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Formation of Long Island Sound
| Tens of millions of years ago, at a time when sea level was much lower than it is today, an ancient river carved out the basin that would later become Long Island Sound. When the glaciers of the last ice age advanced on southern New England, they flowed through this basin, carving it out even deeper before coming to a halt some 21,000 years ago (see steps 1-4 below).
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| At this point, the glacier's rate of advance was in equilibrium with its rate of melting, and for perhaps a thousand years, the ice remained in this position, constantly discharging meltwater along with millions of tons of rock, sand and gravel. This sediment built up a ridge called a "moraine," which formed the south fork of Long Island (see steps 5-6 below).
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As the climate began to warm around 21,000 years ago, the ice margin began to retreat, then paused again forming a second moraine in line with Long Island's north fork. As the ice retreated further, this second moraine acted as a dam for glacial runoff. The Long Island Sound basin became a massive glacial lake, which persisted for centuries, depositing tons of sediment at its bottom, before draining rapidly around 15,000 years ago (see steps 7-10 below).
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 | Step 7 |
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At around the same time, the massive ice sheets that covered the North American continent were beginning to melt rapidly, and the huge volume of water they were releasing began returning to the oceans. As the sea rose to the level of the present-day Long Island Sound, the recently drained glacial lake filled with salt water, and the Sound as we know it today began to take shape (see steps 11-14 below).
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 | Step 11 |
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