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Contour map of georges bank

Submarine valleys of Georges Bank Contoured by Dr. Francis P. Shepard, "the father of marine geology"
from the NOAA Photo Library




the origin of the submarine valleys along the southern edge of georges bank


contour map of georges bank (From the U.S. Coast and Geodetic Survey Association Of Field Engineers Bulletin No. 3, June 1931, pp. 87-90.)

(I) N. H. Heck, U. S. Coast and Geodetic Survey

Chief, Division of Terrestrial Magnetism & Seismology.

(II) F. P. Shepard

Professor of Geology, University of Illinois

With reference to remarks about the mysterious nature of submarine valleys and other structures on Georges Bank appearing in a recent bulletin of Field Engineers, I would like to call attention to the fact that in a recent book, "The Sons of the Earth" by Prof. Kirtley Mather of Harvard, the great ice sheet is shown as extending off to Georges Bank. It is therefore probable that this was terminal moraine. It would seem, however, that if streams were to be gouged out, the area must have been above water at the time. I have never seen any statements about the condition of this area. I understand that it is generally held that much of the continent was depressed under the load and that the otherwise unexplainable earthquakes of the region are related to readjustment. Recent activity as shown by the Grand Banks Earthquake of November 18, 1929, might indicate that a process of adjustment is still going on.

(II) F. P. Shepard

Professor of Geology, University of Illinois

Shortly after the occurrence of the Grand Banks earthquake of November 18, 1929, Captain Bone of the Steamship Transylvania reported that he had found deep water in a position which, according to the chart, should have been well within the margin of Georges Bank. He came to the conclusion that the earthquake had altered the sea bottom at this point. However, the position was about 500 miles from the epicenter of the earthquake and 340 miles from the nearest cable break, which naturally led one to doubt whether there was any connection between the earthquake and the supposed change in the shelf margin.

During the summer of 1930 the United States Coast Survey started operations in a resurvey of Georges Bank. Crossing the vicinity where the change of depth had been reported, they found a submarine valley penetrating the shelf edge for about seven miles (see plate). Since this valley lies across one of the approaches to Nantucket Lightship from Europe, it assumes considerable significance from the point of view of navigation.

The discovery of the valley did not prove that there had been a recent displacement of the edge of the shelf. Examination of the old soundings made by the Coast Survey about 1874 showed that the soundings were so far apart that the valley might easily have been missed. On the other hand, vessels coming in from Europe have been taking soundings in this general vicinity for years, so that it seems a little strange that it was not till after the earthquake that the depression was suspected. Captain N. Johnson, as commander of the North German Lloyd Steamer Columbus, had taken a particularly large number of soundings across this area. The Columbus is equipped with the Fathometer echo sounding device, and this was regularly set in operation as the edge of the shelf was approached. As a result, Captain Johnson became so familiar with the topography that he could tell his position very accurately in approaching Nantucket Lightship. In the Marine Review for June, 1928, some of his soundings are plotted on a chart and curves have been drawn to show the topography. Several of the lines of soundings on this chart cross the valley area, but there is no indication that such a valley existed. Accordingly, it seems to the writer that there is some reason to believe that the valley developed at the time of the earthquake.

If we assume for the sake of argument the contemporaneity of valley formation and the earthquake, we must look for some explanation of the feature in terms of the earthquake. If a block of the earth's crust at this place had dropped suddenly to the extent of about 2,000 feet (the depth of the valley), there should have been at least two important observable effects. In the first place, the shaking of the crust should have made a record on the various seismographs of the vicinity showing epicenters nearer than that of the Grand Banks disturbance. Secondly, a large tidal wave (tsunami) should have swept the adjacent coasts. Since there is no evidence of either of these features, this explanation appears to be unlikely. On the other hand, it has been observed that landslides are a frequent accompaniment of earthquakes, even when the shocks originated at considerable distances. Such phenomena accompanied the San Francisco and Yakutat Bay earthquakes, for example. Also, many of the changes in Sagimi Bay found after the great Japanese earthquake are probably a result of submarine landslides. Some of the cables which were broken at the time of the Grand Banks earthquake were found buried deeply in sediment showing the effect of landsliding. Since this earthquake was felt as far west as Albany, it probably produced very appreciable shaking in the outer part of Georges Bank. When conditions are favorable for landsliding, a small shock is often the trigger effect which sets the slide in motion. Since slides do not commonly produce any record on seismographs, the objection to the down faulting suggestion cited above does not apply to landsliding. Also, if the slide moved rather slowly, as do many slides on land, it would not have produced surface waves perceptible at any distance from the center of disturbance.

