@misc{indiciae8c8994f98ead,
  title = {Exploration intensity map of the Upper Cretaceous Niobrara Formation, Denver Basin, Colorado, Nebraska and Wyoming},
  author = {D.K. Higley and D. L. Gautier and R. F. Mast},
  year = {1987},
  doi = {10.3133/ofr87351b},
  url = {https://pubs.usgs.gov/publication/ofr87351B},
  note = {Source identifier: ofr87351B}
}