SIMILARITY OF SAND IN RIMS AND IN BOTTOMS OF BAYS
The reported similarity between material in the rims and material in the bottoms of the bays appears to be an inference based on occasional records of sand encountered in drilling through the bay deposits. If the inference were correct for the bays as a whole, it would not seem to be significant of the origin of either bays or rims since, no matter how these were formed, sand in the rims could slump or wash or be blown into the adjacent depressions.
It should be fully understood, however, that there is no consistent similarity between material recently accumulated in the bottoms of the bays and material in the rims. Both types of deposit are abundantly described in many reports of soil surveys throughout the bay country, and the distinctions between them are made clear. The two are mapped independently, named and described as different soil types, and represented on maps by different colors. The differences observed are not merely those due to the presence of decaying vegetation within the marshy bays, although the presence of clay, freshwater shells, and other deposits does reflect the special conditions under which the bay deposits were formed.
If Melton and Schriever meant to imply that the material underneath the peculiar deposits of the bays, the “bedrock” material in which the oval craters were excavated, is essentially similar to that found in the rims of the bays, then observations go throughout the bay country as a whole indicate a situation altogether different from that reported by these authors for the Myrtle Beach area. Exposures in walls of many bays, in near-by road cuts or shallow wells, in the slopes of adjacent valleys cut below the levels of the bays, and in the walls of artificial ditches dug for the purpose of draining the bays (14) almost invariably reveal a strong contrast between the material composing the Coastal Plain deposits in which the craters are found and the superficial sands of the bordering rims. Most frequently the Coastal Plain deposits consist of sandy or clayey loam, usually red but occasionally buff, pink, or purple, or a mottling of these colors, more rarely, of gravel, orange-colored sand, iron-cemented red or brown sands, marl, clay, and other beds. Even where the Coastal Plain surface deposits are of sand, one can usually distinguish readily between the slightly clayey, less perfectly sorted sands of the plain and the peculiar coarse, loose, white, gray, or faintly buff sands of the oval rims. Only when recently wind-blown sand forms a coating on the Coastal Plain sand is one in doubt, and then the doubt relates to the possible topographic development of a rim rather than to the distinction between rim and plain material when both are exposed in section.
In the Myrtle Beach area, conditions are somewhat unusual because here the bays occur on a beach plain composed of sand extensively reworked by wave action. But even here the parallel beach ridges of sand alternate with swales in which silt, clay, and marsh deposits are sufficiently abundant to cause marked contrasts in vegetation and to give muddy roads in which cars easily may be bogged. Yet the rims consist uniformly of clean white sand; and the silt and other material which should have been thrown out to help form the rims, were these the product of meteoritic impact, are not found in rim deposits. Prouty, (15) who supports the meteoritic hypothesis, has recognized the contrast between Coastal Plain deposits and the pure sand of the rims, and has suggested that rain wash may have re moved liner material from the rims, thus bringing about a contrast in ejected rim deposits and underlying Coastal Plain deposits which did not originally exist. There can be no doubt that such separation does take place on the Coastal Plain surface, giving a superficial layer of sand quite unlike the underlying loam or similar material from which it was derived. Weathering could, furthermore, easily remove organic material from rims thrown up in such an area as the Myrtle Beach plain. But there are serious objections to applying Prouty’s suggestion to the rims in general. If rims of red loam five, ten, and fifteen feet high could be leached of their finer material and otherwise altered to give white rims of pure coarse sand, adjacent Coastal Plain ridges or swells of the same material should similarly be leached; but these latter remain unaltered close to the surface. Commercial sand pits excavated deeply into the rims of white sand never, within the writer’s experience, reveal an inner core of loam or other material differing from the true rim of sand. Drainage ditches cut through rims reveal not a gradation from white sand above to red loam below but a sharp contact, at the level of the adjacent plain, separating pure white sand above from distinctly different Coastal Plain deposits below. That the rims surrounding the oval craters are uniformly composed of coarse white, gray, or buff sand, regardless of what may be the color and composition of the Coastal Plain beds in which these craters are formed, is a fact fully established by abundant field evidence. This fact seems to be fatal to any hypothesis which would explain the rims as portions of the Coastal Plain deposits ejected by the impact of meteorites. Melton and Schriever in some measure appreciated the force of this objection to the meteoritic hypothesis, for they wrote: “In at least one respect the authors are not convinced that the facts are adequate to substantiate theory. In the rims thus far examined there is a noteworthy absence of bed-rock fragments larger than sand grains.”(16) But they pointed out that such fragments might be found in the rims of bays not yet examined, that perhaps such fragments should not be expected from the unconsolidated, water saturated clastic sediments of the Coastal Plain, and that, even if they did occur, the time since formation of the bays might have been sufficiently long for weathering to reduce such fragments to their constituent elements.
