USGS Geologic Map viewer and data download
Some Colorado geologic Maps are available for free download through the United States Geological Survey (USGS) National Geologic Map Database (NGMD).
In the fall of each year, the CGS submits newly-produced geologic maps to the USGS NGMD via the STATEMAP Program. However there may be a significant delay before they are available. For the most recent maps and publications, check the CGS store for hundreds of free map downloads.