Erosion at Montauk Point bluffs reveals a cross section of...

Erosion at Montauk Point bluffs reveals a cross section of the glacial till, reflecting layers of earth, sand and clay deposited by a glacier. Credit: Newsday / Bill Davis

A half-billion years ago, it was a chain of volcano islands adrift in a tropical sea. Over untold millennia, it took new forms, like a lump of clay forever being reshaped.

Three hundred million years ago, the place that would become Long Island was a dinosaur swampland at the edge of a towering mountain range cast up by the slow-motion collision of continents. And just 20,000 years ago, it was a wasteland of woolly mammoths and iceberg lakes in the menacing shadow of a retreating glacier as tall as a skyscraper and as wide as a continent.

But not until 11,000 years ago — an eyeblink in the 4.6-billion-year life of Planet Earth — did the rising sea finally encircle a fish-shaped pile of sand pushed together by the newly departed ice sheet.

In geological terms, Long Island was born yesterday.

The oversize sandbar where we live is only the latest, temporary incarnation of a corner of the world that has been continuously reshaped by colliding continents, crumbling mountains, shifting sea level, pounding waves and titanic glaciers. And though Long Island is brand new by the time scale of history, the way we live upon it is profoundly influenced by the remarkable series of transformations that occurred here over hundreds of millions of years.

Skyscrapers are harder to build here because the bedrock, pressure-cooked in a series of ancient continental collisions, is buried too deeply to support them. Our underground water supply is abundant, but fragile, because it rests in porous sand from the eroded Appalachian Mountains. The hills and finger bays of the North Shore, and the rich plains to the south, were shaped by the great Canadian glaciers that came here at least twice. And thousands of waterfront homes teeter on the edge of disaster because of the erosion and rising sea level that have continued to make Long Island shimmy and shrink ever since the last ice sheet began retreating 22,000 years ago.

Despite these powerful legacies, a surprising amount of basic information about Long Island’s past is either unknown or controversial. There are gaps in the time line when scientists have no idea what happened, and periods for which there are competing theories but no consensus among the experts.

The problem is that researchers — mostly geologists, since rocks and fossils are just about the only clues to Long Island’s early history — have remarkably little to go on as they try to piece together a credible chronology. Plant and animal fossils usually aren’t well preserved here because the climate is too wet, and many potential digging sites have been paved over. Worse still, the last glacier to visit the region changed the landscape so drastically that much of the earlier geological record is either buried or bewilderingly scrambled.

“It’s like trying to put together a jigsaw puzzle with 70 or 80 percent of the pieces missing,’’ said Ralph Lewis, an expert on the history of Long Island Sound and an associate state geologist at the Connecticut Geological and Natural History Survey. “Every time you get a new piece, you have to do more detective work. You have to modify the picture.’’

But there are a few places on Long Island — such as the rocky cliffs of Montauk or sandpits of Port Washington — where the landscape reveals buried secrets that geologists have struggled to decipher for more than a century.

On a muggy morning last summer, two men who have been exploring the Port Washington site for more than 25 years pondered its mysteries from a rocky perch overlooking the main pit.

For geologists Les Sirkin and Herb Mills, natural history is an almost mystical pursuit, and the abandoned sand pit just west of Hempstead Harbor is a temple. The mining stopped here decades ago, but lasted long enough to scrape away millions of tons of sand deposited by the last glacier, exposing sediments that have been in place for as long as 90 million years. The cliffs that the mining machines cut along the sides of the pit also have a story to tell: the jagged bands of white, gray and black on the cliff faces offer hints about the chaotic events that, layer by layer, built Long Island.

The sand pit is a portal to a time when Long Island did not exist as a distinct entity, but was instead an undifferentiated portion of a broad coastal plain that moved with North America as the continent drifted north from the tropics.

The bottom layer of that plain is bedrock from a series of continental collisions that began 450 million years ago. The slab was covered by a thick wedge of sandy sediment deposited over hundreds of millions of years by streams that ran to the coast from the eroding Appalachians. But what finally created Long Island was a glacier. Like a bulldozer pushing together a sand pile, the ice scraped along the top edge of the sandy wedge to form the Island’s fish shape.

When he was younger, Sirkin, a research professor of earth sciences at Adelphi University, spent long days chipping at rocks while hanging suspended on ropes slung over the cliffs at the sand pits. Mills, too, has spent much of his life studying the jumbled cliff layers, trying to identify the choreography of ice, ocean and rock that created those markings over millions of years.

“In the sand pits, you can really start to see how all the pieces fit together,’’ said Mills, who is curator of geology for the Nassau County museum system.

Understanding the evolution of Long Island requires a grasp of two key concepts, that the Earth’s climate, and its land masses, are forever changing.

At various times, scientists believe, the world’s average temperature has ranged from as low as 59 degrees to as high as 77 degrees, probably because of variations in the Earth’s tilt and orbit, in sunspot activity, and in the relative positions of the other planets. That 18-degree difference may not seem like much, but it has been enough to launch ice ages in which so much ocean water was trapped in glaciers that sea level dropped by hundreds of feet.

