywhere east of the Rockies. Buffalo gets the same treatment from Lake Erie with one difference: Erie is shallow enough to freeze over, which shuts the snow machine off partway through a cold winter.

The state record low is,52 °F, set at Old Forge on 18 February 1979, in an Adirondack hollow that pools cold air on clear nights. The record high is 108 °F, at Troy on 22 July 1926. Both extremes came from valleys rather than summits, which is the usual pattern in terrain that traps air.

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