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. Using the scale given, what is your estimate for the rock’s grain size?
a. Coarse sand to clay (0.5 - < 0.004 mm)
b. Pebbles to granules (64 – 2 mm)
c. Cobbles to boulders (> 64 mm)
d. Silt to boulders (0.004 to > 64 mm)
4. Which of the following best describes the sorting of the material in the outcrop?
a. Poorly sorted
b. Well sorted
5. Which of the following best describes the rounding of particles in the outcrop?
a. Angular
b. Sub-rounded
c. Well-rounded
6. Which Big Ideas (1-9) are connected (list)?
7. Which GeoPrinciples are relevant (list)?
8. 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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TRAIL OF THE GARGOYLES GEOLOGY
Trail of the Gargoyles (TOG) is a fairly well known hiking destination among locals in the Strawberry area and is located off of Herring Creek Road, or Forest Road 4N12, in the Stanislaus National Forest. The EarthCache, sited on the edge of a bowl-shaped ridge, overlooks the Middle Fork of the Stanislaus River watershed.
During much of the Cretaceous (145–66 million years ago) and Paleogene (66–23.03 million years ago) Periods, Konigsmark (2003) states:
“the Sierra Nevada was the site of a large mountain range that extended into Nevada and covered an area much larger than the present-day Sierra Nevada. The crest of these ancestral mountains was far east of the crest of the present-day Sierra. As the ancestral mountains were uplifted, they were also being eroded. Most of the erosion products from these ancestral mountains were carried into the Great Valley, where they formed thick layers of sedimentary rocks that now underlie much of the Great Valley.
During the Oligocene, these volcanoes in central Nevada erupted rhyolitic ash, which was deposited as layers of tuff (light, porous rock formed by consolidation of volcanic ash).
Konigsmark (2003) goes on to summarize more of the geologic history of the Sierra:
“During Miocene time, volcanic material continued to be ejected from volcanic centers along the crest of the Sierra. Eventually, the central and northern Sierra Nevada was covered with a thick blanket of andesitic volcanic rocks known as the Mehrten Formation. These rocks include andesite conglomerates, welded tuffs, lava flows and lahars. While the Mehrten Formation was being deposited, new river channels were formed from time to time in the thick blanket of volcanic rocks that covered the Sierra. The positions of these channels changed often as the old channels were clogged by new volcanic rocks.”
The younger andesitic volcanic strata of the Mehrten Formation were recently mapped, intensively studied, and reinterpreted by Dr. Cathy Busby and her graduate students from U.C. Santa Barbara between 2006 and 2016. These andesitic rocks include: 1) intrusive plugs and lava flows; 2) block and ash flow tuffs; 3) debris flows; 4) streamflow deposits; and 5) high potassium rocks. Their high-resolution mapping has clarified the relationships between these different lithofacies (Note: a lithofacies is a body of rock with specified physical characteristics (grain size, mineralogy, etc.) – distinct from neighboring rock). The Mehrten Formation has generally described the group of younger andesitic rocks covering the older granitic rocks in the Sierra Crest region. As our geologic knowledge base has grown, the picture has become more clear, accurate, and precise. Figure 1 shows Sonora Pass stratigraphy and the relationships between the different lithofacies mapped near Sonora and Ebbetts Passes. The Mehrten Formation is now subdivided into the Relief Peak and Disaster Peak Formations. The Stanislaus Group has different characteristics and isn’t grouped into the Mehrten by Busby and coworkers. Note that these rocks all overlie the Valley Springs Formation, rhyolite ignimbrites (volcanic pyroclastic ash flow and pumice deposits). This stratigraphic subdivision is based on the current understanding that the Little Walker Volcanic Center was active, just east of Sonora Pass (Figure 4 in the Dragoon Gulch EarthCache (link here)), during the late Miocene, along a transtensional fault system in the Walker Lane Shear Zone. The crust was pulled apart, allowing magma to rise to the surface in a series of vents, which opened up and produced the different lithofacies observed in the mountains today. At Trail of the Gargoyles you will observe several lithofacies associated with the Mehrten/Relief Peak/Disaster Peak Formations.
Figure 1: Sonora Pass Stratigraphy. The Relief Peak Formation, Stanislaus Group, and Disaster Peak Formation make up the Mehrten Formation. The formations consist of andesites and potassium-rich lava flows.
Intrusive domes and block and ash flow tuffs are found closer to the Sierra Crest. These are coarser grained rocks one would logically expect to see proximal (nearer) to explosive volcanic vents. Medium grained rocks are found at moderate distances from the crest, while finer and finer grained rocks (like andesitic sandstones and siltstones) are found at the distal (furthest) distances downstream from the source vents. One exception includes volcanic mudflows, called lahars, or volcanic debris flows, which can carry large particles many, many miles since they are so dense and liquid – like fast, giant flows of concrete. The 1980 eruption of Mt. St. Helens in Washington State melted 3 glaciers on the volcano. The glacial meltwater mixed with ash, rocks, and trees, formed multiple debris flows, one of which extended 70 miles downstream along the Toutle River, flowing into the Columbia River. TOG is approximately 30 miles west of the Little Walker Volcanic Center.
One of the techniques geologists used to identify different lithofacies is to assess the physical characteristics of the rocks. Rocks with individual grains visible can be evaluated on their grain sizes, shapes, and sorting (see Figure 2). Grain size and shape give a clearer understanding of the distance from the source the rocks were transported by different physical processes. In general, the further downstream, the smaller and rounder the grains are. Sorting refers to the distribution of the various sizes of the grains in the rock. Well-sorted indicates similar sized grains (low variance), while very poorly sorted indicates grain sizes that vary greatly (high variance). Generally, more well-sorted sediment has been transported further from its source.
