Piecing Together the Puzzle of Long Island Sound’s Formation

Vol. 1, No 2, April 1971
By  Edward Riccluti

More than a million years ago a river carved its valley 100 miles long to the south of where Connecticut is now. In places the trough went down 150 feet, making this primeval cellar the deepest depression in the region that became New England. At length, after the valley had been bulldozed into shape by glaciers, it became Long Island Sound.

For about half a century now, scientists have labored to solve the puzzle of the Sound’s formation and they have pieced together a still incomplete picture out of many disparate bits of evidence; details of terrain sculpted by the glaciers, mud dredged from the Sound’s bottom, sonar echoes that reveal prehistoric landscapes buried beneath the present bottom.

But despite the probing, aboard research vessels and in field and laboratory, much mystery remains. For example, which way did the ancient river run, east or west?

The geologists speculate that 70 million years ago or more, millennia before the river existed, when Tyrannosaurus Rex stalked other dinosaurs, the sea covered much of Southern Connecticut.

In those days, from today’s New York to Georgia, the surf crashed on beaches in the foothills of the Appalachians. Along in there a massive geologic upheaval tilted proto-Connecticut to the southeast. Whether this upset took place under the sea, above it, or both is not certain.

Eventually, over the millennia, the sea receded and much dry land appeared on the eastern tip of the continent. Some went under again and is still under now, forming the continental shelf.

Meanwhile, forests and grasslands covered the flat lands. Mastodons, reindeer and wolves foraged where fish swim now. Fossils of extinct elephants have been dredged from the submerged shelf miles off the northeast coast. The more romantic members of the scientific community yearn to see an arrowhead or some other human artifact brought up from under the waves.

During these times of relatively low sea level, the stream geologists have named “Sound River” was excavating its valley between the hard rock of Connecticut and the sand and clays to the south, where a low ridge stood on the site of Long Island.

In the decades around the turn of (the previous) century, geologists debated the course that the Sound River followed. Did it flow through what is now the Mattituck neighborhood? Or did it cut between Fishers Island and Greenport? A few scientists have tinkered with the idea that the river drained not eastward but to the west. No one really knows.

Which ever way Sound River flowed however, it probably formed part of a drainage system that included some of the present day streams of Long Island and Connecticut. The Connecticut River, probably part of this network, may have flowed southeast from its’ existing mouth, furrowing a deep channel where “The Race” is now. The course of the ancient Connecticut might have resembled the southeasterly channels depicted at the eastern end of ON THE SOUND’s model of the bottom.

Sound River was obliterated by the ice sheets that ground over the region during the Pleistocene epoch, beginning a million years ago. This was the time in earth’s history when glaciers spawned in mountains and in the polar regions expanded and covered vast stretches of the continents. The glacial ice, which reshaped the landscape as it moved, was the key force in molding the Long Island Sound basin.

A deep basin dug by nature’s tools

At least twice during the final 100,000 years of the Pleistocene epoch, the ice sheets plowed over New England. They gouged grooves, still visible, in the rock. The glaciers picked up and carried along millions of tons of gravelly rubble, some of which was bulldozed into heaps on Long Island, where the ice attack ran dead both times. Other glacial rubble was dumped on the island while the glaciers later were receding.

These mounds of glacial debris, called moraines, run the length of Long Island and give it much of its topographical character. It has been estimated, in fact, that if all the glacial debris were stripped from the island, its silhouette would fall 250 feet and much of the eastern end would be under water.

The Long Island moraines are called Harbor Hill and Ronkonkoma. Harbor Hill parallels the North Shore to Orient Point. The Ronkonkoma moraine runs up the Island’s spine and continues largely submerged in a loop past Montauk to Nantucket, at Block Island, and at Martha’s Vineyard.

The last advance of the ice to Long Island began about 30,000 years ago. Mountains of frozen water probably towered hundreds of feet high in and above Sound Valley. About 12,000 years ago, however, global climatic changes ended the ice ages. The planet warmed and Sound Valley emerged for good.

The warming of the world released the water locked up in glaciers. The ocean washed over the eastern part of the Ronkonkoma moraine, leaving only islands as reminders, that dry land once stretched far beyond Montauk. When an arm of the Atlantic Ocean reached into Sound Valley, the great depression became Long Island Sound.

Gradually the Sound took on the look it has today. Marshes began to form along the new coastline. Seaside plants and animals took possession of high land vacated by ice. But, at the same time, the rising sea took the low lands. Parts of western Connecticut claimed so quickly by the ocean that coastal valleys were drowned. Shallow bays and lagoons developed, their bottoms covered with sediment carried from the depths of the Sound by waves and tides. By a thousand years before the Christian Era the Sound appeared much as it did when Europeans first arrived. This much is generally agreed by geologists. But they do not agree on the procedures  by which the ice originally loosed its hold on Sound Valley and the surrounding region.

It is popularly assumed that when the ice ages ended the glaciers simply shrank back in a south-to-north direction. This was true in may areas, but may not have been the case in parts of northeastern North America.

Some geologists suggest that the New England glacier literally died where it stood and melted down in place. Others say that the glacier melted directionally along a shrinking front, but from north to south, ironically, the same way it had come from the polar region. Belief that the glacier simply melted is supported in Connecticut by the absence of moraines strewn by a melting ice front.

Long Island’s glacial debris, however, upholds the theory that the ice front shrank rapidly back to the north from its southernmost penetration. This is the opinion of Dr Walter Newman, a Queens College geologist considered by his colleagues to be particularly knowledgeable about the Sound’s past.

Some researchers speculate that ice in the Sound, sheltered by the valley and insulated by a blanket of rubble from Connecticut’s rivers, lingered after other ice had melted.

Most of what is known about the geology of the sound derives from research over the last two decades. In one project, a team of oceanographers from the Lamont-Doherty Geological Observatory in Palisades, spent 10 years crisscrossing the Sound in sonar equipped research vessels. Sound wave echoes bounced off the bottom were recorded. These helped define not only the Sound floor but also the ancient topography beneath today’s sand, muck and gravel .A study published last year related how the team charted a buried channel that may be the bed of Sound River.