The gas-bearing sandstones of the Medina Group in western New York are the basal Whirlpool Sandstone and the Grimsby Sandstone. Toward the west, these are separated by the Cabot Head Shale and the Manitoulin Limestone. Farther east, the Grimsby overlies and interfingers with the Whirlpool. The reservoir is underlain by the Ordovician Queenston Shale and is capped by shales of the Silurian Clinton Group. The depositional sequence of the Medina Group may be summarized as a marine transgression toward the east that yielded the Whirlpool, Cabot Head, and Manitoulin formations, followed by westward progradation of the deltaic Grimsby formation.

Aspects of internal stratigraphy noted in subsurface studies may be correlated with anomalies in units much higher in the Silurian section and with disturbances on the Silurian-Devonian unconformity. Repetitive adjustments of large-scale structural features are inferred. These changes mark the transition from the westward-facing depositional front of the Ordovician to development of the isolated basins that dominated the Silurian Period.

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