Canyons, Submarine

narrow, gently sloping hollows in the sea floor within the shelf region. In some cases submarine canyons are continuations of major land river valleys. They do not begin at the shore, however; instead they start at depths of up to several dozen meters because the coastal zone of the floor is leveled out by wave action. In a longitudinal profile they often show troughs and sills. The relief and sediments of submarine canyons testify to their surface origin. The inundation of river valleys by the sea is caused by either a rise in the level of the world ocean after the thawing of inland ice formed during an ice age or by the recent submersion of particular segments of the land. Submarine canyons have been found on the edges of all the continents where detailed fathoming has been done. Examples include the submarine canyons that continue the valleys of the Hudson, Mississippi, and Congo rivers and the ancient erosion depressions on the floor of the English Channel and along the eastern shores of Asia.