TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. What is the hardness of the rock: A) lower than 3; B) between 3 and 5.5; C) greater than 5.5.
4. The rock is most likely: A) marble; B) quartzite.
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples are relevant (list)?
7. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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COUGAR ROCK GEOLOGY
Cougar Rock is a fairly well known hiking destination among locals in the Arnold area and is located along the Arnold Rim Trail (ART). The EarthCache is sited on a promontory overlooking much of the lower elevations of the county with spectacular views on a clear day. The rock here is in the Calaveras Complex (see Figure 1), a subduction complex, composed of mostly argillite (hardened, or lithified, slightly metamorphosed mud and clay), quartzite (quartz sand grains fused together), and chert/metachert (hard, usually dark, microcrystalline, silica-rich rock). It also contains lesser amounts of metamorphic schist, phyllite, slate, and lenses of recrystalized limestone and dolomite metamorphosed to marble. Finally, there are minor marine metasedimentary and metavolcanic rocks that have been mapped. Most of the complex formed offshore in warmer seas and relatively deeper water as evidenced by the paucity, or lack, of coarse-grained rocks – lots and lots of mud and clay being eroded and deposited along the continental slope at the time.
The marble of the Calaveras Complex is composed of calcite and dolomite, carbonate minerals that dissolve readily. The body of marble, the largest of its kind in the Sierra Nevada, forms a long, thin, fairly continuous belt, stretching from the town of Volcano, in Amador County, to south of Sonora, in Tuolumne County, over 65 miles. The marble is mined in Columbia by Blue Mountain Minerals and their geologist, Carey Haughy, found fossils resembling Amphipora, an important rock-building invertebrate organism, which was a type of stromatoporoid (sponges that are similar to modern day spaghetti coral). This indicates the rock initially formed during Devonian time (~416 to 358 million years ago) as a limestone reef-like deposit, in a shallow, warm sea. The limestone has largely been altered to marble, recrystallized from the limestone. The reef-like structure(s) once fringed a volcanic island (part of an island arc complex) that was subducted, then accreted onto the western edge of North America during the Jurassic period, approximately 175-150 million years ago.
Figure 1: Geologic map of Calaveras County showing the Calaveras Complex. It ranges from 8 to 30 miles across and covers a significant portion of the county.
Following deposition of the sediments, the rock was then metamorphosed by subduction, uplifted, faulted, and fractured. The Calaveras Complex is a group of rocks that form part of the Western Sierra Nevada Metamorphic Belt and extend from the northern Sierra southeastward through Calaveras and Tuolumne Counties. If the complex had been tectonically accreted onto North America, it would have been called a terrane – a fragment of crust from one plate sutured onto another plate. However, it was not labeled a terrane, like the Northern Sierra (which includes the Shoo Fly Complex), the Don Pedro, and the Foothills terranes, because it generally formed in place and was not rafted in, like these other island arc complexes. Most of the Calaveras Complex consists of fine-grained metamorphic rocks, whose protoliths, or parent rocks, were deeper continental slope deposits, with lesser island arc pieces that were incorporated into the sediments along the subduction zone complex.
To complete this part of the EarthCache you will need to hike to the GPS coordinates, then observe and locate a sample of the rock outcrop, and determine whether the rock is carbonate or silicate. To do this, you will need a pocket knife, or other piece of hardened steel. The knife will help you to be able to assess the hardness of the minerals in the rock. The Mohs Hardness Scale was developed by Friedrich Mohs in 1812 in Germany. The scale is used in mineralogy to determine one of the 7 physical properties of minerals, called hardness. Hardness has been used for thousands of years, dating back to the Greeks, and is simply the ability of one mineral to scratch another. Mohs ranked 10 commonly found minerals in order of increasing hardness, from talc = 1, to diamond = 10. The scale is shown below in order of increasing hardness (see Figure 2):
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond
Figure 2: Hardness scale showing a fingernail (H = 2.5), penny (H = 3.5), nail (H = 4.5), knife blade (H = 5.5), and streak plate (H = 6.5).
In the Sierra Foothills, there are two common (mainly) white minerals that form many of the light colored rocks found. The minerals are calcite and quartz. One way to quickly and easily distinguish the two is by assessing their hardness. Calcite = 3 and quartz = 7 on Mohs’ scale. Gold miners were interested in finding quartz veins in the foothills since gold and quartz were found to be associated with one another, therefore, this would have been a useful tool for miners trying to distinguish between a calcite vein and a quartz vein. To complete this EarthCache (and learn to distinguish calcite from quartz) you will need to test the hardness of a piece of the whitish colored rock at the outcrop.
