1. Geologic map of the Silverton and Howardsville quadrangles, southwestern Colorado [cartographic material] Yale Public content 2000. 2002-06-01T04:00:00Z. 2016-07-21T21:15:56Z. Geological Survey (U.S.). The Survey ; Information Services [distributor]. English. cartographic image. Public. Colorado, San Juan County, and San Juan County.