The evolution of Long Island Sound

It has been an ancient river, a fertile valley, a vast ice field, and a milky, iceberg-laden lake almost 200 miles long. What it hasn’t been, until recently, is the saltwater estuary that makes Long Island a long island.

Long Island Sound is only about 11,000 years old — born yesterday, by the standards of geology. But it runs deep into the distant past. In fact, experts say, were it not for a river that formed tens of millions of years earlier when dinosaurs were still roaming the area, the Sound probably wouldn’t exist today and Long Island would be part of Connecticut.

The signs of the Sound’s varied history are everywhere, if you know where to look. Under its muddy bottom are beach ridges that radiate from the waterway’s center like bathtub rings and mark its gradual expansion as sea level has risen. Embedded in its shoreline cliffs are dark-colored ribbons of clay from a now-vanished freshwater lake. Buried deep in its sediments are the shells of animals that thrived when the Sound was a valley laced with streams, and deeper still are the shadowy vestiges of the ancient river channels that first carved the valley in the time of the dinosaurs.

Geologist Ralph Lewis has been studying those signs for 16 years. Using submarines, sonar, drilling machines and even remote-controlled vehicles to explore the Long Island Sound’s depths, Lewis and other experts have compiled a detailed chronology of the waterway’s relatively recent birth, and its ancient antecedents.

“What’s fascinating about Long Island Sound is that so much of the story happened in the last 12,000 years, when humans were here,’’ said Lewis, an associate state geologist at the Connecticut Geological and Natural History Survey. “The first people who came to this area saw a completely different world than we see today. They watched Long Island Sound evolve.’’

The story actually begins tens of millions of years before the first Indians arrived, when the valley that would one day become Long Island Sound was carved by a river, or perhaps two rivers, that drained a broad, sandy coastal plain. Smaller tributary streams extended to the south onto present-day Long Island, and carved similar valleys that today are still recognizable as the harbor inlets of the North Shore, from Little Neck Harbor in Queens all the way out to the gently curved bays of the North Fork.

But as with almost every other natural feature in the region, it took a series of huge ice floes descending from Canada — the glaciers — to transform that ancient valley into the shape we would recognize as Long Island Sound.

At least twice over the past 150,000 years, ice sheets with imposing front walls that may have been 1,000 feet tall plowed across that river valley.

As they moved, the glaciers widened and deepened the valley, scooping up a massive amount of rock and sand and carrying it south onto Long Island. Some of the scooped-up material ended up as a long ridge, called a terminal moraine, that marks the line where each glacier finally stopped and began receding north again as it melted. The glaciers, however, did not retreat steadily. Instead, they stuttered, creating new ridges called recessional moraines wherever they paused. Today, the locations of two of those moraines are marked by the elevated spines of the north and south forks of Long Island.

Each time a glacier retreated north, it left behind an extraordinary calling card: a large but temporary lake formed by the melting ice. On their north sides, these glacial lakes were bounded by the towering ice wall of the receding ice sheet, and on the south by the bouldery ridges of the moraines.

The last glacier, which probably arrived on Long Island about 23,000 years ago, is the one that researchers know the most about. As it slowly receded into New England about 2,000 years later, the glacier left in its growing wake a huge lake, or perhaps series of lakes, extending from Queens to Martha’s Vineyard. Scientists call the Long Island Sound portion of that lake Glacial Lake Connecticut.

“You may have been able to canoe all the way from New York City to Buzzards Bay [in Massachusetts] in this one big freshwater lake,’’ Lewis said.

Lake Connecticut was unlike anything Long Islanders would know today. It was deeper and colder than today’s Sound and probably had no fish. Icebergs likely floated on its waters, and even its color was different: dim and milky because it contained so much “rock flour’’ — the powdery residue of rocks ground down by the glacier. Mastodons and giant sloths probably roamed the barren tundra of the lake’s southern shoreline, while the towering, gray ice wall loomed on the opposite shore. As the ice continued to recede, short-lived glacial lakes later formed near Albany and Hartford, among other places.

About 3,000 years after it was born, Lewis said, Lake Connecticut drained through an eroded gap in the moraine ridge near Fisher’s Island. For a short time, starting about 16,000 years ago, the ancient but newly broadened valley was once again exposed. But not for long, because about 1,000 years later, rising ocean waters came in through the same eroded gap — this time in the opposite direction. Eventually, the ocean broke through on the valley’s western edge, too, and the Sound began to take shape.

But the Indian hunters who began arriving in the area soon afterward saw a waterway that looked very different than it does today. Long Island Sound at first was slender and small, and its shorelines were bare. Sea level was rising so quickly that there wasn’t time for marshes — which can take decades to fully develop — to appear along its edges. Indeed, it wasn’t until about 4,000 years ago, when the rate of sea level rise slowed, that the wetlands we know today began to appear along its coasts.

Since then, the Sound’s waters have risen another 20 feet, enough to reclaim huge chunks of land that had not been submerged since the days of Lake Connecticut. But even today, Long Island Sound isn’t finished growing. Erosion and rising sea level continue to cut into its shore cliffs and beaches, slowly expanding the boundaries of this young and ever-changing waterway.