If we assume, on the other hand, that the valley was not developed at the time of the earthquake, the possibility that it was due to an earlier landslide may still be entertained. If it is a landslide excavation, one might find a hummocky zone beyond the mouth of the depression, such as usually occurs where a mass of alluvium has slid out of a gully. As only four lines of soundings cross the valley, there are not sufficient soundings to show such topography. However, the outermost line has what appears to be a hill along its course. This might have been caused by a landslide. Such hills are found in association with many of the landslides of the west. A more detailed survey would probably make it possible to test this landslide suggestion.

One might ask why a landslide should leave a depression which is so suggestive of a river valley in shape. Speculation as to the earlier history of the continental shelf in this area may serve as an answer to this question. In the first place, there are many reasons for believing that the continental shelf here, as off many other coasts of the world, has been elevated at some time during the past allowing deep entrenching by rivers. Many submarine valleys, such as that off the Hudson, are still found on the shelves and are too long and narrow to appear to be explainable by landsliding. Possibly one of these valleys was cut in the region under discussion. We can assume in turn that it was submerged at a later period. Still later came the development of the great continental ice sheets. It happens that Georges Bank is in line of continuation with the terminal moraine of the ice sheet which extends north and east through part of Cape Cod and the Island of Nantucket. Also the topography of the inner portion of Georges Bank is suggestive of a moraine with its hummocky character. The finding of various igneous rock fragments on the surface of the bank is also corroborative evidence that the bank is in part a moraine. This would indicate that the front of the Great Glacier stood on the bank. As a result, great quantities of debris should have been washed out from the melting ice onto the outer part of the continental shelf. The sediments distributed in this way should have filled pre-existent depressions in the outer shelf. It is quite likely that the deeper part of the valleys would have received muddy sediments which would in turn have been covered with coarser debris. After the glaciers had retreated, conditions would have become quite ideal for slides. Outside was the steep continental slope, inside a valley filled with loose sediment and plastic mud at the base. Given a shaking of the ground, and it is easy to understand how a great stream of sediment might have glided out of the filled valley accumulating on the continental slope below.

Hundreds of submarine valleys have been found cutting the continental shelves of the world. Many of these are clearly of a different character from the one under discussion. Some of the valleys, for example, are broad troughs of great length like the St. Lawrence trough and the valley heading into the Bay of Fundy. Others of these valleys appear to be quite similar to the Georges Bank valley. Unfortunately, the position of the soundings in most of these cases can not be relied upon since they were made before the adoption of radio acoustic means of location. The recent surveys along the west coast of the United States have shown that there are a considerable number of valley-like indentations in the shelf margin. The valley outside of the Columbia River is quite comparable with the Georges Bank example. Directly beyond this valley is an elevation which might perhaps be interpreted as a result of the sliding of material out of the valley and accumulating as a ridge beyond....

(This article continues with discussion of the origin of the Georges Bank canyon by another investigator with Shepard's rebuttal. It is accompanied by a print entitled "SUBMARINE VALLEY ON GEORGES BANK." The first view shows soundings in the vicinity of the valley. The second shows the valley in cross section. Below this print is a reproduction of a map showing the Northeastern United States, and captioned as follows:

During the Pleistocene Period the northeastern states were invaded by five successive ice sheets. The last of these, the Wisconsin ice sheet, is represented here as being at its maximum extent. The areas covered with drift of the preceding glacial states are also shown. (Based on maps prepared by Ernst Antevs in 1929)

From SONS OF THE EARTH by Dr. Kirtley F. Mather, published by W. W. Norton & Company.)

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