New York’s main landforms are the Adirondack Mountains in the northeast, crowned by Mount Marcy at 1,629 m (5,344 ft), the state’s highest point; the Catskills and the wide Allegheny Plateau across the south; the Hudson Highlands and the Taconic hills along the eastern edge; the Hudson and Mohawk valleys; the Great Lakes lowlands on Lake Erie and Lake Ontario, where Niagara Falls drops over the Niagara Escarpment; the eleven Finger Lakes; and the moraines and outwash plains of Long Island. The lowest point is sea level on the Atlantic. Those features were not made at once. Across the state’s 141,297 km² (54,555 sq mi), the rock records more than a billion years of collisions, seas, rifting and ice, and the easiest way to understand the map is to read it in order, oldest first.
Grenville roots: the billion-year-old core of the Adirondacks
The oldest story in New York is written in the Adirondacks. Their rocks began as sediments laid down roughly two billion years ago on the floor of a sea near the equator, about 15,000 m (50,000 ft) thick. During the Grenville orogeny, a long mountain-building episode dated to roughly 1,250 to 980 million years ago and tied to the assembly of the supercontinent Rodinia, those sediments were crushed against Laurentia, the ancestor of North America, and cooked into metamorphic rock.
That Grenville rock carries the iron ore once mined at Lyon Mountain, Mineville and Tahawus, and the garnet of the Barton Mine near Gore Mountain. Geologically, the Adirondacks are a continuation of the Canadian Shield, not part of the Appalachian chain that built New York’s other ranges, which is why they look and behave so differently from the Catskills to the south.
The Grenville mountains themselves were worn flat long ago. Around 600 million years ago, as Laurentia pulled away from Baltica and the Iapetus Ocean opened, faults running north to northeast cracked the old surface. Some of those down-dropped blocks, or grabens, hold water today: Lake George and Schroon Lake both sit in them. A shallow sea then flooded the eroded surface and laid down the Potsdam sandstone, which still preserves trilobite trackways.
The Taconic and Acadian collisions, 450 to 360 million years ago
New York’s eastern border records the next chapter. Beginning around 550 million years ago the Iapetus Ocean started to close. An offshore volcanic island arc, the Bronson Hill arc, drifted toward the continent and by the Late Ordovician had slammed into it. That Taconic orogeny, which ended about 440 million years ago, folded and faulted the sediments along the old margin and raised a mountain chain from eastern Canada down toward the Piedmont. The worn-down remnants are the Taconic Mountains, a 240 km (150 mi) sub-range of the Appalachians on the New York border with Massachusetts and Vermont; their highest summit, Mount Equinox at 1,170 m (3,840 ft), lies just over the line in Vermont.
A second collision, the Acadian orogeny, followed in the Devonian. It raised a new range roughly where the Taconics stand today, and much larger. Rivers carried their debris west into a shallow inland sea, and that debris became most of the southern half of the state.
The Hudson Highlands, which straddle the river mainly in Orange County on the west bank and Putnam County on the east, between Newburgh Bay and Haverstraw Bay, are far older than either collision. Their bedrock is part of the Reading Prong, more than a billion years old and formed in the Grenville orogeny. The highest summit, the south peak of Beacon Mountain, reaches only 486 m (1,595 ft), yet the hard rock squeezes the Hudson into a narrow gorge where the river reaches its deepest point, 66 m (216 ft), near Garrison.
The Catskill Delta and the Allegheny Plateau
Between about 395 and 325 million years ago, sediment eroded from the Acadian Mountains spread west in a vast wedge now called the Catskill Delta. Coarse gravel dropped first, near the mountains; finer sand and mud travelled farther, and farthest west the river deposits interfingered with marine sandstone, shale and, in deeper water, limestone.
The Catskill Mountains are therefore not a folded range at all but a dissected plateau: flat-lying delta rock later lifted and carved into peaks by streams. Their highest summit, Slide Mountain in Ulster County, is generally mapped at about 1,270 m (4,180 ft), making it the highest natural point in the New York metropolitan area. One peak has a stranger origin. Around 375 million years ago a meteorite struck the shallow Devonian sea and left a crater about 10 km (6 mi) across; it filled with sediment, and after uplift and erosion the harder fill now stands as Panther Mountain.
West of the Catskills the same rock forms the Allegheny Plateau, which underlies most of the southern part of the state; the section along the Pennsylvania border is known as the Southern Tier, a country of broad, rolling hills cut by deep stream valleys. The Shawangunk Ridge along the Catskills’ southeastern edge is a different animal: a hard, tilted ridge of the Ridge-and-Valley province, the same ridge called Kittatinny Mountain in New Jersey and Blue Mountain in Pennsylvania. Beneath much of the plateau lies the Marcellus Shale, a gas-bearing formation that extends into Pennsylvania and Ohio.
