shelf

A shelf is solid rock, or bedrock, found beneath alluvial soil or deposits, a flat-surfaced stratum. A shelf can also be a flat ledge of rock or a projecting rock layer, as on a slope. The term is most commonly used to refer to a continental shelf, a gently sloping, nearly horizontal marine platform that was dry land during the last ice age and is now beneath the waves, projecting gradually away from the present-day shoreline to a depth of between 60 and 1,800 feet.

Donna Seaman