Outcrop exposures along the Attawapiskat River in the Hudson Bay Lowlands of northern Ontario clearly reveal the morphology of Silurian (upper Llandoverian) reefs of the Attawapiskat Formation and the relationships between reef cores, flanking beds, and post-reef beds. These relationships indicate that the reefs had a syndepositional relief of at least 8–10 m. The relief of the reefs is indicated by debris-flow lenses encased within flanking beds, a debris-flow fan at the base of a reef, thin reef-flanking beds truncated against a vertical reef face, penecontemporaneous brachiopod beds on the flanks of reefs, and three large slide blocks that apparently slid over the side of a reef.Reefs of the Attawapiskat Formation are largely limestones with a wide range of lithologies, from stromatoporoid-, coral-, and cement-rich boundstones to alga–cement-rich boundstones. Thick, laterally extensive Nuia grainstone beds, a product of widespread Nuia monocultures in supratidal to intertidal ponds, are present above the reefs. The most important diagenetic processes were early marine cementation (predominantly radial-fibrous calcite), shallow burial diagenesis, and pervasive neomorphism.Schematic reconstructions of the depositional history of the Attawapiskat Formation in outcrop, constructed from observations of outcrop relationships, show a stratigraphic succession that was controlled by relative-sea-level changes. Reefs growth was terminated by a relative-sea-level fall, but subsequent minor relative-sea-level fluctuations resulted in alternating deposition of supratidal to shallow subtidal sediments for a short time before the final retreat of the Silurian seas from the Hudson Bay Platform; only supratidal evaporite facies are present in the remainder of the Silurian section above the Attawapiskat Formation.