Rifting and the Palisades Sill, 200 million years ago
When the supercontinent Pangaea began to break apart at the end of the Triassic, the crust of the Northeast stretched and cracked into rift basins. The Newark Basin, mostly in New Jersey, is one of them, and molten rock rising into its sandstone about 200 million years ago cooled underground into a thick sheet of diabase. Erosion of the softer sandstone around it has left that sheet standing as the Palisades, the columnar cliffs on the west bank of the lower Hudson. They run about 32 km (20 mi) north from Jersey City to near Nyack, New York, rising from roughly 90 m (300 ft) at Weehawken to about 160 m (540 ft) at their northern end, and they form a canyon wall opposite Manhattan north of the George Washington Bridge.
After the rifting, New York settled into a long quiet phase of uplift and erosion. The Adirondack dome is the exception. About 10 million years ago the region began to rise again, and it has been lifted about 2,000 m (7,000 ft) since, currently at around 2 mm a year, faster than erosion can wear it down. The cause is still debated; one explanation is a column of hot, seismically slow material 50 to 80 km beneath the massif.
The Laurentide ice sheet and the Finger Lakes
About 2.5 million years ago the ice ages began, and the Laurentide Ice Sheet advanced over New York repeatedly. During the last advance, ice up to about 3,000 m (10,000 ft) thick buried the Adirondacks, and after the climate warmed it took nearly 10,000 years to melt away completely. In the Adirondacks it left eskers, winding ridges of gravel dropped by rivers flowing under the ice; the Rainbow Lake esker runs discontinuously for about 137 km (85 mi). High on the slopes of Mount Marcy sits Lake Tear of the Clouds, the pond conventionally cited as the source of the Hudson.
The Finger Lakes are the showpiece. Before the ice, streams flowed north across the plateau toward the Ontario lowland. Glacial ice and meltwater under great pressure deepened and widened those valleys into troughs, and moraine left by the retreating ice dammed their southern ends. Seneca Lake is 61 km (38 mi) long and reaches 188 m (618 ft) deep; Cayuga reaches 133 m (435 ft). Both floors lie well below sea level, yet neither lake is more than 5.6 km (3.5 mi) wide. The ice cut the main valleys so deeply that side streams were left hanging as much as 120 m (390 ft) above the valley floors, which is why gorges such as Watkins Glen and Taughannock around the lakes are strung with waterfalls. The lakes became ice-free about 14,400 years ago.
At the other end of the state, the ice built rather than carved. Long Island is essentially two moraines, gravel and boulders dumped at the ice front, with a sandy outwash plain spread south of them. The northern Harbor Hill moraine hugs the North Shore; the Ronkonkoma moraine forms the island’s spine, roughly along the Long Island Expressway. Jayne’s Hill, at 122 m (401 ft), is the highest point on either. Kettle lakes such as Lake Ronkonkoma mark where buried ice blocks melted.
After the ice: Lake Iroquois, the Hudson fjord and Niagara
As the ice sheet melted back, it blocked the St. Lawrence outlet near the Thousand Islands, and the Ontario basin filled with Glacial Lake Iroquois, about 30 m (100 ft) higher than Lake Ontario is now. Around 13,000 years ago it drained southeast through a channel near Rome and down the Mohawk Valley to the Hudson, the same corridor the Erie Canal would follow. When the ice dam finally failed, the lake dropped suddenly, and the resulting flood is thought to have cut the Narrows between Staten Island and Brooklyn through a moraine that had joined them.
The Hudson itself is a drowned river. Rising sea level after the ice age flooded its lower valley, so the lower half of its 507 km (315 mi) course is a tidal estuary occupying the glacially carved Hudson Fjord, deeper than the harbour it flows into. The tide reverses the current as far north as Troy, and the river’s old channel continues across the continental shelf as the Hudson Canyon. The wider geography of New York still follows this glacial plumbing, with cities strung along the Hudson and Mohawk.
Niagara Falls is one of the youngest major landforms in the state. When the upper Great Lakes began draining through the Niagara River, the water spilled over the Niagara Escarpment, a north-facing cuesta capped by hard Lockport dolomite of Silurian age, which runs about 1,050 km (650 mi) in an arc from New York through Ontario, Michigan and Wisconsin. Beneath the caprock lies softer Rochester shale; the river undercut it, the dolomite collapsed in blocks, and the falls moved upstream. About 10,900 years ago they stood between Lewiston and Queenston, where the river crosses the escarpment. Since then they have retreated about 10.9 km (6.8 mi) south, leaving the Niagara Gorge behind them. The geography of Niagara Falls covers how engineering has slowed that retreat.
The oldest and youngest chapters meet in the Adirondacks, where billion-year-old Grenville rock is still being pushed upward while glacial boulders sit on its slopes. For how ice sheets such as the Laurentide grow and move in the first place, see how glaciers are formed.