@misc{indiciae2867a007d961,
  title = {Geologic map showing thickness of the upper Cretaceous Pierre Shale in the north half of the Powder River basin, southeastern Montana and northeastern Wyoming},
  author = {N.M. Denson and M.L. Gibson and G.L. Sims},
  year = {1993},
  doi = {10.3133/i2380a},
  url = {https://pubs.usgs.gov/publication/i2380A},
  note = {Source identifier: i2380A}
}