@misc{indiciae47c4c778676d,
  title = {Maps showing natural aggregates of the conterminous United States},
  author = {William H. Langer and Donna A. Belval},
  year = {1983},
  doi = {10.3133/ofr83743},
  url = {https://pubs.usgs.gov/publication/ofr83743},
  note = {Source identifier: ofr83743}
}