The Birth of Long Island

A half-billion years ago, it was a chain of volcano islands adrift in a tropical sea. Over untold millenia, 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.