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 SÄo 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.
Their shape and apparent relation to existing rivers have led Shepard to suggest that the submarine canyons were cut by rivers at some time when their gorges were above sea level. The relative youth of the canyons seems to relate them to some happenings in the world of the Ice Age. It is generally agreed that sea level was lowered during the existence of the great glaciers, for water was withdrawn from the sea and frozen in the ice sheet. But most geologists say that the sea was lowered only a few hundred feetânot the mile that would be necessary to account for the canyons. According to one theory, there were heavy submarine mud flows during the times when the glaciers were advancing and sea level fell the lowest; mud stirred up by waves poured down the continental slopes and scoured out the canyons. Since none of the present evidence is conclusive, however, we simply do not know how the canyons came into being, and their mystery remains. *
The floor of the deep ocean basins is probably as old as the sea itself. In all the hundreds of millions of years that have intervened since the formation of the abyss, these deeper depressions have never, as far as we can learn, been drained of their covering waters. While the bordering shelves of the continents have known, in alternative geologic ages, now the surge of waves and again the eroding tools of rain and wind and frost, always the abyss has lain under the all-enveloping cover of miles-deep water.
But this does not mean that the contours of the abyss have remained unchanged since the day of its creation. The floor of the sea, like the stuff of the continents, is a thin crust over the plastic mantle of the earth. It is here thrust up into folds and wrinkles as the interior cools by imperceptible degrees and shrinks away from its covering layer; there it falls away into deep trenches in answer to the stresses and strains of crustal adjustment; and again it pushes up into the conelike shapes of undersea mountains and volcanoes boil upward from fissures in the crust.
Until very recent years, it has been the fashion of geographers and oceanographers to speak of the floor of the deep sea as a vast and comparatively level plain. The existence of certain topographic features was recognized, as, for example, the Atlantic Ridge and a number of very deep depressions like the Mindanao Trench off the Philippines. But these were considered to be rather exceptional interruptions of a flat floor that otherwise showed little relief.
This legend of the flatness of the ocean floor was thoroughly destroyed by the Swedish Deep-Sea Expedition, which sailed from Goteborg in the summer of 1947 and spent the following 15 months exploring the bed of the ocean. While the Swedish Albatross was crossing the Atlantic in the direction of the Panama Canal, the scientists aboard were astonished by the extreme ruggedness of the ocean floor. Rarely did their fathometers reveal more than a few consecutive miles of level plain. Instead the bottom profile rose and fell in curious steps constructed on a Gargantuan scale, half a mile to several miles wide. In the Pacific, the uneven bottom contours made it difficult to use many of the oceanographic instruments. More than one coring tube was left behind, probably lodged in some undersea crevasse.
One of the exceptions to a hilly or mountainous floor was in the Indian Ocean, where, southeast of Ceylon, the Albatross ran
Their shape and apparent relation to existing rivers have led Shepard to suggest that the submarine canyons were cut by rivers at some time when their gorges were above sea level. The relative youth of the canyons seems to relate them to some happenings in the world of the Ice Age. It is generally agreed that sea level was lowered during the existence of the great glaciers, for water was withdrawn from the sea and frozen in the ice sheet. But most geologists say that the sea was lowered only a few hundred feetânot the mile that would be necessary to account for the canyons. According to one theory, there were heavy submarine mud flows during the times when the glaciers were advancing and sea level fell the lowest; mud stirred up by waves poured down the continental slopes and scoured out the canyons. Since none of the present evidence is conclusive, however, we simply do not know how the canyons came into being, and their mystery remains. *
The floor of the deep ocean basins is probably as old as the sea itself. In all the hundreds of millions of years that have intervened since the formation of the abyss, these deeper depressions have never, as far as we can learn, been drained of their covering waters. While the bordering shelves of the continents have known, in alternative geologic ages, now the surge of waves and again the eroding tools of rain and wind and frost, always the abyss has lain under the all-enveloping cover of miles-deep water.
But this does not mean that the contours of the abyss have remained unchanged since the day of its creation. The floor of the sea, like the stuff of the continents, is a thin crust over the plastic mantle of the earth. It is here thrust up into folds and wrinkles as the interior cools by imperceptible degrees and shrinks away from its covering layer; there it falls away into deep trenches in answer to the stresses and strains of crustal adjustment; and again it pushes up into the conelike shapes of undersea mountains and volcanoes boil upward from fissures in the crust.
Until very recent years, it has been the fashion of geographers and oceanographers to speak of the floor of the deep sea as a vast and comparatively level plain. The existence of certain topographic features was recognized, as, for example, the Atlantic Ridge and a number of very deep depressions like the Mindanao Trench off the Philippines. But these were considered to be rather exceptional interruptions of a flat floor that otherwise showed little relief.
This legend of the flatness of the ocean floor was thoroughly destroyed by the Swedish Deep-Sea Expedition, which sailed from Goteborg in the summer of 1947 and spent the following 15 months exploring the bed of the ocean. While the Swedish Albatross was crossing the Atlantic in the direction of the Panama Canal, the scientists aboard were astonished by the extreme ruggedness of the ocean floor. Rarely did their fathometers reveal more than a few consecutive miles of level plain. Instead the bottom profile rose and fell in curious steps constructed on a Gargantuan scale, half a mile to several miles wide. In the Pacific, the uneven bottom contours made it difficult to use many of the oceanographic instruments. More than one coring tube was left behind, probably lodged in some undersea crevasse.
One of the exceptions to a hilly or mountainous floor was in the Indian Ocean, where, southeast of Ceylon, the Albatross ran
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