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GC133J7
Dillon PinnaclesType: Earth | Size: Not chosen 
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| Terrain: By: Johnicache @ | Hide Date: 05/22/2007 | Status: Available Country: United States | State: Colorado Coordinates: N38° 27.914 W107° 15.848 | Last updated: 08/30/2019 | Fav points: 50 
Site is at a pull-out on Highway 50 in Curecanti National Recreation Area. Fully Handicapped accessible. No fees associated with this site. During the Cenozoic era, approximately 26 to 35 million years ago, the area that is now Western Colorado was subjected to extensive volcanic activity. None of the volcanoes that existed then are present today, but evidence of there existence is all around. When a volcano erupts to the extent that it empties its chamber and collapses, it forms what is called a “caldera”. Southwest of the Dillon Pinnacles, the remains of a caldera which was uplifted by the Gunnison Uplift can be observed. The caldera now forms the peaks of the rugged San Juan Mountains, such as Uncompahgre Peak – the sixth highest peak in Colorado. Volcanic activity in the West Elk Mountains, to the north of Dillon Pinnacles, covered this area with volcanic ash and lava. This lava, along with mud flows, ash, and rock fragments, formed a sedimentary rock called “breccia”. The West Elk Breccia settled at a depth of approximately 600 feet over the harder igneous and metamorphic rock of the Dillon Pinnacles. The breccia is composed of hardened molten rock (magma) “clasts” and an ashy material called “matrix”, formed by the mud flows and ash, that cement the clasts together. The volcanic activity of the San Juans to the south later covered this area with incandescent ash which retained sufficient heat to fuse together and form a hard “cap rock” over the tops of the mesas in this area. The harder igneous rock clasts and “cap rock” are more resistant to erosion than the ashy matrix of the breccia. The result is the formation of the interesting spires and pinnacles of Dillon Pinnacles. To claim this cache you must email me the answer to the following question which is addressed by the interpretive sign at the coordinates: What is the scientific name of the “cap rock” that was formed over the mesas of this area by the incandescent ash deposits from the San Juan volcanic activity?
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Thanks for the earthcache.
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