@misc{indiciae9914e28d489a,
  title = {Stratigraphic significance and petrology of phosphate nodules at base of Niobrara Formation, east flank of Black Hills, South Dakota},
  author = {H. A. Tourtelot and W. A. Cobban},
  year = {1968},
  doi = {10.3133/pp594l},
  url = {https://pubs.usgs.gov/publication/pp594L},
  note = {Source identifier: pp594L}
}