Continental shelf

Continental shelf, submarine rim around most of the earth's continents, extending on average about 50 mi (80 km) beyond the shoreline before dropping steeply to the ocean floor. They are relatively flat and shallow, between 300 ft (90 m) and 600 ft (180 m) below the ocean surface, although some are cut by deep submarine canyons. They are covered with layers of fine sediment. The shallower portions of the continental shelf were at one time dry land, since the sea level has risen about 300 ft (90 m) from the time of the Ice Ages.