Though the oceans have been traversed for centuries very little is really known about them. This is one reason why the performance of duty at sea sometimes leads to ends far from those originally intended. As an instance, no one connected with the work had any idea that a routine survey of Georges Banks could bring to light a geological puzzle that would provide scientists with an entirely new field of argument. Such new topography was shown by the survey that an overhaul of existing geological theory was clearly called for.
By 1930 radio acoustic ranging had become an accurate means of position control in marine surveying. Sonic sounding had largely replaced the lead and wire in both speed and accuracy. By these means it was possible not only to make more soundings per unit of area but also to carry the traverses much farther offshore. In other words, modern developments of sound duplicated the exactness of inshore charts in areas where the navigator formerly found soundings far apart and none too accurate. The recharting of Georges Bank by sonic methods was accomplished by the U. S. Coast and Geodetic Survey in 1932. The unexpected result was the discovery of the great underwater cuts into the continental shelf, which from their forms made the name “canyon” self-evident.
For many years the existence of a submerged river valley off the approaches to New York has been well known. Many a former junior officer on the South American run probably remembers sweating over a sounding machine trying to find it. Until the 1932 survey, however, similar topography on the European track was more or less unsuspected. Due to the early start of excellent charting and the amount of study devoted to them, the canyons of Georges are best known and most deserve discussion.
When detailed drawings of the contours of these gorges had been made, they compared favorably in size with some of the great ones that lie above the sea. As such they provided excellent guides for the navigator and equally excellent headaches for the geologist. Had any scientist the temerity to suggest that canyons as great as the Grand Canyon of the Colorado could be found in bunches in a prosaic and unromantic area of fishing banks, his imagination would have drawn unkind criticism and his sanity would have been somewhat doubted. Yet the charts clearly showed such phenomena existed. This was the situation that faced the men working on submarine geology at the Woods Hole Oceanographic Institution in 1933.
Several factors favored the immediate study of these underwater wonders. Georges Bank is not far from Woods Hole as ocean distance is regarded. Submarine geology was being studied at the Institution, so men were available who could handle the problem efficiently and a ship built for just such a contingency was ready for their use. A research problem that promised such far-reaching results seemed like the right order left at the right address at the right time.
Close inspection of the detailed charts showed the magnitude of the work and also gave clues to the best attack. There were some outstanding characteristics that applied to all the gorges. In general, these cuts extend into the shelf from 5 to 12 miles; they vary in width from 2 to 6 miles and the floors range from 1,200 to 12,000 feet below sea level. Unlike the more familiar Hudson Gorge, all trace of them is lost inside the 50-fathom contour. As the walls tower above the floors, they also recede in a series of steps, in some cases so precipitously that sediments cannot stick to them; that is, the walls are steeper than the angle of repose for muds (30° to 35°). It also means that these sections of wall are bare rock.
Rock in its original formation functions for the geologist somewhat as the chronometer for the navigator, if the ratio of seconds to millions of years is considered. Could some of the bare spots on the canyon walls be reached with an instrument that would break or chisel off a few lumps, the rock could be dated if it contained fossils and substantial evidence obtained on the original formation—or to put it briefly, one could tell how the gorges got there and when.
Gear, then, was the primary consideration, since the scientists, ship, and crew were ready and the focus of attack decided upon. Already available was an electric winch, capable of an 8-ton direct pull, with 30,000 feet of specially designed wire hoisting rope. This was part of the permanent equipment of the research ship Atlantis. Two types of dredges were decided upon, similar in construction but differing in the way they cut, one having the spiked lip similar to that used on steam shovel buckets, the other a smooth chisel-like edge. To enhance the cutting effect, the mouths were flared. Three-quarter inch steel plate 18 inches wide welded into oblong shape, somewhat on the order of twice as long as wide, provided a dredge mouth comparable in strength to the rest of the gear. A bag of ring and link steel mesh such as that used by oyster dredgers was attached to collect whatever was cut by the lips. Bails of heavy steel strip were used to connect to the towing wire, weights having been added to keep them in cutting position. To take care of hard and fast hookups, a preventer of rope with a breaking strain of 12,000 pounds was so made fast that when the rope parted, the dredge pulled out sideways on a tripping bridle.
Along with the dredges, coring tubes were made. These were lengths of steel pipe, weighted with lead at the upper ends and cutting edges on the lower. When allowed to run freely from the surface to the bottom, the weighted top caused the cutting edge of the tube to be driven into the mud. As stratification of bottom sediments tells much of what has happened in geologic time, this type of gear was included to take care of the valleys and other locations where mud might be obtained.
