Abstract Fourteen cores from deeper, shelf-edge reefs on St. Croix, USVI, record an impressive ability to build substantial thicknesses of carbonate material, both horizontally and laterally. At Salt River submarine canyon, over 24 m of Holocene reef material has been added laterally to a reef now in 30 m of water. Most of that thickness is caused by the accumulation of displaced rubble and larger talus blocks from shallower reefs above. At Cane Bay, the amount of in-place framework decreases with depth, while the actual rate of reef accretion increases. Like at Salt River, much of this accretion was caused by the incorporation of debris derived from shallower water. Both of these modern reefs are substantially or wholly collections of displaced reefal material derived from somewhere upslope. With the percentage of framework down near 30%, and much of that being out of place, it would clearly be difficult to distinguish these as true reefs in the ancient based on core data alone.