Geologists generally consider the islands to be terminal moraines —material left by glaciers—deposited about 17,500 years ago as the ice cap paused in its retreat northward. Above water, the moraines are characterized by various rocks, gravel, sand, silt and clay, sometimes sorted out by waves. The Captain islands in Greenwich to the west are part of the same moraine (but not the Fish islands in Darien), and submerged parts of the same moraine are located between the Norwalk Islands and Charles Island, off Milford, to the east. (That island is probably part of the Hammonasset-Ledyard Moraine.)