The recession of the ice-sheets was marked by a series of halts and occasional re-advances of the ice sheet. During periods of stand-still, beach ridges formed along the existing shores of glacial lakes. The beaches are long narrow ridges of sand and gravel, smoothly rounded, rising 10 to 20 feet above the surrounding land. In some places, gravel beaches were not formed, but the till surface of the lake bed was terraced by wave action. |