Schist, Granite, and Pegmatite EarthCache
Schist, Granite, and Pegmatite
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(other)
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Parking for this EarthCache is in a gravel pull-off area on the east side of the road at N 40 25.088, W 105 21.524. There is plenty of room for parking, and you will want to park your vehicle as far from the road as you can.
From the parking area, walk to the site of the given coordinates. This is about 200’ down canyon (to the north) using the trail on the east side of the road. The intent of this EarthCache is to have you cross the road to examine the rocks on the west side. However, if you feel uncomfortable crossing the road, please only answer question 1 and make your estimate from the east side of the road. This EarthCache is NOT child friendly due to the proximity of the road. Be warned that vehicles tend to move fast and sightlines are shortened in the canyon.
At the site of the given coordinates, look at the road cut on the west side of the highway. In this road cut, schist containing biotite, sillmanite, and muscovite, has been cut by several small white veins of pegmatite that grade into granite. According to Braddock (1976), “The area of biotite-muscovite granite is restricted to this general vicinity, and is probably a late phase of the Silver Plume intrusive complex.” Placing this granite in the Silver Plume intrusive complex allows us to date this rock at about 1.4 billion years old.
At this point, if it is safe, cross the road to get an upclose view of the schist, granite, and pegmatite. Notice the silvery sheen of the schist. This is due to the small mica flakes which are incorporated into this rock. The pegmatites are much lighter in color than the schist, as they grade from white to light pink. The granite here and 100’ to the south has larger crystals of white quartz and pink feldspar, as well as inclusions of mica.
To log this EarthCache, send me an email with the answers to the following questions: 1.) Measure (or estimate) the veins of pegmatite at this location and tell me the width of the largest vein; 2.) Examine closely the mica in the schist at this location and the mica in the granite located about 150’ to the south on the west side of the road. Tell me the differences you see in the mica in these rocks.
Please consider posting photos of yourself, or the local geology, when you log this EarthCache. Photos can be an additional rewarding part of your journey, but posting them is not a requirement for logging this EarthCache, and is strictly optional.
The above information was compiled from the following sources:
1976. Braddock, W.A. Road Log, Precambrian Geology of the Northern and Central Front Range, Colorado; First Day, Northern Front Range Professional Contributions of the Colorado School of Mines, Studies in Colorado Field Geology. No. 8, P. 1-8.
1987. Hutchinson, R.M., and W.A. Braddock. Precambrian Structure, Metamorphic Mineral Zoning, and Igneous Rocks in the Foothills East of Estes Park, Colorado. Geological Society of America Centennial Field Guide – Rocky Mountain Section, 1987.
Thanks to the Roosevelt National Forest for allowing placement of this EarthCache!
Additional Hints
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