@misc{indiciae91f14bd9768b,
  title = {The Helderberg group of parts of West Virginia and Virginia},
  author = {F.M. Swartz},
  year = {1930},
  doi = {10.3133/pp158c},
  url = {https://pubs.usgs.gov/publication/pp158C},
  note = {Source identifier: pp158C}
}