Around the coasts of all the land masses, between low-tide level at about the 100-fathom mark is a shallow platform known as the continental shelf, from which the higher parts project as islands. The shelf slopes gently seaward with an angle of less than one degree. It is well developed off Western Europe, where it extends westward for 200 miles from south-west Britain, and off north-eastern North America, while off the Arctic coast of Siberia it is about 750 miles wide. Detailed hydrographic surveys in the Canadian Arctic have outlined a continental shelf 75 to 100 miles in width to the north of Canada, covered with water to 100 fathoms, and abruptly dropping by the continental slope to deep water. Around the continents it is much narrower, or almost completely absent, especially along coasts where fold mountains run parallel and close to the sea, as along the edge of the eastern Pacific.
Detailed soundings have revealed that the valleys of many
rivers seem to continue across the continental shelf. This could be accounted
for either by rising sea level or by a sinking of the land; in other words,
continents really end at the edge of the shelf. This would explain the fact
that the continental shelf is at its widest around the shores of lowland areas,
where a slight change of level involves a considerable extent of land. Some
continental shelves may be due to in part to wave erosion, or to the building
up of an offshore terrace by deposition during long periods of geological time;
some authorities even claim deposition by the Quaternary ice sheets may have
helped to build up the shelves in the Atlantic.
One difficult problem is that some submarine channels cross
the shelf and then continue beyond its edge into deeper water. One of these
lies off the Hudson in North America, another in the Bay of Biscay. Some
authorities ascribe them to faulting, others to erosion by powerful submarine
turbidity currents, or to former river erosion, involving therefore enormous
changes in sea level. Another explanation is that a river current, continuing
into the sea, keeps its channel clear by depositing material at the sides, thus
leaving a trough as sediment accumulates. All these explanations are open to
criticism, particularly in the case of the deeper troughs. More difficult still
to understand are troughs that do not cross the whole shelf, but are found near
its oceanward margin; sometimes they form deep gorges cut into the edge of the shelf.
A number have been surveyed in detail by echo-sounding off the New England
coast. One explanation attributes these gorges to the sapping action of submarine
springs bursting out far down the continental slope.
© John Welford