Developing a technique for handling the gear could be done only at sea. Discussion of this aspect of the problem, especially when salted by sailor and scientific pessimism, presented far more difficulty than was ever encountered. Like most jobs at sea, practice ironed out the wrinkles and made the work seem easy. The use of dredges finally resolved into three stages, getting down on the right location, dragging over a selected area, and getting the gear aboard.
The survey had been carried out so accurately that hitting a spot on a canyon wall a mile or so from the surface was simpler than it sounds. Immediately after stellar observations, the ship was headed for the initial canyon and when this was reached, soundings were taken continuously with the fathometer directly across to the plateau above the opposite side. A marker buoy established this spot for later reference. Then the course was reversed until the ship was over the greatest depth of the valley. Here the dredge was dumped overboard and the wire allowed to pay out freely as the ship returned over the wall toward the marker; the intent being to lay out enough wire to allow the dredge to drag over outcroppings of rock as it was pulled up the wall of the canyon. The heavy weighted bails functioned perfectly when used this way but the tooth-mouth dredge dug in too solidly, one trial or so showing the chisel-mouth to be superior.
A dynamometer on the wire showed the strain as the dredge worked over the bottom. Often hard and fast hoopups occurred on rocky ledges and when this happened the engine was stopped while gentle heaving on the winch breasted the ship over the anchored dredge. This tended to loosen it, not only on account of the more vertical pull but also because of the heaving motion imparted to the hull by wave action. In one such instance the wire itself sawed a piece of sandstone from a cliff face and the stone was brought up holding firmly in the cut.
The dynamometer readings were also indicative of the kinds of material being dredged below. Cutting through recent deposits exerted a tension of about 3,000 pounds, more consolidated ones increased this a ton or so, while breaking bedrock ran the strain to 10,000 pounds or caused the preventer to part. This approximation of material was borne out in the catches. Where strains fluctuated a great deal, the collected rock often showed identical kinds, some with fresh and weathered surfaces, others with no sign of fresh fracture at all, indicating the dredge had picked up talus at the foot of a cliff, as well as rock from its original formation. As rock samples were considered more important than any other, much time was saved by gauging the hunting grounds by the dynamometer readings.
Bringing the dredge to the surface and getting it aboard was sometimes awkward, for the mesh bag collected considerable mud and clay to add to the already considerable weight. However, forewarning of what was coming up helped immensely. When necessary, part of the catch was washed out of the bag to bring the weight down where it could be handled.
One drawback in this technique of bottom sampling is the exact location of the specimens. Obviously the position of the dredge can only be located at the beginning and end of a haul except in the few cases where the wire angle became vertical due to hookups. Paying out enough wire to allow the dredge to reach bottom and the ship to gain the plateau above the canyon wall results in a line from the valley to the top. The material could come from anywhere on the line. Navigational positions have little value in this sense, for the vertical distance is the controlling factor rather than the horizontal. The geologist is more interested in the height at which these rocks were dredged than the exact yardage from the marker. The middle or upper sections where the walls are steepest are the points where this difficulty is greatest.
The coring tubes, used where the bottom is muddy, do not present trouble of this sort, for the gradients being less steep, the surface position becomes more pertinent.
Sampling bedrock from the ocean bottom has been the dream of geologists for years. When the Atlantis brought in the first load from Georges, the dream became actuality. The gathered material was not only rock from the abyssmal bottom but it certainly was submarine rock that had formed where it was found. And that from the oceanographer’s standpoint “was something.” Instead of substantiating or disproving existing theories, however, this new evidence tended to confuse them.
Men who study coast lines are fairly well agreed that the continental boundaries are, technically speaking, the edge of the ocean basins, and the submerged portion of the continents, called “continental shelves,” are really enormous embankments building seaward continually from waste material of the land masses. The extent of this action varies a great deal, for on the northeast coast of North America the shelf extends from 30 to 200 miles seaward, north of Eurasia toward the Polar Basin some 500 miles, while on the west coast of North America there are many places where there is no shelf at all. Land drainage and age are controlling factors.
Wave action planes the shelf more or less smooth and also constantly transports the erosional products to deeper water. At about the 100-fathom curve, however, the soundings increase rapidly as a rule and the embankments descend in steep grades to the ocean abyss some 2,000 fathoms or more below sea level. This is the real boundary of a continent and is known as the “continental slope.” Canyons are found in this region. That these phenomena are not confined to the northeast coast of North America was being borne out in new discoveries when war precluded further research on them.
Study of the dredging results on Georges as well as those farther south divided expert opinion into two camps. Some scientists preferred to think that the canyons had been formed while the strata were out of water, while others thought the geological evidence pointed to their formation under the sea. It is interesting to follow the reasoning.
