@misc{indiciae41708324c416,
  title = {Contour map of the bedrock surface, Ludlow Quadrangle, Massachusetts},
  author = {Clark J. Londquist},
  year = {1973},
  doi = {10.3133/mf503a},
  url = {https://pubs.usgs.gov/publication/mf503A},
  note = {Source identifier: mf503A}
}