Pawnee Buttes - Ogallala Formation EarthCache
Pawnee Buttes - Ogallala Formation
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This EarthCache is found on the Pawnee National Grassland, a part of the Arapaho & Roosevelt National Forests and Pawnee National Grassland, and is located on the trail on the east side of the West Pawnee Butte. This trail is open year-round, though please note that the Overlook Trail and the Lips Bluff Trail are closed from March 1 – June 30 each year for nesting raptors. Cultural and paleontological features on the Grassland are protected by law and must not be removed.
The Pawnee Buttes are a small erosional remnant of the High Plains Escarpment, which can be seen to the east. The Buttes are formed from the lighter White River Formation on the bottom, and the more resistant Ogallala Formation forming the cap rock on the top. The White River Formation is composed of mudstones and siltstones, and was formed during the Oligocene Epoch around 25 – 30 million years ago. The Ogallala Formation was formed from sediments deposited 5 – 10 million years ago during the erosion of the Rocky Mountains during the Miocene Epoch. Sandstones and conglomerates are the components of the Ogallala Formation.
The coordinates will take you to a large boulder of Ogallala Formation material that has broken off and rolled down the Butte. This occurs when rocks that are resistant to erosion (the harder sandstones and conglomerates of the Ogallala Formation) overlie weaker rocks (the White River Formation). The weaker rock on the flank of the hillside erodes out, but the resistant cap rock serves as a barrier which slows erosion. Eventually, undercutting of the softer rock allows large boulders to erode out and cascade down from the weathering cap rock.
To receive credit for this EarthCache, you must:
1) Measure the largest particles that you can find in the boulder of conglomerate at this site. Email me with this information.
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:
2003. Colorado Geological Survey. Messages in Stone. Matthews et al., eds.
2004. Hopkins R.L., and Hopkins L.B. Hiking Colorado’s Geology.
2004. U.S. Forest Service. An Introduction to the Pawnee Buttes.
Thanks to the Pawnee National Grasslands, for allowing placement of this EarthCache!
Additional Hints
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