The shallow coral reef and algal ridge systems on the eastern shelf of Croix are described and mapped in some detail. Based on present morphology, a section through the barrier reef in a ship channel, various sand probes and C14 dating, Holocene growth patterns of the reefs are determined and a model of Holocene evolution developed. Based on many drill cores through the algal ridges, C¹⁴ dating and paleoecologic relative to modern ridge and reef surfaces on St. Croix, growth kerns during the late Holocene are also developed for the algal ridges. Lithophyllum congestum, Porolithon pachydermum, and several Neo-oniolithon species are the primary algal ridge builders on St. Croix. L. congestum requires turbulent water and high light intensity to achieve the branching form which characterizes its occurrence in the algal ridges. also, coralline accretion rates of 3-6 mm/year necessary for ridge construction are achieved only if intensive parrot fish and Diadema grazing is prevented by consistent and intensive wave action. A dead coral surface or pavement at a depth of 0 to 2 m, will be colonized by crustose corallines and in turbulent water, can develop progressively by coralline algal accretion into an incipient mound, a high boiler and eventually by boiler fusion, into a linear algal ridge. Off open, easterly shores in St. Croix, coral reefs, on building to surface, develop algal ridges. The present morphology of the ridge-complex has developed primarily as a result of the control exerted pre-existing shelf and bench levels and changing rates of Holocene sea level rise on coral-coralline and grazer ecology. Special emphasis is placed here on the importance of pre-existing shelf level in determining form and developmental stage of ridge-reef systems.