Thousands of years ago Long Island Sound was a large oval lake, carved out by the last Ice Age. At some point in time, the easternmost wall of this large lake eroded or otherwise collapsed, allowing the ocean to ebb and flow throughout the sound. The wall from Long Island to Connecticut, running northeast, consists of Plum, Little Gull and Fisher Islands as well as Valient Rock and Race Rock reefs. The remainder of this "wall" is below water, mostly 40 to 83 feet below low water. However, there is a comparatively narrow opening in the undersea wall southwest of Race Rock which is around 200 feet deep. It is through this two mile gap that the water of Long Island Sound passes during each tide change. At the height of the ebb tide the water passes Race Rock at speeds up to 4 knots or 4 1/2 miles per hour. A navigator, especially in the days of sail, who made a wrong calculation when passing through the Race, found his vessel swept onto Race Rocks or Valient Rock to the southwest, usually with disastrous results.
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