Lake Connecticut was a large glacial lake that filled what is now Long Island Sound and parts of coastal Connecticut as the Laurentide Ice Sheet retreated roughly 18,000 to 20,000 years ago. Held back by the terminal moraine that became Long Island, Fishers Island, and Watch Hill, it reached about the size of the modern sound. Fine meltwater sediments settled across its floor, helping create the sound’s relatively shallow depth. Around 15,000 years before present, the moraine dam failed and the lake drained through what is now The Race. Its disappearance was gradual, as alternating freshwater and saltwater conditions transformed the basin into today’s marine sound.