Here's why:

* Geological History: Pennsylvania's geology is heavily influenced by the Appalachian Mountains. The formation of these mountains involved the deposition of massive amounts of sediment (like mud, sand, and organic matter) in shallow seas.

* Shale Formation: This sediment compacted and solidified over time, forming sedimentary rocks like shale.

* Prevalence: Shale layers are found across much of Pennsylvania, particularly in the Appalachian Plateau region.

While shale is the most common, you'll also find other rock types in Pennsylvania, including:

* Sandstone: Found in areas where sand was deposited.

* Limestone: Formed from the accumulation of marine organisms.

* Coal: Found in layers associated with ancient swamp environments.

* Igneous Rocks: While less common, there are some igneous rocks in Pennsylvania, especially in the southeastern part of the state.