In stead, the environment conducive to coal formation moves and migrates,
like sand bars in a stream. The result is that a coal bed can sometimes be a series of horizontal lenses interspersed
in a vertical unit of rock. Coal can also form in one distinct layer, but even then the thickness of the coal is
highly variable. Coal deposits usually have a much smaller regional extent than the Marcellus Shale. This is be
cause they are deposited in areas near streams, lakes, and rivers, which are geographically smaller than shallow
continental seas.