The Pennsylvania Anthracite region contains numerous thick, extensive, low-sulfur coal beds of Pennsylvanian age. These coal beds are the result of the accumulation of swamp vegetation, and deposition of fine- to coarse-grained clastics in a terrestrial, rapidly sinking asymmetric basin, whose source area lay to the southeast of the Anthracite region. The beds in this basin were extensively folded and faulted in Permian-Triassic time as the strata above a basal décollement were thrust northwestward.