@misc{indiciae67ec81ff8e14,
  title = {The Helderberg Group and the position of the Silurian-Devonian boundary in North America},
  author = {J.M. Berdan},
  year = {1964},
  doi = {10.3133/b1180b},
  url = {https://pubs.usgs.gov/publication/b1180B},
  note = {Source identifier: b1180B}
}