With respect to the first point, it may be noted that studies by Prouty and others, and the present writer’s own extended examinations of bays in seventeen counties of three states, fully substantiate the observation of Melton and Schriever that “there is a noteworthy absence of bed-rock fragments” in the rims. Indian arrowheads and associated small fragments of Hint and other materials used in making such arrowheads, evidently of recent human importation, are the only fragments larger than sand grains thus far reported.
With respect to the second point, it should be noted that the Coastal Plain deposits include, close to the surface in many places as well as in depth, layers of well-consolidated, iron-cemented sandstone, beds of limestone, coquina, silicified limestone and shell rock, layers of chert, and other hard material. Large fragments of bedrock are found on the slopes of shallow valleys and ravines in areas where bays are present, and there appears to be no reason why they should not have been ejected with other rim materials if the rims were the product of meteoritic impact.
With respect to the third point, the long period of weathering invoked by Melton and Schriever is based on their assumption that the bays are relatively ancient, antedating the period of Coastal Plain terracing and the formation of the Myrtle Beach plain with its parallel beach ridges and swales. In an earlier chapter we have demonstrated that this assumption of great age is invalid and that the bays are of later date than the surface on which they are found. Consequently, the opportunity for weathering has been more limited than was supposed. Furthermore, much of the material forming consolidated beds in the Coastal Plain is of a nature to resist weathering very effectively. Silicified shell rock found near the surface in valley Walls and occasionally encountered in the marshes of baylike depressions is extremely resistant. Layers of even more resistant chert or of silicified foraminiferal limestone are found close to the surface in parts of the bay region, and the hard fragments are collected by the natives for use in stone walls or as decorations for flower gardens. All things considered, there seems to be no reason to doubt that, if the bay rims were the product of meteoritic impact, there would be an abundance of bedrock fragments in the ejected material. Thus the meteoritic hypothesis fails to explain satisfactorily the striking contrast between the composition of the Coastal Plain beds and the composition of the rims, and the absence of bedrock fragments in the latter.
(14) Bays showing red loam or other Coastal Plain beds exposed in their walls are too numerous to record but not all of these possess rims. Examples possessing rims and showing contrasted Coastal Plain deposits exposed in bay walls are: several small bays near Shell Bluff village, center of Greens Cut quadrangle, Ca.»S_C.; bay northeast of McBride Church, Hilltonia quadrangle, Ca.-S.C.; large bay east of Blackville and hay 3 miles south of Elko, Williston quadrangle, S.C.; Swallow Savanna Bay, Peeples quadrangle, Ga.-S.C.; Coles Bay, Dial Bay, and Woods Mill Bay, Mayesville quadrangle, S.C. Examples of bays possessing white sand rims and known to he located in red loam or other contrasted Coastal Plain deposits revealed in neighboring road cuts, shallow wells, or valley walls are: the oval bays on either side of the Little Salkehatchie River, Olar quadrangle, S.C.; Sand Hill Bay, 2% miles north of Elim, Florence County soil map, S.C.; Mossy Bay, southeast of Blenheim, Marlboro County soil map, S.C.; Dial Bay and associated bays previously mentioned, Mayesville quadrangle, S.C.; small bay southwest of Sumter, on road to Pinewood, S.C. Examples of bays in which the walls of recently cut drainage ditches reveal red loam or other Coastal Plain deposits overlain by sharply contrasting white sand of rims are the bay northwest of McBride Church, Hilltonia quadrangle, Ga.-S.C., and the Devil’s Woodyard Bay near Springerville, north of Darlington, S.C.
(15) William F. Prouty, personal communication.
(16) A. Melton and William Schriever. The Carolina “Bays” – Are They Meteorite Scars? Jour. Geol., 41:52~66, 1933. See p. 65.