The depositional environment of the A and B units of the Upper Silurian Salina Formation in the Appalachian and Michigan basins has been a contentious issue for the past century. In large measure, this is due to the fact that exposures of the Salina Formation are rare and that many published models are based only on data obtained from drill cores. Also, most studies have concentrated on the basins themselves and have largely ignored the intervening area across the positive high of the Algonquin arch.The Domtar Construction Materials gypsum deposit is situated in the Niagara peninsula, on the eastern flank of the Algonquin arch. Stratigraphic correlation from drill cores and underground mapping indicates that the mined gypsum bed is the Salina Formation B anhydrite unit. This is overlain by shales and gypsiferous rocks of the Salina Formation B unit and underlain by shales, dolomite, dolomitic shales, and gypsum of the Salina A-2 unit, dolomitic shales of the Salina A-1 unit, and dolomite of the Guelph Formation. At the Domtar deposit, the Salina A and B units comprise five distinct dolomitic shale to gypsum cycles that increase progressively upward in gypsum content. Underground mapping at mines 2 and 3 reveals that a wide range of facies and textures are present in the mined gypsum bed and that the enclosing sediments were deposited in a relatively anoxic subaqueous environment. Although primary evaporite textures have been modified by both dehydration of gypsum to anhydrite on burial and later rehydration of anhydrite to gypsum on post-Pleistocene uplift, they are consistent with subaqueous deposition in a coastal salina or lagoon.It is suggested that the Salina A and B units of the Niagara peninsula were deposited in a sedimentary setting similar to the modern evaporitic environment of the northern Sinai. Here, gypsum is precipitating in coastal lagoons and salinas, adjacent to the Nile estuary. In the same area, evaporites are also present in sabkhas and continental playas. Thus, comparison of the Salina Formation with the modern environment of northeastern Egypt may also explain the apparently contradictory evidence, from other studies, of adjacent subaqueous, supratidal, and subaerial evaporitic environments in the Salina A unit of the Michigan and Appalachian basins.