While much of the Devonian rock surrounding the Marcellus Shale is also shale, there are many layers
of silt, limestone, and sandstone, as well. Even the different shales surrounding the Marcellus are unique
in their composition. These different kinds of rock layers are called lithologies, and each lithology has its own
unique set of physical characteristics that react differently to the pressures that cause hydraulic fractures. Each
lithology has been deposited under slightly different marine and terrestrial conditions, which can affect its
grain size and provenance (where the grains came from and what types of grains are present), composition of
the mineral glue cementing grains together, and the quantity of organics present in the shale. For example,
some layers contain evidence of hurricane-force storms, and others record times of little or no sediment deposition.