The Earth’s surface keeps changing because it consists of seven large plates of solid rock, and two dozen smaller ones, that slide in different directions very slowly — usually a few inches per year. Earthquakes in California, volcanoes in Japan, and the growing Himalayan Mountains are all responses to colliding plates. Conversely, huge trenches such as the ones in Africa’s Great Rift Valley appear where plates are pulling apart.

It was an ancient continental collision that formed the oldest, and hardest, rocks buried beneath Long Island.

For hundreds of millions of years, North America was located in the tropics, on or below the equator. The continent was rotated on its side so that the present-day East Coast faced south. Offshore, an arc of volcanic islands that resembled modern-day Japan slowly approached that coastline near the place where Long Island would take shape.

That island chain collided with North America about 450 million years ago, and the volcanic rocks were pushed deep into the Earth’s crust, where intense heat and pressure hardened them. That super-hard bedrock eventually moved closer to the surface, and today probably lies beneath much of Long Island. It can be seen in the walls of a New York City water tunnel being dug 700 feet beneath Queens.

An entire continent, Africa, loomed off the North American coast when the next key event began, roughly 300 million years ago. The two continents slowly ground into each other for the next 100 million years, buckling at their edges and pushing up a vast mountain range, the Appalachians, that was taller than the present-day Rockies.

Dinosaurs were dominating life on Earth by the time Africa and North America began drifting apart about 200 million years ago. The great reptiles probably thrived on the edge of the widening Atlantic Ocean, where majestic rivers swept down from the Appalachians into swampy deltas flecked with islands thick with ferns.

One of those rivers carved the valley that would become Long Island Sound, and they all carried away huge volumes of sediment from the eroding Appalachians. That sand piled up on the eastern flank of the mountain range and gradually formed a thick wedge on top of the older bedrock. That wedge still survives on Long Island, where the porous sand performs the vital job of storing most of our underground water supply.

The Long Island area was probably underwater much of the time that the dinosaurs lived, since the worldwide climate was so warm that relatively little ocean water was locked up in ice caps. But geologists know the area was exposed at least part of the time, because rocks found in Port Washington and elsewhere contain fossilized leaves from land plants similar to today’s tulip and magnolia trees, leading Mills and other researchers to surmise that dinosaurs probably roamed here.

The Bahamas-like weather in which the dinosaurs thrived ended suddenly 65 million years ago. According to the current leading theory, a giant comet or asteroid hit the Earth, raising enormous dust clouds that blocked the sun. The dinosaurs died off quickly, and the cooling trend gradually picked up speed as North America continued to drift north and west toward its current position.

When it was in the tropics, North America had avoided several ice ages that struck other parts of the world. But this time, it would feel the full force of the big chill. Ice caps, developing first in Antarctica, eventually formed in Canada as well and began expanding south from Hudson Bay about 2.4 million years ago.

The Canadian ice sheet probably expanded and receded across the entire northern half of North America at least 16 times, lowering and raising sea level by hundreds of feet with each advance and retreat. Just 22,000 years ago, when the last major glaciation was at its zenith, sea level was more than 300 feet lower than it is now, and the Atlantic shoreline was 80 miles south of present-day Fire Island.

Exactly how many times glaciers reached Long Island is uncertain. Most experts say there’s no evidence the Island was glaciated more than twice, although some geologists think it happened five times or more. The first ice sheet was here either about 150,000 or 60,000 years ago, and the second 22,000 years ago, according to the majority view, based in part on Sirkin’s research.

Monumental in size and power, the ice sheets changed everything in the region, and effectively built Long Island.

Before the glaciers arrived, Long Island was an indistinct part of a broad and flat plain that was often underwater. By the time the last ice sheet left, Long Island had an elevated central spine and distinctive north and south forks, all high enough to avoid the encroaching sea and survive as an exposed island.

Perhaps as much as 1,000 feet tall at its intimidating front edge, the last glacier, which is the one geologists know the most about, may have been even thicker farther north — thick enough to cover the tallest mountains in the Northeast, including 6,288-foot-tall Mt. Washington in New Hampshire.

The ice was so heavy that it distorted the Earth’s crust, and probably also caused ice-free areas just beyond the glacier’s southern edge to bulge up slightly. Thousands of years after that ice had retreated, land that had been squashed beneath the thickest parts of the glacier was still gradually rebounding. Newfoundland, for example, has bounced back more than 600 feet.

Like a snowplow clearing off a concrete driveway, the glacier’s leading edge scraped away the sandy deposits of the coastal plain wedge and exposed the bedrock below, which is why so many outcroppings of bedrock are visible today in New England but not Long Island.

