This first-reported subsurface examination of a Caribbean shelf-edge feature off St. Croix, indicates: (1) The basic morphology of the St. Croix feature was formed during the Pleistocene, 27,000 years B.P. and older. However, eastern parts of the feature have additional relief formed by mid to later Hocene deep water coral communities (2) An extensive, narrow ridge fronts the feature along most of the southern shelf This ridge was a flourishing shallow water, Acropora cervicornis reef that developed during the early Holocene. It stopped growing from 8000 to 9000 years B.P., about the time the broad shelf itself was submerged by rising sea level. Turbidity derived from erosion of shelf regolith probably killed off the reef. Therefore, throughout the Caribbean continuous Holocene reef building may have been prevented wherever shelf depths are greater than about 10 m. However, in areas where Acropora cervicornis was a major framework builder, reef construction has occurred in the later Holocene from depths greater than 10 m. Previous studies suggest that barrier coral reefs and algal ridges are abundant in the eastern Caribbean where basements are less than 10 m deep. The extent of Holocene barrier formation is thus a function of both shelf depth, which is general!y dependent upon tectonic factors, and factors controlling the ability of Acropora cervicornis to build an extensive framework.