land at the edge of the continents. It begins at the shoreline and gently slopes underwater to an average  depth of about 430 feet (130 m). The width of the continental shelf averages 47 miles (75 km). In certain areas, such as parts of the arctic region, the shelf extends as far as 1,000 miles (1,600 km). In some other areas, particularly those bordering much of the Pacific, it measures only 1 mile (1.6 km) or less. Valleys of varying depths cut through the shelf.