@misc{indiciaefbd3e2cf5540,
  title = {Geologic map of the Grand Canyon 30' x 60' quadrangle, Coconino and Mohave Counties, northwestern Arizona},
  author = {G. H. Billingsley},
  year = {2000},
  doi = {10.3133/i2688},
  url = {https://pubs.usgs.gov/publication/i2688},
  note = {Source identifier: i2688}
}