As you embark or continue your journey to discover and explore beautiful and historic rocks, waterfalls, peaks, creeks and other wonders, please keep in mind that these places need to remain wild and protected so that they may be enjoyed by others for generations to come. Please be diligent in respecting these sites by doing the following:
- Please keep trash with you at all times, do not leave it behind in these pristine places.
- Bury human waste 6 inches deep, make certain you are at least 50 paces away from any water source and PLEASE bring your toilet paper and sanitary items back out with you.
- Keep food and all scented items on your person at all times.
- Support wildlife by allowing them to find their own food, do not feed them.
- Allow plants to grow and water to stay clean by staying on trails, bike paths and roads.
Thank you, Yosemite Wilderness Management
Yosemite Valley is famous for its granitic rocks. Much of the rock in the Yosemite region is a type of granitic rock called granodiorite which is often mistaken for granite. Even among the granodiorite there are different types. Prior knowledge of what a glacial erratic is needed to answer the logging question.
Granitic rocks include granite, granodiorite, tonalite, and diorite. All are intrusive igneous rocks and contain lighter-colored minerals of quartz, potassium feldspar, and plagioclase feldspar, and darker-colored minerals are mostly biotite and hornblende The relative percentage of each mineral determine the type of granitic rock it is. The rock at this location is granodiorite.
Granodiorite is made up of biotite (black), hornblend (dark gray), plagioclase (off-white), and quartz (translucent gray). Even within rocks classified as granodiorites, geologists can differentiate different intrusions of magma based on the size of the grains of each of the minerals in the rock and the relative percentage of each mineral. The two granodiorites present at this location are the Half Dome Granodiorite and the Cathedral Peak Granodiorite (both part of the Tuolumne Intrusive Suite).
| Half Dome Granodiorite |
Cathedral Peak Granodiorite |
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The Half Dome Granodiorite is made up of medium-grained biotite and hornblend with some parts having large crystals of white plagioclase (though not on this outcrop).
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The Cathedral Peak Granodiorite medium-grained biotite with large blocky crystals of pink potassium feldspar. |
One granodiorite is the glacial erratic while the other is the bedrock. For a refresher on glacial erratics see Pothole Dome - Glacial Features
Logging questions:
- The text "GC5P6TK Half Dome and Cathedral Peak Granodiorites" on the first line
- The number of people in your group.
- Using the pictures as a guide to identify the different granodiorites, determine which type of rock is the erratic and which is the bedrock.
The following sources were used to generate this cache:
- http://www.yosemite.ca.us/library/geologic_story_of_yosemite/rocks.html
- Matthes 1962. Marks of Time: Yosemite and the High Sierra. EL CAPITAN MORAINE AND ANCIENT LAKE YOSEMITE. Sierra Club Bulletin, January, 1913, pages 7-15. http://www.yosemite.ca.us/library/matthes/lake_yosemite.html
- CLYDE WAHRHAFTIG. GEOMORPHOLOGY OF THE YOSEMITE VALLEY REGION, CALIFORNIA. Bulletin 182Geologic Guide to the Merced Canyon and Yosemite Valley, California U.S. Geological Survey, Menlo Park, California and University of California, Berkeley, California http://www.nps.gov/history/history/online_books/geology/publications/state/ca/cdmg-bul-182/sec4.htm
- http://www.yosemite.ca.us/formation/
- Roadside Geology of Yosemite Valley. Road Guide is adapted from "The Living Geology of the Sierra Nevada, Great Valley and Coast Ranges of California" edited by Garry Hayes. http://virtual.yosemite.cc.ca.us/ghayes/roadside.htm
- Leavitt, Amanda. 2008. Glaciers – A Force Like No Other Evolution of Yosemite: A Tale of Glaciers Past. January 1. http://www.indiana.edu/~sierra/papers/2007/leavitt.pdf
- http://www.nps.gov/yose/learn/nature/granite.htm