@misc{indiciaeecee2028de2b,
  title = {Map showing high-purity silica sand of Middle Ordovician age in the Midwestern states},
  author = {Keith B. Ketner},
  year = {1979},
  doi = {10.3133/mf1097},
  url = {https://pubs.usgs.gov/publication/mf1097},
  note = {Source identifier: mf1097}
}