Figure 2: Rounding, Grain Size, and Sorting Classification.
This EarthCache is on public land managed by the U.S. Forest Service (USFS). Please follow the rules and regulations posted on their visitor signs. You can obtain a USFS brochure from the Summit Ranger Station at Pinecrest that gives interpretive information for landmarks along each of the trails. There are 11 stops on the North Rim Trail and 9 stops on the South Rim Trail. As you hike along the 2 trails, you will see various layers of different volcanic and volcaniclastic (volcanic sediment) deposits of the Mehrten/Relief Peak/Disaster Peak Formations. They have been weathered and eroded into various shapes and forms, some of which look like Gargoyles. Hike to the GPS coordinates given and the debris flow deposit there. Using the classification system from Figure 2 above, observe the rock’s grain size, sorting, and degree of rounding, then record your observations.
To log this part of the EarthCache, answer the following questions:
1. Using the scale given, what is your estimate for the rock’s grain size?
A) Coarse sand to clay (0.5 - < 0.004 mm)
B) Pebbles to granules (64 – 2 mm)
C) Cobbles to boulders (> 64 mm)
D) Silt to boulders (0.004 to > 64 mm)
2. Which of the following best describes the sorting of the material in the outcrop?
A) Poorly sorted
B) Well sorted
3. Which of the following best describes the rounding of particles in the outcrop?
A) Angular
B) Sub-rounded
C) Well-rounded
After estimating the grain size, sorting, and roundness of the particles in the rock outcrop, do you think the source of the particles was relatively close or far away?
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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:
- Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet.
- Big Idea 2: Earth is 4.6 billion years old.
- Big Idea 3: Earth is a complex system of interacting rock, water, air, and life.
- Big Idea 4: Earth is continuously changing.
- Big Idea 5: Earth is the water planet.
- Big Idea 6: Life evolves on a dynamic Earth and continuously modifies Earth.
- Big Idea 7: Humans depend on Earth for resources.
- Big Idea 8: Natural Hazards pose risks to humans.
- 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:
- Superposition – the oldest strata are at the bottom of the sequence
- Original Horizontality - layers of sediment are originally deposited horizontally
- Lateral Continuity - layers of sediment initially extend laterally in all directions
- Faunal Succession - fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances
- Law of Intrusive Relationships - the geologic feature which cuts another is the younger of the two features
- 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
- 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
The Trail of the Gargoyles EarthCache is located approximately 6.75 miles up Herring Creek Road/USFS Road 4N12 at approximately 7,400 feet elevation and is closed for half of the year or more, depending on the amount of snow in the area. The area is rugged and without cell phone service. EarthCachers should take great care in terms of safety while enjoying this cache. The Trail of the Gargoyles consists of 2 parts: 1) the North Rim Trail; and 2) the South Rim Trail. Both trails together total approximately 1.5 miles in length. You can pick up a trail guide at the Summit Ranger Station in Pinecrest on your way up Highway 108 from Sonora. The USFS guide says:
“Each (trail) provides a complete interpretive experience, but each contains unique features. Take good walking shoes or boots. Although the walks are relatively easy, there are a couple of portions with moderate steepness. Points of interest are numbered along the path and explained in this guide. Please stay on the trail and avoid the cliffs both for safety reasons and to prevent erosion of these delicate and loose soils. You are about to undertake a fascinating journey into the world of geologic beauty, to experience a spectacular example of the processes that forever sculpt and alter the faces of our planet.”
Note: Some of the sign posts mentioned in the USFS trail guide (small, numbered 4X4’s) may be missing. However, you may still be able to find each location based on the USFS brochure descriptions. Enjoy!
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. Using the scale given, what is your estimate for the rock’s grain size?
a. Coarse sand to clay (0.5 - < 0.004 mm)
b. Pebbles to granules (64 – 2 mm)
c. Cobbles to boulders (> 64 mm)
d. Silt to boulders (0.004 to > 64 mm)
4. Which of the following best describes the sorting of the material in the outcrop?
a. Poorly sorted
b. Well sorted
5. Which of the following best describes the rounding of particles in the outcrop?
a. Angular
b. Sub-rounded
c. Well-rounded
6. Which Big Ideas (1-9) are connected (list)?
7. Which GeoPrinciples are relevant (list)?
8. 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. Creely, Scott and Eric R. Force, 2007, “Type Region of the Ione Formation (Eocene), Central California: Stratigraphy, Paleogeography, and Relation to Auriferous Gravels”, U.S. Department of the Interior, U.S. Geological Survey, Open-File Report 2006-1378.
4. Domokos G, Jerolmack DJ, Sipos AÁ, Török Á, 2014, “How River Rocks Round: Resolving the Shape-Size Paradox”, PLoS ONE 9(2): e88657, https://doi.org/10.1371/journal.pone.0088657.
5. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
6. East Carolina University, 2010, “Grain Size Distribution”, Introduction to Depositional Systems and Grains, Geology 4010/4011, date last revised: 08/20/2010, GEOLOGY 4010/4111, http://core.ecu.edu/geology/rigsbyc/rigsby/Sedimentology/2010/sedsyl.htm.
7. Hines, Gary, c. 1995(?), “Trail of the Gargoyles”, Trail Guide ROG 16-22, 02/05, Stanislaus National Forest, Pacific Southwest Region, United States Department of Agriculture, www.fs.fed.us/r5/stanislaus.
8. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
9. Portland State University, “Glaciers of California”, last updated 08-04-2011, http://glaciers.research.pdx.edu/glaciers-california.
10. 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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