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.Prouty, "Carolina Bays andTheir Origin." Bulletin of the Geological Society of America, LXIII (1952), 167-224.Peculiar elliptical depressions, or "oval craters," locally called "bays," are thickly scattered overthe Carolina coast of the United States and more sparsely over the entire Atlantic coastal plainfrom southern New Jersey to northeastern Florida.These marshy depressions are numbered inthe tens of thousands and, according to the latest estimate, their number may reach half amillion.1Measurements made on more prominent ones, seaward from Darlington, show that the largerbays average 2200 feet in length, and in single cases exceed 8000 feet.A remarkable feature ofthese depressions is their parallelism: the long axis of each of them extends from northwest tosoutheast, and the precision of the parallelism is "striking." Around the bays are rims of earth,invariably elevated at the southeastern end.These oval depressions may be seen especiallywell in aerial photographs.Any theory as to their origin must explain their form, the ellipticity ofwhich increases with the size of the bays; their parallel alignment; and the elevated runs at theirsoutheastern ends.In 1933 a theory was presented by Melton and Schriever of the University of Oklahoma,according to which the bays are scars left by a "meteoric shower or colliding comet." ^ Sincethen the majority of the authors who have dealt with the problem have accepted this view, and ithas found its way into textbooks as the usual interpretation.8 The authors of the theory stressthe fact that, "Since the origin of the bays apparently cannot be explained by the well-knowntypes of geological activity, an extraordinary process must be found.Such a process issuggested by the elliptical shape, the parallel alinement, and the systematic arrangement ofelevated rims."The comet must have struck from the northwest."If the cosmic masses approached this regionfrom the northwest, the major axes would have the desired alinement." The tune when thecatastrophe took place was estimated as sometime during the Ice Age.The bays are "filled to aconsiderable extent by the deposition of sand and silt, a process which doubtless occurred whilethe region was covered by the sea during the terrace-forming marine invasion of the Pleistocene[glacial] period."4 But the possibility was also envisaged that "the collision took place" through"the shallow ocean water during the marine invasion." The swarm of meteorites must have beenlarge enough to hit an area from Florida to New Jersey.2 F.A.Melton and W.Schriever, "The Carolina Bays Are They Meteorite Scars?" Journalof Geology, XLI (1933).3 Cf.Johnson, The Origin of the Carolina Bays, p.4.4 Melton and Schriever, Journal of Geology, XLI (1933), 56.Some critics disagree with the idea that the bays originated in the Ice Age or "are relativelyancient," and place their origin in a more recent time.'* The craters were produced by meteoricimpact, either by direct hits or by explosion in the air close to the ground, thus causing theformation of vast numbers of depressions.Some of the bays, it is assumed, are on the bottom ofthe ocean.It was also stressed that "a very large number of meteorites have been discovered inthe southern Appalachian region, in Virginia, North and South Carolina, Oeorgia, Alabama,Kentucky, and Tennessee.'* *The Bottom of the AtlanticIn the fall of 1949, Professor M.Ewing of Columbia University published a report on anexpedition to the Atlantic Ocean.Explorations were carried on especially in the region about theMid-Atlantic Ridge, the mountainous chain that runs from north to south, following the generaloutlines of the ocean.The Ridge, as well as the ocean bottom to the west and to the east,disclosed to the expedition a series of facts that amounts to "new scientific puzzles.*"One was the discovery of prehistoric beach sand.brought up in one case from a depth oftwo and the other nearly three and one half miles, far from any place where beaches existtoday." One of these sand deposits was found twelve hundred miles from land.Sand is produced from rocks by the eroding action of sea waves pounding the coast, and by theaction of rain and wind and the alternation of heat and cold.On the bottom of the ocean thetemperature is constant; there are no currents; it is a region of motionless stillness.Mid-oceanbottoms are covered with ooze made up of silt so fine that its particles can be carried suspendedin ocean water for a long.time before they sink to the bottom, there to build sediment.The oozecontains skeletons of the minute animals, foraminifera, that live in the upper5 Johnson, The Origin of the Carolina Bays, p.93.e Cf.C.P.Olivier, Meteors (1925), p.240.l M.Ewing, "New Discoveries on the Mid-Atlantic Ridge," National Geographic Magazine, Vol.XCVI, No.5 (November 1949).THE BOTTOM OF THE ATLANTIC93waters of the ocean in vast numbers.But there should be no coarse sand on the mid-ocean floor,because sand is native to land areas and to the continental shelf, the coastal rim of the oceanand its seas.These considerations presented Professor Ewing with a dilemma: "Either the land must havesunk two to three miles, or the sea once must have been two or three miles lower than now.Either conclusion is startling.If the sea was once two miles lower, where could all the extrawater have gone?"It is regarded as an accepted truth in geology that the seas have not changed their beds with theexception of encroachment by shallow water on depressed continental areas.Thus it wasdifficult to accept the startling conclusion that the bottom of the ocean was at some time in thepast dry land.But there was another surprise in store for the expedition.The thickness of the sediment on theocean bottom was measured by the well-developed method of sound echoes.An explosion isset off and the time it takes for the echo to return from the sediment on the floor of the ocean iscompared with the time required for a second echo to return from the bottom of the sediment, orfrom the bedrock, basalt or granite."These measurements clearly indicate thousands of feet ofsediments on the foothills of the Ridge.Surprisingly, however, we have found that in the greatflat basins on either side of the Ridge, this sediment appears to be less than 100 feet thick, afact so startling." Actually, the echoes arrived almost simultaneously, and the most that couldbe attributed in such circumstances to the sediment was less than one hundred feet of thickness,or the margin of error."Always it had been thought the sediment must be extremely thick, since it had beenaccumulating for countless ages.But on the level basins that flank the Mid-Atlantic Ridge oursignals reflected from the bottom mud and from bedrock came back too close together tomeasure the time between them [ Pobierz całość w formacie PDF ]