@misc{indiciaef21ce7b1909a,
  title = {A digital resource model of the Upper Pennsylvanian Pittsburgh coal bed, Monongahela Group, northern Appalachian basin coal region, USA},
  author = {L.F. Ruppert and S.J. Tewalt and L. J. Bragg and R.N. Wallack},
  year = {1999},
  doi = {10.1016/s0166-5162(99)00009-9},
  url = {https://pubs.usgs.gov/publication/70021874},
  note = {Source identifier: 70021874}
}