Gently seaward-sloping surface that extends between the shoreline and the top of the continental slope at about 150 m depth. The average gradient of the shelf is between 1:500 and 1:1000 and, although it varies greatly, the average width is approximately 70 km. Five major types of shelves may be recognized: (a) those dominated by tidal action; (b) those dominated by wave and storm action; (c) those dominated by carbonate deposition; (d) those subject to modern glaciation in polar areas; and (e) those floored by relict sediments which constitute up to 50% of the total shelf area.