@misc{indiciae216b084e2c7b,
  title = {Preliminary bedrock geologic maps of the Westford and Billerica quadrangles, Middlesex County, Massachusetts},
  author = {Donald Clayton Alvord},
  year = {1975},
  doi = {10.3133/ofr75387},
  url = {https://pubs.usgs.gov/publication/ofr75387},
  note = {Source identifier: ofr75387}
}