If the land is a Federal Reservation, where complete sovereignty was either never surrendered to the state, or has been given back, then the Federal Government has the same police power as a state would. I've seen a couple of guys with Federal DWIs, because they were arrested on the National Seashore on Padre Island, Texas.
It is not enough that the Federal Government own the land. Walking into a field office owned by the Department of Agriculture with a joint in your pocket is not a Federal crime.
Most of the land in that part of the Rockies is “national forest” and/or under the jurisdiction of the Bureau of Land Management.