Porous grainstones deposited as offshore carbonate sand shoals are important hydrocarbon reservoirs, but little is known about their internal architecture and its effect on reservoir heterogeneity. The Middle Silurian Amabel Formation, exposed along the Niagara Escarpment of southern Ontario, provides an unparalleled opportunity to examine such a deposit in three dimensions with photomosaics and ground-penetrating radar. The Amabel is a dolomitized bioclastic grainstone unit up to 25 m thick, composed mainly of crinoid ossicles. It thins and grades westward to thin-bedded argillaceous dolostones. Although previously assumed to be reefal, the undulating and oblique bedding that dominates the internal geometry was caused by vertical and lateral accretion on hummocky topography and scour surfaces. Some scours are huge—as much as 15 m deep—and attest to the dramatic nature of sediment movement by storms and hurricanes. Ground-penetrating radar successfully detects the large-scale bedding pattern and, in conjunction with outcrop observations, has potential for mapping the three-dimensional architecture of ancient lithified deposits.