Weston Pass Fault

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Container Type: Ammo
Code Location: Inside container
Access Info: This cache is located on USFS property.
Winter Access: Same, dependent on road conditions.

Along the Weston Pass Fault system, Paleozoic dolomite (500-300 million years ago) abuts ancient metamorphic rock that may be up to 2 billion years old. The geocache lies roughly on the border or “contact” between the younger dolomite (similar to limestone) and the older metamorphic rock. This contact is described as an “unconformity” because there is a significant difference in age between the two types of rock and the older metamorphic rock lies on top of the younger dolomite. The Weston Pass Fault system—fractures in the earth’s surface that shift due to plate tectonics—is the cause of this unconformity. In this case, the older rock units were thrust up and over the younger rock. The Weston Pass Fault system consists of many fractures throughout this area. During Park County’s mining boom, several of these fractures were mined for silver. Abrupt changes in rock type and age indicate the location of the faults. Notice how the dolomite outcrops (gray-colored rock) give way to metamorphic rock as you move along the trail.

Take the Weston Pass Road (County Road 22) west from Highway 285 past the Weston Pass Campground until you reach an unmarked 4WD road on your right. Most vehicles should park across the road and hike up to the cache. See if you can spot the two rock types along your way!