But granting that gullies and canyons in shallow continental shelves were in fact eroded while the shelves were exposed during glacial episodes, we are left with no explanation of how submarine canyons in the deeper continental slopes were eroded to depths of two miles below present sea level. Glacial ice may have been sufficient to lower oceans 500 or 600 feet—the maximum depth of the shelves—but nobody expresses a conviction that the ice could have lowered the oceans 10,000 or 12,000 feet.
The considerable length of time which must have been required for stream erosion of deep canyons in solid rock, plus the fact that Pleistocene ice was insufficient to lower the oceans two miles, seem to be decisive proof that the gorges necessarily must have been eroded prior to Pleistocene glaciation; also during a long interglacial interval rather than during a short glacial episode.
Origin of Canyons Unsolved
Volumes of guesses have been made and published as to how submarine canyons could have been produced. Theories include submarine “turbidity” currents of muddy water; artesian-spring sapping; submarine solution; tsunamis; slumping; diastrophism. All students of the mystery continue to mull over those theories; but it is safe to say that not a single one has been accepted as valid. The prevailing feeling is that the canyons have been incised by rivers working over a very long geological period while the slopes were above water. Yet it seems that most students cannot confidently accept such an explanation because nobody has produced a plausible theory as to how all continental shores, world-wide, could have been raised simultaneously, or how ocean level could have been lowered by 10,000 or 12,000 feet.
P. H. Kuenan, in Marine Geology (p. 499), definitely rejects the theory of subaerial erosion of canyons, followed by drowning due to down-warping in very recent time. He says the theory would mean that up to the end of Tertiary time continental slopes were in approximately the same position as they are at present and that a temporary emergence occurred during the cutting of the canyons. He says that no one could attribute such swift, regular and universal uplifting, followed by a downward movement of like extent, to normal diastrophism.
Shephard (op. cit.) says that evidence points to world-wide changes of sea level, but the magnitude of the changes causes hesitation in accepting the conclusion. He says further:
still greater difficulty appears to lie in the path of any conclusion that the continental margins and ocean basins have been undergoing tremendous oscillations in comparatively recent geological times and especially that they have now all been submerged. This is a serious predicament and requires much thought and still more investigation in the field. Whatever we find, the indications are that something has been radically missing from our knowledge of past geological conditions.
Von Engeln and Caster in Geology (p. 320) say:
If the submarine gorges, world-wide in distribution, are truly stream-eroded, geologic science is confronted with alternatives none of which seems possible of realization. One is that sea level in recent times has been 10,000 feet lower than now. The other is that coastal lands of all the continents have been 10,000 feet higher than now; or possibly that sea level was 5,000 feet lower and the lands were 5,000 feet higher than now; or some other proportion of the lowering of sea level and the raising of coastal lands to account for the 10,000 feet.
They label all these concepts as fantastic, yet admit that greatly lowered sea levels are clearly indicated.
Professor Nevin of Columbia, in Principles of Structural Geology, says with reference to the canyons:
Since they are restricted to the marginal zone, are of world-wide extent and attain about the same accordant levels, the evidence appears to be in favor of an origin controlled by a eustatic change of sea-level. The stumbling block, of course, is a valid cause that would change sea-level the necessary thousands of feet.
Thereupon he discounts the possibility of such great eustatic change by saying: “The facts point clearly to the very old age and permanence of the continental blocks. . . He points out that if uplift of the continents or of the marginal zones is imagined, so as to permit subaerial erosion, all the marginal zones must have been up-lifted the same amount and to the extent of ten or twelve thousand feet.
After erosion of the canyons, all the marginal zones must have been lowered the same amount and returned precisely to their former (and also present) positions. All this must have happened very recently.
He says further:
The volume of the ocean must be taken as constant, unless an ice age is assumed. But there does not seem to have been nearly enough ice during the glacial period to account for a drop in sea-level of several thousand feet. . . . The problem of the origin of submarine canyons is apparently unsolvable with the present data. Unfortunately we cannot retreat into the security of the distant past, when called upon for an explanation of their origin. The actuality of the submarine canyons savors almost of a joke on the part of Nature.
Other Evidence of Lower Sea Levels
Evidence of former lower ocean levels is not limited to submarine gullies and canyons. In all oceans there are submerged islands, mountain ranges and volcanic peaks which bear unmistakable effects of subaerial sedimentation and weathering. In the Pacific, there are hundreds of volcanic peaks, many of which are submerged to approximately uniform depths of about 800 fathoms—nearly a mile. These sea mounts, called “Guyots.” have flat truncated tops believed to indicate subaerial ablation at a time when the peaks stood above sea level. The great mid-Atlantic submerged ridge which extends for 9,000 miles, almost from Iceland to Antarctica, is believed to have once been above water. It stands some 15,000 to 20,000 feet above the ocean floor and many portions are covered by only a few hundred feet of water. The Azores, Ascension and many other islands are protruding portions of this ridge. Thousands of islands throughout all oceans are but small, exposed portions of much larger land areas which formerly stood above water.
- Mysteries of glaciation
- The true origin of ice ages