@misc{indiciae91b95fd9693c,
  title = {Map showing contours on the base of saprolite, District of Columbia},
  author = {A.J. Froelich},
  year = {1975},
  doi = {10.3133/ofr75539},
  url = {https://pubs.usgs.gov/publication/ofr75539},
  note = {Source identifier: ofr75539}
}