The bedrock at McConnells Mill State Park was formed over 300,000,000 years ago as layers of peat, mud and sand were compressed. This part of Pennsylvania was a coastal area at the time. The layers of bedrock were built into mountains as the continental plates shifted. The layers of bedrock had different strengths. Some of the layers eroded quickly and others eroded at a much slower pace. The various erosion rates account of the variety of geologic features at McConnells Mill State Park. The geology of the park was also heavily influenced by the glacial effects of the ice age: a large glacial lake (water that was held in place by a glacier), broke about 140,000 years ago. This break in the glacial dam caused a dramatic change in the flow of the water. The lake that had previously drained in a northward direction was now draining to the south. The drastic change in direction and very high volume of water flow created Slippery Rock Creek Gorge.