Seaward border of a continental shelf .
The world's combined continental slope is about 200,000 mi (300,000 km) long and descends at an average angle of about 4k from the edge of the continental shelf to the beginning of the ocean basins at depths of 330–10,500 ft (100–3,200 m). The slope is most gradual off stable coasts without major rivers and is steepest off coasts with young mountain ranges and narrow continental shelves. Slopes off mountainous coastlines and narrow shelves commonly have outcrops of rock. The dominant sediments of continental slopes are muds; there are smaller amounts of sediments of sand or gravel.