In the process of making alongshore and offshore seismic refraction studies of the geologic structure beneath the submerged Atlantic Coastal Plain from north of the Cape Fear arch in North Carolina to the vicinity of the Florida-Georgia boundary, it was noted that the measurements between Charleston, South Carolina, and Doboy Sound, Georgia, indicated that the basement (pre-Cretaceous) surface sloped towards the continent rather than towards the ocean. This reversal in the normal seaward slope of the basement surface was interpreted as being caused by a basement ridge, and named the Yamacraw Ridge of Meyer and Woollard. This past year it was possible to investigate this area

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more fully and 12 reversed seismic refraction measurements were made in the emerged Coastal Plain area lying between Ridgeland, South Carolina, Walthourville, Georgia, and the coast. These measurements plus the previous seismic measurements in the submerged Coastal Plain area and well data, show that the Yamacraw Ridge is a well defined topographic feature on the buried pre-Cretaceous surface and that it forms the eastern boundary of the northern portion of the South Georgia basin. The ridge has an overall length of about 90 miles and a width of about 25 miles. It strikes about 22° west of south and its axis coincides roughly with that of the present coastline. The ridge originates at the general basement level near Charleston, S. C., and has the form of a spur which plunges to t e south with about 2,000 feet change in elevation along its length. The cross-section relief over most of its length is about 1,200 feet. On the basis of the observed seismic velocity values (16,000-19,500 feet per sec.), the ridge is composed of varying crystalline rock material. On the landward side of the ridge there is a marked basement embayment and local deepening alongside the ridge. This trough-like depression on the basis of the seismic velocity value (15,000-19,000 ft. per sec.) appears to be floored with crystalline rock material. The basement surface at the head of the trough is very irregular with indications of two incised valleys extending southward from the basement plateau area in South Carolina that stands about 3,000 feet above the floor of the trough in Georgia. In vi w of the relief and physical form of the Yamacraw Ridge it is suggested that it is of tectonic origin.

The available well data suggest the ridge was completely buried before the end of Lower Cretaceous time. Because of the discontinuous nature of the seismic horizons in the overlying sediments plus the scarcity of well data it is not clear whether there has been any tectonic movement associated with the ridge since its burial.

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