@misc{indiciae6acd294e0985,
  title = {Coal regions and fields of the United States in digital form},
  author = {S.J. Tewalt and C.L. Oman},
  year = {1993},
  doi = {10.3133/ofr93396},
  url = {https://pubs.usgs.gov/publication/ofr93396},
  note = {Source identifier: ofr93396}
}