Since it is impractical to bring the whole Mohs hardness scale kit into the field, geologists have developed a simpler method that you will use, to determine hardness using the following common objects:
• fingernail (hardness = 2.5)
• copper penny (hardness = 3)
• glass plate or steel knife (hardness = 5.5)
• steel file (hardness = 6.5)
The following principles should also be taken into account when measuring hardness:
1. “That which scratches is harder than that which has been scratched.”
2. “Objects of equal hardness can be scratched with difficulty.”
Once you have located a rock sample, use your pocket knife (or other steel object, like a nail) and try to scratch it. Assume the rock is made of all one type of mineral (i.e. either quartz or calcite). Observe whether the knife scratches the rock, or whether the rock is harder than the knife. If the knife is harder, try to scratch it with your fingernail (hardness = 2.5). If your fingernail cannot scratch the rock, then try using a penny to narrow down the range of hardness. If the range is between 3 and 5.5, then the mineral making up the rock is likely calcite (or it could be dolomite, a calcium magnesium carbonate). Both calcite and dolomite make up the marble found in the Calaveras Complex. If, however, the rock cannot be scratched by the knife, then the hardness is greater than 5.5. If you have a steel file handy, use that to try to scratch the knife. If not, you still have enough information to solve the problem – quartz is harder than 5.5, while calcite and dolomite are both softer. Therefore, the rock will likely be quartzite if you can’t scratch it with your knife. Note your observations for the EarthCache.
After making your observations, look out over the landscape to the west and notice the relatively horizontal layering of the younger superjacent series volcaniclastic sedimentary rocks (generally called the Mehrten Formation) that overlie the older subjacent series Calaveras Complex rocks. The source volcanoes for the Mehrten Formation erupted from east of Ebbetts Pass less than 10 million years ago, then eroded over geologic time and their sediments filled the river valleys toward the west. Those deposits have been preserved as the ridges you now see, due to inverted topography. Enjoy your hike back to the parking lot!
Geoscientists have to be skilled in several different areas. They need to be somewhat proficient in: math, physics, chemistry, biology, surveying, geospatial technologies (GPS, GIS, remote sensing, and webmapping), lab technologies (microscopes, spectrometers, etc.), computer use, and more. Career opportunities in the earth sciences include: geologist, hydrologist, mapping/surveying and geospatial technologist, watershed analyst, mineralogist/mining engineer, petroleum geologist, environmental scientist, natural resources scientist, and more. If you are interested in learning more, take a course at your local community college to get started.
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EARTH SCIENCE BIG IDEAS
The Earth Science education community put together 9 “Big Ideas” for the Earth Science Literacy Initiative (ESLI), shown below. Their purpose was to highlight the main concepts and ideas a person should understand to be literate in the earth sciences:
An Earth-science-literate person:
• understands the fundamental concepts of Earth’s many systems
• knows how to find and assess scientifically credible information about Earth
• communicates about Earth science in a meaningful way
• is able to make informed and responsible decisions regarding Earth and its resources
Which of these Big Ideas below do you think are most relevant to this EarthCache?
Earth Science Literacy Project:
1. Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet.
2. Big Idea 2: Earth is 4.6 billion years old.
3. Big Idea 3: Earth is a complex system of interacting rock, water, air, and life.
4. Big Idea 4: Earth is continuously changing.
5. Big Idea 5: Earth is the water planet.
6. Big Idea 6: Life evolves on a dynamic Earth and continuously modifies Earth.
7. Big Idea 7: Humans depend on Earth for resources.
8. Big Idea 8: Natural Hazards pose risks to humans.
9. Big Idea 9: Humans significantly alter the Earth.
For more details see: Earth Science Literacy Initiative
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GEOPRINCIPLES
There are several fundamental principles, developed over time, that guide geological reasoning and critical thinking, listed below. Read each short description, then use your best judgement to determine which principle, or principles, best relate to this EarthCache.
