@misc{indiciaeae969770c3ec,
  title = {Map showing mineral resources of Montgomery County, Maryland},
  author = {A.J. Froelich},
  year = {1975},
  doi = {10.3133/i920e},
  url = {https://pubs.usgs.gov/publication/i920E},
  note = {Source identifier: i920E}
}