@misc{indiciaebeccf4473729,
  title = {Bedrock exposure map of the Billerica Quadrangle, Massachusetts},
  author = {D.C. Alvord},
  year = {1973},
  doi = {10.3133/ofr735},
  url = {https://pubs.usgs.gov/publication/ofr735},
  note = {Source identifier: ofr735}
}