Standardization of Silurian stratigraphic terminology (exclusive of the Cayugan) in southern Ontario and western New York, including the type section in the Niagara Gorge, discloses that the strata are best suited to a tripartite division, both from a faunal and physical standpoint. Accordingly, the Silurian system is divided into the Medina, Clinton, and Niagara groups, the three comprising the Ontarian series.

Medinan time includes an early short transgressive history represented by the deposition of the Whirlpool sandstone, the Fish Creek shale, and the Manitoulin dolomite, followed by a longer regressive history represented by the deposition of the Cabot Head shale and the Grimsby sandstone. Continental sedimentation occurred during lower Whirlpool deposition and upper Grimsby deposition. The change from transgressive to regressive tendencies is marked by a phosphate zone in the upper Manitoulin dolomite of the Niagara Gorge; the Manitoulin-Cabot Head contact is essentially a time-line. The faunal and lithic characteristics of the Medinan stratigraphic units are discussed in detail.