Frenchman Mountain Great Unconformity EarthCache
Frenchman Mountain Great Unconformity
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An unconformity is a gap in the rock record – a period of time when erosion was occurring. Then much younger rocks were deposited on top of the eroded surface of the older rock. The Great Unconformity was named by Clarence Dutton in 1882 to describe the dramatic exposures he studied in the Inner Gorge of the Grand Canyon. This Earthcache in the Frenchman Mountain area east of Las Vegas is a more western exposure of that famous geologic feature and requires significantly less effort to visit. The coordinates mark the beginning of the Great Unconformity Interpretive trail. This site was developed by the Citizens for Active Management of the Frenchman-Sunrise Area (CAM) and the Bureau of Land Management. Interpretive panels were developed and placed, with considerable effort, on the trail to educate visitors about the geological significance of this area. Sadly, the panels were senselessly destroyed by vandals. I do recommend that prior to finding this Earthcache visit Dr. Stephen Rowland’s Frenchman Mountain Great Unconformity web site (visit link) where you can read and print out the information that was on the destroyed interpretive panels and bring it along.
The Interpretive panel base that remains at the coordinates marks the location of an exposed portion of the Great Unconformity. Extending to the west are Precambrian age igneous granites and metamorphic schist and gneiss of the Vishnu Group that are about 1.7 billion years old. These rocks made up the core of an ancient mountain range. To the east of the marker and on top of the Precambrian granite lie the sedimentary rocks of the Cambrian Tapeats Sandstone formation, deposited about 520 million years ago. These sedimentary rocks were deposited in a shallow-water marine environment. This contact between the Tapeats and the Vishnu represents a gap of 1.2 billion years in the rock record, or roughly 25% of Earth’s geologic history. Place your hand on that contact and try to imagine what must have occurred during that length of time. This information has been reviewed and changes made as suggested by Dr. Stephen Rowland.
To log this Earthcache:
1. Send me an email with a description of the differences you see between rocks of the sedimentary Tapeats Sandstone and the igneous Vishnu Group.
2. Post a photo of yourself and your GPSr with the Great Unconformity visible behind you when you log your find (Optional).
References:
Tingley, J.V. et al. Geologic Tours in the Las Vegas Area. Nevada Bureau of Mines and Geology Special Publication 16.
Rowland, Stephen, Frenchman Mountain Great Unconformity web Site, (visit link)
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