@misc{indiciae5177c7a36b98,
  title = {Bedrock exposure map of the Westford Quadrangle, Massachusetts},
  author = {D.C. Alvord},
  year = {1973},
  doi = {10.3133/ofr736},
  url = {https://pubs.usgs.gov/publication/ofr736},
  note = {Source identifier: ofr736}
}