Like the rest of this part of the
Appalachian Mountains, Spruce Knob began to form with the breakup of
Pangea I. The African Plate separated from the North American Plate opening the Proto-Atlantic Ocean. The North American Plate stretched and thinned, allowing it to backfill with a shallow inland sea. About 50 million years later, with the
Taconic Orogeny, the two plates reversed course and began to move towards each other. Mid-ocean subduction created a volcanic arc (now known as the
Blue Ridge Mountains) which eventually collided with the North American Plate. The arc fused onto the continent and the land to the west was uplifted. The accumulation of shells and other hard parts of marine organisms (made of CaCO3,
calcium carbonate) at the bottom of the shallow inland sea cemented into a layer of
Greenbrier Limestone. The shallow inland sea began to retreat with the uplift. This caused fine grains of mud and silt to settle out and lithify into a layer of Mauch Chunk Shale on top of the Greenbrier Limestone. As the Blue Ridge eroded, rivers carried sediment down to the low-lying areas that formed a layer of
Pottsville Conglomerate on top of the shale. The large boulders on the summit are remnants of this layer, and outcrops of both Mauch Chunk Shale and Greenbrier Limestone can be found lower on the mountain. When the North American and African Plates finally collided around 250 mya, it caused a massive uplift that folded and faulted these layers of sedimentary rock. Spruce Knob was originally in the bottom (syncline) of one of these folds, but over time cracks in the Pottsville Conglomerate in the higher elevations allowed it to erode quickly, and the softer layers of shale and limestone were quick to follow. This left Spruce Knob as the highest point in the landscape. Spruce Knob (Spruce Mountain) is the westernmost extent of this intense folding and faulting. To the west, the
Allegheny Plateau is composed of more gently sloping hills and dendritic drainages. == Climate ==