Quick answer: Long Island is a pile of debris left by glaciers. The Ronkonkoma moraine (the island’s spine and South Fork) and the Harbor Hill moraine (the North Shore and North Fork) mark where the ice sheet paused roughly 20,000 years ago, and the sandy South Shore is meltwater outwash. Nearly every beach stone was carried south from New England and upstate New York.
Long Island has no mountains and almost no exposed bedrock. What it does have are two long ridges, a flat southern plain, beaches full of rounded stones, and scattered boulders the size of cars or even houses. All of it was delivered by ice.
An island made of glacial debris
During the last ice age, a vast ice sheet pushed south out of Canada and New England. Its southern edge stalled over what is now Long Island around 20,000 years ago. A glacier acts like a conveyor belt: ice keeps flowing forward, carrying ground-up rock, gravel, and boulders, and dumps it where the ice melts at its front edge.
That dumped material formed two ridges called terminal moraines:
- The Ronkonkoma Moraine, the older and more southerly ridge, running through the middle of the island and out along the South Fork.
- The Harbor Hill Moraine, left slightly later as the ice paused during its retreat, running along the North Shore and out the North Fork.
The ridges give Long Island its two eastern “forks.”
The outwash plain
Meltwater streams poured south from the ice, carrying sand and gravel and spreading it in a broad, gently sloping sheet. That is the outwash plain that makes up much of the South Shore, and it is why towns there sit on flat, sandy ground. Much of the island’s groundwater is stored in these porous sands.
Glacial erratics
An erratic is a boulder carried by ice from its home bedrock and dropped somewhere with completely different rock. Long Island is full of them. Shelter Rock in Manhasset is one of the largest and best known, a single boulder estimated at well over a thousand tons. Others show up in parks, on beaches, and in backyards across the island. Many are granite and gneiss from Connecticut and New England, carried across what is now Long Island Sound.
What the beach stones are
Pick up a handful of Long Island beach stones and you are holding a sample of the Northeast:
- Milky and clear quartz, the toughest survivor, from veins in New England bedrock.
- Granite and gneiss, speckled with pink or white feldspar, black mica, and quartz.
- Red sandstone and dark basalt-like rock, especially toward the western end, carried from the Newark Basin and Palisades area of New Jersey and New York. See the Watchung Mountains for that geology.
- Occasional jasper and other microcrystalline quartz, sorted in agate, jasper, and carnelian.
And the petrified wood?
Long Island also turns up petrified wood, which has its own story. We tell it in how petrified wood ended up on Long Island, and the broader process in petrified wood: a tree that became a rock. For collecting trips farther afield, see rockhounding the Northeast.
Long Island’s two moraines, explained
A moraine is a ridge of rock, gravel, sand, and clay dumped at the edge of a glacier. Long Island has two big ones, and together they are its backbone:
- The Ronkonkoma moraine runs through the middle of the island and out along the South Fork to Montauk. It marks an earlier, farther-south position of the ice front.
- The Harbor Hill moraine runs along the North Shore and out the North Fork to Orient Point. It formed later, when the ice front paused again a little farther north.
That is why the island’s east end splits into two forks: each fork is a different moraine.
What is the outwash plain?
South of the Ronkonkoma moraine, meltwater streams pouring off the glacier spread out sheets of sand and gravel, sorted by flowing water. That gently sloping, sandy apron is the outwash plain, and much of the South Shore sits on it. It is also why South Shore soils are sandy and well drained, while the moraines are lumpier and full of boulders.
When did the glaciers leave Long Island?
The ice sheet of the last glacial period reached its southern limit here roughly 20,000 to 25,000 years ago and then melted back north. Long Island has essentially been shaped by that ice and its meltwater ever since, with waves and wind reshaping the shoreline.
Where did Long Island’s rocks come from?
Almost none of the stones on Long Island formed here. Granite, gneiss, schist, basalt, and sandstone pebbles on its beaches were carried south by ice from Connecticut, New York, and New England. Large boulders left behind are called glacial erratics. You can even find pieces of New Jersey’s Palisades basalt on western Long Island. For a local example of what washes out of this glacial mix, see our Petrified Wood Specimen — Long Island, New York.
Keep reading: Can you find gemstones on Long Island beaches? Garnet sand, magnetite sand, and Montauk’s faintly radioactive black sand.
