@misc{indiciaec18051f70826,
  title = {Map showing potential sources of gravel and crushed-rock aggregate in the greater Denver area, Front Range Urban Corridor, Colorado},
  author = {D. E. Trimble and H. R. Fitch},
  year = {1974},
  doi = {10.3133/i856a},
  url = {https://pubs.usgs.gov/publication/i856A},
  note = {Source identifier: i856A}
}