@misc{indiciae41014c33b5c7,
  title = {Contour map of the base of saprolite, Montgomery County, Maryland},
  author = {A. J. Froehlich},
  year = {1975},
  doi = {10.3133/i920c},
  url = {https://pubs.usgs.gov/publication/i920C},
  note = {Source identifier: i920C}
}