Holy Cross is EXREMELY difficult! A Rubicon model will have trouble if not be incapable and a stock vehilce will absolutely not be able to do it.
Colorado is one big "park." A significant portion of our state is public land, open to all as long as you abide by the rules. There are enoug old mining roads that even a resident could spend a life time attempting all of them.
I suggest looking up "Colorado Backroads and 4-Wheel Drive Trails" (ISBN 10: 0-9664976-6-x or 13: 978-0-9664976-6-3) by Charles A. Wells. It is a thorough book of Colorado trails.
I also suggest looking up the National Geographic maps for the area. They will show foot trails, vehicle, multi use, etc... For Avon you are looking at map 108 or 121. They are tear resistant and water resistant. Colorado trails are big enough to get lost, turned around, and generally screwed...
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