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The events that created Long Island Sound began about 200 million years ago, when the land masses that are now North America and Africa began to drift away from each other. Sediment eroded from the land was washed down to the newly forming Atlantic Ocean. These sediments accumulated along the coast to form the Atlantic Coastal Plain and its extension under the ocean known as the continental shelf.
At the same time, rivers flowing to the ocean cut valleys through the sedimentary deposits. The Long Island Sound basin started out as a broad river valley lying between the coastal plain in Connecticut and hills along what is now Long Island.
Three million years ago, glaciers pushed into the area and covered the land from Canada to New York and Connecticut. As the amount of water trapped in glaciers increased, the sea level dropped dramatically. The powerful glaciers scoured the land, carrying along sediment, stone, and eroded debris as they advanced. Many familiar features of our landscape were created by these glaciers.
About 22,000 years ago, the last of the glaciers reached its southernmost point in the middle of what is now Long Island when the forward movement of the ice was halted by a warming global climate. At this time, the ocean was 350 feet lower than today and the coastline was 150 miles south of Long Island.
The melting edge of a glacier deposits sand and stone in moraines, which vary in size with how long the edge holds steady in one place. One large moraine (the Ronkonkoma moraine) formed in the middle of Long Island where the glacier first halted. Another (the Harbor Hills moraine) formed where the retreating glacier paused at the north shore of Long Island, which today is still mainly moraine. It has been said that all the concrete in New York City has been made with sand from the moraines of Long Island.
The Harbor Hill moraine stretched the length of the old river valley, which had probably been widened and deepened by the grinding of the glaciers, and formed a barrier around it. Consequently, the area that is now Long Island Sound filled with water as the glaciers melted. For several thousand years, Long Island Sound was a freshwater glacial lake.
Meanwhile, sediment carried by the melting glaciers also flowed into the lake, accumulating to a depth of almost 500 feet. Eventually, the lake water overflowed the surrounding moraine. It is thought that the force of the escaping water cut a deep channel at the eastern end of the Sound and the glacial lake was partially or completely drained.
As glaciers continued to melt, the sea level rose. Eventually, about 8,000 years ago, seawater flowed into what had been the glacial lake. At this point, the Sound was an arm of the sea, with water entering and leaving at the eastern end. Soon, however, the rising sea level reached a second opening to the ocean near New York City. Long Island Sound had become an estuary.
The Sound, as we know it, began to evolve about 4,500 years ago. By this time, the climate as it is today had been established; and sea level rise had slowed enough for plant and animal habitats to start forming on the shores of the Sound.
Natural forces took millions of years to form Long Island Sound. Although it is difficult to predict what will take place in the future, the Sound will without doubt continue to change. Both natural and human forces are at work.