TO LOG THIS EARTHCACHE INCLUDE:
1. The name of this EarthCache on the first line of your email.
2. The number of people in your group.
3. Estimate or measurement of: 1) the horizontal orientation (strike) of the tombstone rock features (number of degrees between 0 (north) and 360 (also north)); and 2) the vertical orientation (dip) of the tombstone rock features (number of degrees between 0 (horizontal) and 90 (vertical), including the dip direction).
4. Which Big Ideas (1-9) are connected (list)?
5. Which GeoPrinciples (1-7) are relevant (list)?
6. 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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TOMBSTONE ROCKS AND FOOTHILLS TERRANE GEOLOGY
Many people wonder about the rock formations in the Mother Lode that look like tombstones, or gravestones (see Figure 1).
Figure 1: View of "typical" tombstone, or gravestone, rocks observed in the Mother Lode.
They can also resemble shark fins poking out of the earth. These rocks are metamorphic rocks, meaning they were formed from pre-existing “parent” rocks – either sedimentary, igneous, or other metamorphic rocks, under the influence of heat and pressure. According to Mary Hill, author of “Geology of the Sierra Nevada”, tombstone rock is:
“metamorphosed volcanic rock that crops out as isolated slabs in the gold country. It is the product of undersea volcanoes of 140 million years ago, now upended and changed by mountain-building forces. A field of such slabs reminded early miners of an untended cemetery, hence the name “tombstone rocks”, “gravestone slate”, or “gravestone schist”.”
The parent rock, from which tombstone rocks formed, is part of the Western Sierra Nevada Metamorphic Belt, in the Foothills terrane. They are likely part of either the Peñon Blanco, Copper Hill, or Gopher Ridge (all volcanic) Formations. These are mapped as volcanic island arc complexes, chains of volcanic islands with lava flows, volcanic ash, and debris flow deposits similar to Japan, Indonesia, or the Philippines today. They were rafted tectonically, on an oceanic plate, toward the western margin of North America over 150 million years ago. They were then accreted, or welded, onto the continent in a subduction zone, along the Bear Mountains Fault Zone. The volcanic island arc rocks underwent regional metamorphism and were subjected to heat, pressure, and the influence of fluids and were, thus, compressed, foliated, and metamorphosed into greenschists and phyllites. These rocks were then uplifted, weathered, and eroded and are now standing on end as tombstone rocks before you. They get their reddish coloring from oxidation of iron-bearing minerals in the rocks.
For this EarthCache, you will estimate or measure the 3D geometric orientation of the tombstone rock features with a smartphone compass app and with a smartphone clinometer app (see Figures 2 & 3).
Figure 2: Smartphone Compass app (download one of your own choice to use for this activity).
Figure 3: Smartphone Clinometer app (download one of your own choice to use for this activity).
Geologists measure the attitude – called strike and dip – of geologic features in order to infer what stresses they have been subject to or to determine what the subsurface geometry is. This is a significant skill in mineral and oil exploration when trying to locate important resources such as minerals, metals, or hydrocarbons (oil, natural gas, coal, etc.) trapped beneath the earth’s surface.
In this part of the world, the orientation of the fractures in rock is an important control on how water (and other fluids) travel through rock. The Royal Mountain King Mine, an abandoned open pit gold mine, just south of the EarthCache location, has had issues with arsenic-rich waters potentially leaving the mine site. The state of California was concerned the water could be contaminating groundwater wells downstream. These wells tap into water that migrates through fractures in the rock. Geologists are trained to assess the orientations of fractures to understand the paths of groundwater flow. The diagrams below can help you to assess the horizontal and vertical orientation of the rock features (see Figures 4 & 5). Once you have located the tombstone rocks, record the 2 estimates.
Figure 4: Strike (horizontal direction of the rock feature) and Dip (vertical direction of the feature). The value for the strike of the layers in this diagram is east-west, which is 90 degrees. North is always 0 (or 360) degrees, while east = 90 degrees, south = 180 degrees; and west = 270 degrees. The approximate dip of the layers in this diagram = 45 degrees toward the north.
Figure 5: Using the right hand rule: 1. Place your right hand on the surface (if the feature you are interested in is visible). 2. Rotate your hand so that your four fingers point downward, toward the dip direction. 3. Finally, extend your thumb on the same plane. The direction which your thumb is pointing to is the direction of the strike.
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 Copperopolis, then by taking Rock Creek Road to the north. The EarthCache is located near an historical monument where you can park your vehicle. To reach the EarthCache, walk north to the GPS coordinates, which should take you to a spot next to a barbed wire fence. The tombstone rocks on are the other side of the fence - please don't climb over the fence onto the private ranch land. You can estimate the strike and dip from the roadway. The area is very hot and dry in the summer time and the best time of year to visit may be during spring when the wildflowers are in bloom.
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 of your email.
2. The number of people in your group.
3. Estimate or measurement of: 1) the horizontal orientation (strike) of the tombstone rock features (number of degrees between 0 (north) and 360 (also north)); and 2) the vertical orientation (dip) of the tombstone rock features (number of degrees between 0 (horizontal) and 90 (vertical), including the dip direction).
4. Which Big Ideas (1-9) are connected (list)?
5. Which GeoPrinciples (1-7) are relevant (list)?
6. 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., “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. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
4. Hill, Mary, “Geology of the Sierra Nevada”: Revised Edition, University of California Press, May 15, 2006, p. 170.
5. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
6. Poulton, T. P., et al, in “The Jurassic of the Circum-Pacific” International Geological Correlation Programme Project 171: Jurassic of the Circum-Pacific; IGCP, IUGS, UNESCO, edited by G. E. G. Westerman, McMaster University, Cambridge University Press, www.cambridge.org/9780521351539, 1992, p. 74-75.
7. Putnam, Roger, (pers. comm.), May 2017, Professor of Earth Science, 11600 Columbia College Drive, Sonora, CA, 95370.
8. 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.
9. Senanayake, Sanuja, “Right Hand Rule in Geology”, 04-01-2013, http://sanuja.com/blog/right-hand-rule-in-geology.
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