The stratigraphy of the islands consists of the Middle Pleistocene Owl's Hole Formation, overlain by the Late Pleistocene Grotto Beach Formation, and then the Holocene Rice Bay Formation. However, these units are not necessarily stacked on top of each other but can be located laterally. The oldest formation, Owl's Hole, is capped by a terra rosa paleosoil, as is the Grotto Beach, unless eroded. The Grotto Beach Formation is the most widespread.