B.Smith & H.Dewey in Mems.Geol.Svy:Sp.Reps.Min.Res.G.B: vol.22: lead and zinc ores in the pre-Carboniferous rocks of West Shropshire and North Wales, London, 1922, state: shales and slates, with layers of extrusive felspathic ash. All Bala Series in the Upper Ordovician.
BGS 1:50,00 geology of Britain online viewer states:
Snowdon Volcanic Group - Mudstone And Siltstone. Sedimentary Bedrock formed approximately 451 to 461 million years ago in the Ordovician Period. Local environment previously dominated by shallow seas. Setting: shallow seas. These rocks were formed in shallow seas with mainly siliciclastic sediments (comprising of fragments or clasts of silicate minerals) deposited as mud, silt, sand and gravel.
Middle Crafnant Volcanic Formation - Felsic Tuff And Felsic Tuffite. Igneous Bedrock formed approximately 454 to 455 million years ago in the Ordovician Period. Local environment previously dominated by explosive eruptions of silica-rich magma. Setting: explosive eruptions of silica-rich magma. These rocks were formed from viscous and highly gaseous silica-rich magma. It rose to the surface, where sudden pressure relief caused explosive volcanic eruptions, producing fragmentary pyroclastic material or ash.
Upper Crafnant Volcanic Formation - Felsic Tuff And Felsic Tuffite. Igneous Bedrock formed approximately 454 to 455 million years ago in the Ordovician Period. Local environment previously dominated by explosive eruptions of silica-rich magma. Setting: explosive eruptions of silica-rich magma. These rocks were formed from viscous and highly gaseous silica-rich magma. It rose to the surface, where sudden pressure relief caused explosive volcanic eruptions, producing fragmentary pyroclastic material or ash.
At eastern / north eastern end of Parc mine and also Hafna mine:
Unnamed Igneous Intrusion, Ordovician - Microgabbro. Igneous Bedrock formed approximately 444 to 488 million years ago in the Ordovician Period. Local environment previously dominated by intrusions of silica-poor magma.
Setting: intrusions of silica-poor magma. These rocks were formed from silica-poor magma intruded into the Earth's crust. It cooled to form intrusions ranging from large, coarse-crystalline, often gabbroic, plutons at depth to smaller, fine to medium crystalline, often basaltic dykes and sills.