Carbonates of the Middle Ordovician Black River Group across the outcrop in southern Ontario have a fabric of elongate particles trending consistently in NW–SE and NE–SW directions. The origin of the fabric is considered primarily resulting from longshore currents in a shallow basin influenced by tidal activity. Insoluble residues constitute a small portion of the rock mass, but contain a wide range of mineral species. Although their distribution is sporadic, suites of minerals east and west of Marmora contain distinctive species. Generally, the detrital minerals seem to reflect the Precambrian source area to the north of the Paleozoic outcrop. A higher concentration of anhydrite to the east is interpreted as indicative of an area of more restricted marine conditions.