Poudre Falls Pegmatite Veins EarthCache
Poudre Falls Pegmatite Veins
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Please park at the pull out on the west side of the highway at this site. Vehicles drive at a high rate of speed on this road, so please be careful at this location. There is no need to cross the road to answer the questions, but if you would like to see the pegmatite veins up close, it would be better to park about 300’ to the south on the east side of the road and walk up to the road cut.
At this location, the Cache La Poudre River drops through a series of steep falls on the west side of the road. On the east side of the road there is a road cut through rock composed of dark metamorphic amphibolite. The lighter veins that cut through the dark metamorphic material are composed of igneous pegmatite which contains large crystals of quartz and feldspar.
Pegmatites are coarse-grained igneous rocks, granitic in nature, that usually contain feldspar, quartz, and mica. Often they occur in dikes or veins as you see here, crosscutting through the native country rock. A close look at the veins will show banding or zoning from the different minerals which are present in the pegmatite. The banding occurs due to the differential temperature, pressure, and chemical composition that was present during the growth of the minerals.
The pegmatite veins were formed around 1.7 billion years ago when the cracks of the amphibolite were filled with fluids during the Early Proterozoic Orogeny. These rocks, some of the oldest in Colorado, were heated between 500 to 700 degrees Celsius, and buried from 7 to 11 miles deep when they formed.
To receive credit for this Earthcache, send me an email with the answers to the following questions:
1.) Estimate or measure the width of the largest pegmatite vein which you can see in this road cut.
2.) What colors do you see in the pegmatite veins?
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:
2007. Chronic H., and F. Williams. Roadside Geology of Colorado, 2nd Ed.
2003. Colorado Geological Survey. Messages in Stone, Colorado’s Colorful Geology. V. Matthews, K. KellerLynn, and B. Fox, Eds.
2004. Hopkins, R.L., and L.B. Hopkins. Hiking Colorado’s Geology
Thanks to the Roosevelt National Forest for allowing placement of this EarthCache!
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