@misc{indiciaeff2b6b51a353,
  title = {Superposed deposits of thick coal on the eastern edge of the Illinois Basin and their association with underlying geologic features},
  author = {D. Harper and G.A. Olyphant},
  year = {1991},
  doi = {10.1016/0166-5162(91)90035-h},
  url = {https://pubs.usgs.gov/publication/70016692},
  note = {Source identifier: 70016692}
}