Spreading south by perhaps one foot per day — a “glacial pace’’ that is remarkably fast by the timescale of geology — the expanding ice sheet not only bulldozed the sandy debris, it also sheared and carried off boulders as it scraped across bedrock. Some smaller rocks were moved more than 250 miles, which is why Mills has found cobbles in the Port Washington sand pits that came from Whiteface and Gore mountains in the upstate Adirondacks.

Fleeing the advancing ice sheet, an array of exotic cold-weather animals, many now extinct, moved into the Long Island area, including woolly mammoths, mastodons, ground sloths, saber-toothed tigers, elk-moose, caribou, musk oxen and wolves. Today, the teeth and bones of some of those Ice Age beasts are occasionally snared in fishermen’s nets off the South Shore.

Scooping up massive amounts of rock and sand as it moved through Connecticut, the glacier then widened and deepened the ancient river valley that would later become Long Island Sound, and also broadened the narrow stream beds that would become the finger harbors of Long Island’s North Shore.

The wide valley played another key role: It slowed the advancing ice sheet just before the world’s climate grew warm enough to stop the glaciation.

By the time the glacier covered the northern half of Long Island, about 22,000 years ago, its edge was melting as fast as new ice was pushing down from Canada. More than 1,000 miles from where it began in Hudson Bay, the glacier finally stopped. Soon, the melting began to outstrip the production of new ice, and the ice sheet, like a Popsicle on a sidewalk, began to shrink.

In its wake, the retreating glacier left behind a Long Island landscape that it had dramatically transformed. A hilly ridge, or moraine, now marked the line where the ice sheet stopped. Made of bulldozed rock mixed with sand and gravel dumped by streams running down from the melting glacier, that moraine is still visible as Long Island’s elevated central spine, which extends from Brooklyn to Amagansett before curving offshore. The offshore portion of that moraine has since been destroyed by ocean erosion but may once have constituted a third East End fork below the present-day Montauk peninsula.

The south-flowing ice water streams, including the Connetquot and Carmans Rivers, fed short-lived glacial lakes where icebergs floated. The swollen rivers also shaped fertile flatlands south of the moraine — most notably the Hempstead Plain, where the soil was rich with wind-blown silt that was also a gift of the glacier, since the fine powder came from rocks ground down by the ice sheet.

As it receded slowly, the glacier did more sculpting. Additional hilly moraines formed along the North Shore and North Fork, marking places where the ice sheet paused. Chunks of ice that broke free from the glacier and did not immediately melt formed deep depressions in the soil that later filled in with groundwater. The region’s two biggest lakes, Ronkonkoma and Success, and many others, were formed in these ``kettleholes.’’ The glacier also left behind thousands of boulders that the ice sheet had sheared off the bedrock farther north and carried to Long Island.

By about 19,500 years ago, the glacier was off Long Island and slowly receding north. In the shadow of the retreating ice, a huge glacial lake that may have stretched from Queens to Martha’s Vineyard filled the valley that would become Long Island Sound. That lake drained about 16,000 years ago, and the coastal plain was once again exposed. As the weather warmed, tundra vegetation gradually gave way to pine trees, and eventually chestnuts and oaks.

Still, Long Island was not yet an island. That process began roughly 15,000 years ago when the rising sea began entering the valley, probably near Fisher’s Island. A few thousand years later, the ocean probably broke through at the valley’s western edge.

Meanwhile, a new species was arriving in the area: Homo sapiens. Starting about 12,000 years ago, nomadic hunters, whose ancestors had trekked across the Bering Strait to Alaska 100 generations earlier, entered the region. Skillful hunters, these paleo-Indians probably quickly killed off the mammoths, mastodons and other large Ice Age animals.

Long Island finally earned its name about 11,000 years ago, or perhaps slightly later, when the rising ocean waters on the eastern and western edges of the ancient river valley finally joined to form Long Island Sound.

But Long Island’s evolution didn’t end, and continues today, because storm erosion and the rising sea are still reshaping the Island.

The one-foot-per-century rise in sea level, and the pounding storm waves, are gradually shrinking the Island by pushing back the North Shore cliffs and cutting into the South Shore beaches. They also are gradually propelling the barrier islands inland. Fire Island, for example, is moving toward the South Shore at a rate of about 1.7 feet per year.

What happens next is uncertain; the only sure thing is that Long Island will keep changing. Many scientists suggest that emissions of carbon dioxide and other pollutants from burning oil and coal will speed the worldwide warming trend and escalate the sea-level rise, spelling disaster for shoreline communities in the coming century.

That’s not the only possible scenario, however. There have been more than a half-dozen major ice ages in Earth’s history, and each has apparently lasted anywhere from 20 million to 50 million years. But the last one began just 2 million years ago, and has consisted of 100,000-year cold periods — when much of North America was covered by ice — alternating with much shorter intervals of warm climate, usually lasting only about 10,000 years.The current period of warming has lasted more than 20,000 years.

“We may be running out of time,’’ said John Sanders, a retired geology professor at Barnard College. “And the switchover, when it comes, is going to be very dramatic.’’

In other words, the glaciers may not be finished with Long Island just yet.

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