1. Map showing potential sources of gravel and crushed-rock aggregate in the Boulder-Fort Collins-Greeley area, Front Range urban corridor, Colorado [cartographic material] Yale Public content 1974. 2013-05-28T15:54:15Z. 2013-06-14T17:58:03Z. Colton, Roger B. (Roger Burnham), 1924-. U.S. Geological Survey. English. Public. Colorado, Boulder County, Larimer County, and Weld County.