The collision of the tectonic plates carrying present day North America and Africa formed the Appalachian Mountains. What are now Pennsylvania and West Virginia straddled the equator, producing a tropical climate. Western Pennsylvania and West Virginia were part of the foothills of the enormous Appalachian Mountain chain that stretched from Alabama to Newfoundland (and beyond - as far as Scandinavia, once you put all the puzzle pieces of the tectonic plates back together!). The bedrock in the foothills was gently folded into broad, shallow wrinkles miles across. Numerous streams, probably derived from the streams that deposited the bedrock, cut down through the folded bedrock layers.