7 Principles in Geology:
1. Superposition – the oldest strata are at the bottom of the sequence
2. Original Horizontality - layers of sediment are originally deposited horizontally
3. Lateral Continuity - layers of sediment initially extend laterally in all directions
4. Faunal Succession - fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances
5. Law of Intrusive Relationships - the geologic feature which cuts another is the younger of the two features
6. Uniformitarianism - the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe
7. Catastrophism - the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope
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LOGISTICS AND SAFETY
This site can be reached via Highway 4 to Arnold, then by navigating to the parking area GPS coordinates. Please note that the US Forest Service road is unpaved and may be closed during portions of the year in winter months due to snow and mud. After parking at the trailhead (elev. ~=3,900 ft.), you will need to hike approximately 2 miles, climbing approximately 300 feet in elevation on the way to the EarthCache (elev. ~=4,200 ft.). The EarthCache is located at Cougar Rock with a spectacular view and overview of a portion of Calaveras County looking west.
Visitors to this site should plan ahead and prepare by:
• Knowing the regulations and special concerns for the area you are planning to visit (obeying laws that prohibit collection or destruction of artifacts);
• Carrying a map and a GPS unit and/or compass;• Staying on existing roads and trails;• Staying away from any/all mine shafts and adits;
• Planning for extreme weather, hazards, and emergencies;
• Being aware that cell phones DO NOT usually work in the rural areas away from the major highways;
• Leaving your travel plans with a responsible party, including the date and time of your return;
• Being aware of any natural hazards associated with the region (e.g. poison oak, rattlesnakes, mosquitoes, cliffs/steep slopes, etc., etc);
• Carrying a full-size spare tire, extra food, water, and warm clothing;
• Following the “Tread Lightly" and "Leave No Trace” philosophy.
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TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. What is the hardness of the rock: A) lower than 3; B) between 3 and 5.5; C) greater than 5.5.
4. The rock is most likely: A) marble; B) quartzite.
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples are relevant (list)?
7. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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Note: For a brief summary of the geologic history of the Central Sierra, see this EarthCache:
Dragoon Gulch EarthCache
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REFERENCES
1. Busby, Cathy J., Andrews, G.D.M., Koerner, A.K., Brown, S.R., Melosh, B.L., and Hagan, J.C., 2016, “Progressive derangement of ancient (Mesozoic) east-west Nevadaplano paleochannels into modern (Miocene–Holocene) north-northwest trends in the Walker Lane Belt, central Sierra Nevada”, Geosphere 12, p. 135-175, 2016, http://www.geosphere.gsapubs.org.
2. Busby, Cathy J., Koerner, Alice, Hagan, Jeanette, and Andrews, Graham, 2012, “Sierra Crest graben: a Miocene Walker Lane Pull-apart in the Ancestral Cascades Arc at Sonora Pass”, in, N. Hughes and Garry Hayes (eds), “Geological Excursions, Sonora Pass Region of the Sierra Nevada”, Far Western Section, National Association of Geoscience Teachers field guide, p. 8-36.
3. Cronin, Vincent and Dennis G. Tasa, Laboratory Manual in Physical Geology, 11th Edition, AGI American Geological Institute, NAGT - National Association of Geoscience Teachers, Tasa Graphic Arts, Inc., Pearson Publishing, ©2018.
4. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
5. Haughy, Carey, October 1996, Geologist, Blue Mountain Minerals, 24599 Marble Quarry Rd., Columbia, CA, 95310, (209) 533-0127, pers. comm.
6. Higgins, Chris T., 1997, “Mineral Land Classification of a Portion of Tuolumne County, California, for Precious Metals, Carbonate Rock, and Concrete-grade Aggregate”, (with assistance on digital processing from students and staff, Columbia College, Columbia, California, and staff, Tuolumne County Planning Department), California Dept. of Conservation, Division of Mines and Geology, DMG open-file report ; 97-09.
7. Hill, Mary. 2006. Geology of the Sierra Nevada. University of California Press revised edition, Berkeley and Los Angeles, California 468 pp.
8. House, M. R. (n.d.). Devonian Period. Retrieved June 02, 2017, from https://www.britannica.com/science/Devonian-Period#ref585568.
9. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
10. Mohs scale of mineral hardness. (2017, May 30). Retrieved June 02, 2017, from https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness.
11. Putnam, Roger, (pers. comm.), May 2017, Professor of Earth Science, 11600 Columbia College Drive, Sonora, CA, 95370.
12. Schweickert, Richard, 2006, “Accretionary Tectonics of the Southern Part of the Western Sierra Nevada Metamorphic Belt” (modified from a 1999 guidebook article by Schweickert, Girty, and Hanson), in J. Tolhurst (ed), “Geology of the Central Sierra”, National Association of Geoscience Teachers Far Western Section Fall Conference field guide, p. 55-95.
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