Exposures of Silurian to lowermost Devonian strata in the Porcupine River region consist of an unnamed carbonate unit and the Road River Formation. Petrographic studies indicate that these rocks display facies representative of five depositional environments: basin, open sea shelf, deep shelf margin, open platform, and restricted shelf.

The unnamed carbonate unit, exposed in the Linear Ridge area, is 390 ft (126 m) thick and records a history of restricted shelf to basinal sedimentation. Stratigraphic relations and paleontological studies suggest a Middle to Late Silurian (Ludlovian) age for this unit.

The Road River Formation is Late Silurian (Ludlovian) to Early Devonian (Lochkovian) in age and is exposed near the confluence of the Porcupine-Salmontrout Rivers and downstream along the Lower Ramparts. It consists of 30-190 ft (10-61 m) of graptolitic shale with interbeds of siliceous limestone. Petrographic studies of the shales are interpreted to reflect deposition in a basinal setting, whereas the siliceous limestones represent deep shelf-margin debris flows derived from nearby, coeval shallow-water shelf environments. Together, the unnamed carbonate unit and the Road River Formation represent a shelf to basin transition on a carbonate ramp that transcends the Silurian-Devonian boundary.

Petrographic examination of these rocks reveals that they are susceptible to a wide range of diagenetic processes, including (1) micritization, (2) neomorphism, (3) syntaxial overgrowths, (4) pressure solution (stylolitization), (5) trapping of dried hydrocarbons, (6) tensional stress (calcite veining), and (7) silicification.

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