As a fracture grows above or below the Marcellus Shale, it encounters other lithologies, and this can
play an important role in determining the ultimate extent of a hydraulic fracture. Lithologies often change
abruptly, representing events or changing environments in Earth history; such distinct boundaries between lithologies are often more weakly cemented together than the layers, or beds, within each lithology. Because of this, these boundaries are considered planes of weakness. Layers, or beds, within a lithology can also be planes of weakness, but these are usually more cohesive than boundaries.