@misc{indiciae952284b1f2b4,
  title = {Preliminary geologic map, District of Columbia},
  author = {A.J. Froelich and John Tilton Hack},
  year = {1975},
  doi = {10.3133/ofr75537},
  url = {https://pubs.usgs.gov/publication/ofr75537},
  note = {Source identifier: ofr75537}
}