The exposed sedimentary rock layers in the Pittsburgh area were deposited in a deltaic system. A delta is a roughly trianglular pile of sediment up to thousands of square kilometers in area which builds up where a river enters an ocean or a lake. The best-known example in the United States is the Mississippi River Delta. 300 million years ago, western Pennsylvania was a lot like Louisiana is now.
Deltas gradually build outward, forming a very flat surface just barely above sea level.
When the river reaches this very flat area, it forks into several meandering channels. The tremendous weight of sediment in a delta can cause the area to sink. The rocks which make up a continent are more buoyant than those that make up the ocean floor, and so will not sink as deep or as fast under the weight of a delta. An area may sink at a rate equal to the rate of deposition, producing a thick accumulation of similar rocks. If the delta sinks faster or slower than the rate of sediment accumulation, the setting and rock type will change over time.