@misc{indiciaecb7654e1c86d,
  title = {Contour map showing altitude of bedrock surface in southern Philadelphia, Pennsylvania},
  author = {J.B. Graham},
  year = {1950},
  doi = {10.3133/ofr5073},
  url = {https://pubs.usgs.gov/publication/ofr5073},
  note = {Source identifier: ofr5073}
}