A reconnaissance study has been made of the geology of the insular shelf south of St. Thomas and St. John, Virgin Islands. High-resolution seismic-reflection profiling reveals that a buried northeast-southwest striking bedrock ridge controls the shallow structure of the eastern portion of the shelf. This ridge is thought to be related to the Virgin Islands pluton farther north. Bedrock in the western shelf portion is more deeply buried and is thought to be a flatter terrain, probably lithologically similar to rocks exposed on St. Thomas.
The sediment cover appears to consist principally of carbonate grains in the sand-size range. This sand is covered to various degrees by carbonate nodules at depths below about 34 m, but at shallower depths the nodules are not present. Three large areas of sandy bottom were mapped 1) west of Brewers Bay, 2) near Buck Island, and 3) off south-central St. John. Variations in the amounts of land-derived particles, organic matter, and silt/clay sized material were mapped in these bodies.