Leading proponents of the subaerial division can imagine an uplift of the continental slope with conditions like those already existing in the Niagara Gorge; a stream flowing across such a plain would deposit little of its load and do slight cutting until reaching the edge. There it would fall into the ocean, washing a channel at the base and tending to cut back. There would be no deep channel in the plain until the falls had already crossed it. This postulation could very well account for the Hudson. That rugged individualist among the deep-sea canyon family runs into the shelf to Ambrose Lightship, the shallow portion being between this point and the 50-fathom curve. At this depth it descends rapidly to 200 fathoms and 20 miles farther out the depth is 475 fathoms. There are also pronounced irregularities in the depth of the valley. But these characteristics do not apply to any of the Georges Canyons, and furthermore, there are so many and they are so close together that “stream piracy” alone would almost certainly enlarge some at the expense of others.
The structure of the Eastern Coastal Plain is fairly well known due to the knowledge of underlying strata. Well borings and seismic technique have given scientists this knowledge, and while the former is impossible on the deep sea, the latter has been used too little for concrete conclusions. However, the dredged rock has been used for comparison and this has brought out some interesting facts.
The strata of the Coastal Plain have a thin edge inshore at the Fall Line, from which they dip gently seaward becoming thicker as they extend over the shelf. Borings have shown the average slope in this direction to be from 40 to 50 feet per mile off the New Jersey coast, over 2,000 feet thick at Atlantic City by one boring; several points in Maryland were more than a thousand without going through them. Yet some 50 miles offshore in this region, the canyons have been cut in like material that is only 300 to 600 feet below sea level, so it seems the incline has been broken somewhere. The rocks might be the crystalline basement or the thickened part of the coastal wedge. To warp these crystallines upward some 2,000 feet to form canyon walls is at direct variance with other geologic data from this locality.
Dredged materials alone have been used to account for the formation of Georges. While rock collected in this manner does not show the thickness of the strata, this sort of proof gives more weight to the sunken plain theory rather than that of iceberg debris or terminal moraine. None of this knowledge contributes an iota of explanation as to the origin of the canyons.
Those who think the canyons formed beneath the sea have plausible ideas, too. One of these involves the application of terrestrial conditions as in the Coastal Plain postulation. Similar gorges exist in Arizona and have been known a long time.
This is not as far-fetched as it seems for the sapping action of underground streams can form gorges of this sort. It is well illustrated in the Bright Angel District of Arizona. The U. S. Geological Survey topographical sheets show a great similarity between the Transept and one of the Georges Canyons while the Bright Angel is very like two others. If the continental slope were elevated some 7,000 feet, conditions on the southern scarp of Georges would be very similar to those at Cameron, Arizona.
Faulting has also been suggested, but learned opinion holds that where this takes place, it is usual to find steps rather than the levels that obtain on the plateau above the valleys. The scars left by landslides are of quite different shape from the steps of the canyon walls.
The action of submarine currents has been studied as a factor. From what is known of the salinity-temperature relation in ocean water, it would be difficult to reconcile a current powerful enough to cut these gorges either flowing at right angles to the coast or restricted to this locality. Even though the presence of fine sediments worked against this explanation, current meters were placed in some of the gorges. With tidal allowance, the greatest strength measured was but slightly over a knot. Canyons of this magnitude could not be cut by a stream of such low velocity.
The proponents of underwater genesis have one more card to play; one that can’t be overlooked. They contend that shifting sea level to allow for canyon formation implies that either the land of the eastern seaboard was much higher at one time or the sea level much lower. The ebb and flow of former ice would account for some change but the canyons demand a relative shift of some 8,000 or more feet. Certain nonconformities on and in the coastal plain indicate that the continental shelf has oscillated somewhat but a shift of this magnitude runs counter to anything so far discovered. The evenness of the depth of the canyons implies a uniform lift of the entire shelf from Georges to Hatteras. Since no part of the emerged coastal plain shows this, it might have been a sectional elevation with the fulcrum at the Piedmont Boundary. Such a long contingent lift with no differential movement would be unusual to say the least.
The alternative of a lowered sea level is equally hard to swallow considering the magnitude called for. This sort of fluctuation would certainly be shown in other geologic features. Looked at literally it approaches the catastrophic.
Whatever the explanation, one of the many natural wonders of our country lies submerged offshore. These canyons are never likely to provoke the “Ohs” and “Ahs” of any tourists except the finny ones that live with Davy Jones. Neither will they provide angle shots for camera enthusiasts or political arguments as National Parks, but for a long time the Canyons of Georges will provide brain teasers for the geologists and many an argument for the sailors who worked on them.