Bedrock Geology


Cross section through the Slippery Rock Gorge at McConnells Mill, looking north.

The cross-section through the Slippery Rock Gorge shows the bedrock around McConnells Mill and underneath Moraine State Park. These rocks formed during the Pennsylvanian Period of geologic history (300 million years ago), when a completely different landscape was present, and when what is now Pennsylvania was south of the equator. Rivers heavily laden with clay, sand, and gravel flowed from highlands in what is now eastern Pennsylvania, New Jersey, and Delaware across Pennsylvania. At the mouths of the rivers, this sediment formed large deltas along the shoreline of an ancient sea which covered what is now Ohio and western Pennsylvania. Thick layers of sand accumulated in river channels and as beaches and barrier bars along the coast. High energy waves and currents moved some of the sand into large ripples and dunes. Extensive swamps formed on the river floodplains and behind the beaches. In that quiet water of the swamps, with low energy levels, layers of silt and clay accumulated with mats of thick vegetation (peat). As the river systems changed course or the seas rose to cover the low delta areas, early sediments were buried and new sequences of swamps, sand, and mud formed. Several times, the shallow sea spread and deposited marine clays and muds. As each layer was buried, compaction and cementation changed the soft, loose sediment to solid rock. The peat bogs and swamps became coal. The sand became the Homewood and Connoquenessing Sandstones, and the silts and clays formed the Kittanning, Clarion, and Mercer Formations. The thickest marine lime mud became the Vanport Limestone. About 250 million years ago, as the continents moved northward, the area was uplifted and erosion started to create the landscape as we know it today.

The Vanport Limestone exposed along the shore of Lake Arthur near the Hidden River Bridge of the Glacier Ridge/NorthCountry Trail.                 The Homewood Sandstone exposed along the Alpha Pass/North Country Trail, just south of Alpha Falls.

      Vanport Limestone                                    Homewood Sandstone