Now hundreds of vessels are equipped with sonic sounding instruments that trace a continuous profile of the bottom beneath the moving ship (although only a few can obtain profiles at depths greater than 2000 fathoms[11]). Soundings are accumulating much faster than they can be plotted on the charts. Little by little, like the details of a huge map being filled in by an artist, the hidden contours of the ocean are emerging. But, even with this recent progress, it will be years before an accurate and detailed relief map of the ocean basins can be constructed.
The general bottom topography is, however, well established. Once we have passed the tide lines, the three great geographic provinces of ocean are the continental shelves, the continental slopes, and the floor of the deep sea. Each of these regions is as different from the others as an arctic tundra from a range of the Rocky Mountains.
The continental shelf is of the sea, yet of all regions of the ocean it is most like the land. Sunlight penetrates to all but its deepest parts. Plants drift in the waters above it; seaweeds cling to its rocks and sway to the passage of the waves. Familiar fishes—unlike the weird monsters of the abyss—move over its plains like herds of cattle. Much of its substance is derived from the land—the sand and the rock fragments and the rich topsoil carried by running water to the sea and gently deposited on the shelf. Its submerged valleys and hills, in appropriate parts of the world, have been carved by glaciers into a topography much like the northern landscapes we know and the terrain is strewn with rocks and gravel deposited by the moving ice sheets. Indeed many parts (or perhaps all) of the shelf have been dry land in the geologic past, for a comparatively slight fall of sea level has sufficed, time and again, to expose it to wind and sun and rain. The Grand Banks of Newfoundland rose above the ancient seas and were submerged again. The Dogger Bank of the North Sea shelf was once a forested land inhabited by prehistoric beasts; now its ‘forests’ are seaweeds and its ‘beasts’ are fishes.
Of all parts of the sea, the continental shelves are perhaps most directly important to man as a source of material things. The great fisheries of the world, with only a few exceptions, are confined to the relatively shallow waters over the continental shelves. Seaweeds are gathered from their submerged plains to make scores of substances used in foods, drugs, and articles of commerce. As the petroleum reserves left on continental areas by ancient seas become depleted, petroleum geologists look more and more to the oil that may lie, as yet unmapped and unexploited, under these bordering lands of the sea.
The shelves begin at the tidelines and extend seaward as gently sloping plains. The 100-fathom contour used to be taken as the boundary between the continental shelf and the slope; now it is customary to place the division wherever the gentle declivity of the shelf changes abruptly to a steeper descent toward abyssal depths. The world over, the average depth at which this change occurs is about 72 fathoms; the greatest depth of any shelf is probably 200 to 300 fathoms.
Nowhere off the Pacific coast of the United States is the continental shelf much more than 20 miles wide—a narrowness characteristic of coasts bordered by young mountains perhaps still in the process of formation. On the American east coast, however, north of Cape Hatteras the shelf is as much as 150 miles wide. But at Hatteras and off southern Florida it is merely the narrowest of thresholds to the sea. Here its scant development seems to be related to the press of that great and rapidly flowing river-in-the-sea, the Gulf Stream, which at these places swings close inshore.
The widest shelves in all the world are those bordering the Arctic. The Barents Sea shelf is 750 miles across. It is also relatively deep, lying for the most part 100 to 200 fathoms below the surface, as though its floor had sagged and been down-warped under the load of glacial ice. It is scored by deep troughs between which banks and islands rise—further evidence of the work of the ice. The deepest shelves surround the Antarctic continent, where soundings in many areas show depths of several hundred fathoms near the coast and continuing out across the shelf.
Once beyond the edge of the shelf, as we visualize the steeper declivities of the continental slope, we begin to feel the mystery and the alien quality of the deep sea—the gathering darkness, the growing pressure, the starkness of a seascape in which all plant life has been left behind and there are only the unrelieved contours of rock and clay, of mud and sand.
Biologically the world of the continental slope, like that of the abyss, is a world of animals—a world of carnivores where each creature preys upon another. For no plants live here, and the only ones that drift down from above are the dead husks of the flora of the sunlit waters. Most of the slopes are below the zone of surface wave action, yet the moving water masses of the ocean currents press against them in their coastwise passage; the pulse of the tide beats against them; they feel the surge of the deep, internal waves.
Geographically, the slopes are the most imposing features of all the surface of the earth. They are the walls of the deep-sea basins. They are the farthermost bounds of the continents, the true place of beginning of the sea. The slopes are the longest and highest escarpments found anywhere on the earth; their average height is 12,000 feet, but in some places they reach the immense height of 30,000 feet. No continental mountain range has so great a difference of elevation between its foothills and its peaks.
Nor is the grandeur of slope topography confined to steepness and height. The slopes are the site of one of the most mysterious features of the sea. These are the submarine canyons with their steep cliffs and winding valleys cutting back into the walls of the continents. The canyons have now been found in so many parts of the world that when soundings have been taken in presently unexplored areas we shall probably find that they are of world-wide occurrence. Geologists say that some of the canyons were formed well within the most recent division of geologic time, the Cenozoic, most of them probably within the Pleistocene, a million years ago, or less. But how and by what they were carved, no one can say. Their origin is one of the most hotly disputed problems of the ocean.
Only the fact that the canyons are deeply hidden in the darkness of the sea (many extending a mile or more below present sea level) prevents them from being classed with the world’s most spectacular scenery. The comparison with the Grand Canyon of the Colorado is irresistible. Like river-cut land canyons, sea canyons are deep and winding valleys, V-shaped in cross section, their walls sloping down at a steep angle to a narrow floor. The location of many of the largest ones suggests a past connection with some of the great rivers of the earth of our time. Hudson Canyon, one of the largest on the Atlantic coast, is separated by only a shallow sill from a long valley that wanders for more than a hundred miles across the continental shelf, originating at the entrance of New York Harbor and the estuary of the Hudson River. There are large canyons off the Congo, the Indus, the Ganges, the Columbia, the Sāo Francisco, and the Mississippi, according to Francis Shepard, one of the principal students of the canyon problem. Monterey Canyon in California, Professor Shepard points out, is located off an old mouth of the Salinas River; the Cap Breton Canyon in France appears to have no relation to an existing river but actually lies off an old fifteenth-century mouth of the Adour River.
11
The range of echo-sounding instruments has now been so greatly extended that under ideal conditions the most powerful of them are capable of sounding the maximum depths of the sea. Factors such as the nature of the underlying bottom and conditions in the intervening water layers influence the effectiveness with which the sounding devices operate under actual conditions at sea. Nevertheless, the potential range necessary for charting all parts of the sea is now at the command of oceanographers.