TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. Does the "slip surface", or geological contact, between the glacial till and the volcanic mudflow deposit daylight above the road? A) yes; B) no.
4. Which Big Ideas (1-9) are connected (list)?
5. Which GeoPrinciples 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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GEOLOGY OF THE SOURGRASS/DORRINGTON DEBRIS FLOW
During late December of 1996 through the first week in January of 1997 a series of storms dropped snow, then rain, onto the western slopes of the Sierra Nevada. Known as a “rain on snow event”, these types of events can cause widespread flooding and mass wasting. The Sourgrass/Dorrington debris flow occurred during this event and is among the largest mass wasting events in recent memory in the Sierra Nevada (see Figures 1, 2, and 3).
Figure 1: Aerial oblique view of the debris flow from the top, near the ridge, to the channel of the North Fork of the Stanislaus River.
Figure 2: View from Highway 4 looking upslope. The slide passed over the roadway and did not destroy the road.
Figure 3: View of the south side of the Stanislaus River, where some of the debris was deposited. Note the size of the boulders and the 2 humans (circled in red) for scale.
The winter of 1996-97 was an El Nino year that caused extensive flooding in the Sierra and the Central Valley. Several feet of snow were deposited by a cold winter storm on December 21st and 22nd, followed by more than 10 inches of rain from a series of warmer storms a so-called “Pineapple Express”, or now known as an “Atmospheric River”, or AR). The event lasted several days and caused millions of dollars of damage.
The amount of water was so significant that it saturated the Earth, destabilizing the slope, and triggering a type of landslide known as a debris flow that rapidly moved downhill, removing almost everything in its path. Debris flows are masses of loosely consolidated material that consists of water, mud, soil, sand, rock, air, and any other debris the flow picks up (trees, vegetation, buildings, bridges, etc.) that moves downslope under the influence of gravity. Speeds range from 1-2 feet per year (30-60 cm/yr) to over 100 miles per hour (160 km/hr). Their rate of movement and volume can make them very dangerous to those living and working in areas susceptible to debris flows.
Conditions generally found in the source area of debris flows include: 1) a steep slope; 2) loosely consolidated debris; 3) a source of abundant moisture; and 4) a lack of vegetation. They can be triggered by a number of things including: 1) addition of moisture; 2) removal of support or oversteepening of slopes; 3) failure of ancient landslide deposits; 4) removal of vegetation due to logging or wildfire; and 5) volcanic eruptions. Understanding where these conditions occur together and the triggering events are the first steps in the development of mitigation planning.
This particular debris flow, the Sourgrass/Dorrington debris flow on Highway 4, was interpreted to have been caused by the failure of “a block of Tahoe-age (?) till nearly 50 feet thick overlying (a) volcanic mudflow breccia…the slip surface generally coincides with the upper part of the volcanic unit” (DeGraff, 1997). The less permeable volcanic mudflow breccia allowed pore water pressure to build up in the overlying till until the till’s shear strength dropped to zero and the failure occurred. The rain-on-snow event, the steepness of the slope (~40%) , and the weakness of the earth materials combined to cause the failure. Some vegetation had been removed by selective logging, but this was not noted in the briefing paper. Additionally, a logging road cut into the slope was determined not to have played a role in the failure because:
“The slip surface "daylighted" from the slope about 12 to 15 feet above the surface of a timber access road which contoured across the slope below it. The top of the cutslope is 5 feet above the road surface and indicates slope movement was unlikely to be a result of removal of material to construct the road.” (DeGraff, 1997).
The purpose of this EarthCache is to either verify or refute the geological claim that “the slip surface “daylighted” from the slope about 12 to 15 feet above the surface of a timber access road which contoured across the slope below it” (DeGraff, 1977).
You will drive, or hike, to the GPS coordinates on USFS Forest Route 6N59. The debris flow’s head escarpment, or scarp, is located above you, toward the top of the ridge. Hike around to observe the till overlying the volcanic mudflow breccia described above. The till contains loose boulders, gravel, sand, and silt (see Figure 4). The volcanic mudflow breccia contains boulders, gravel, sand, and silt, but the particles are cemented together (see Figure 5). Volcanic ash may also be present and is fine-grained, doesn’t drain well, and weathers into clay minerals that are impermeable (see Figure 6). The color of the till is more yellowish, the volcanic breccia is gray to purplish, and the ash tends to be either white or pinkish.
Figure 4: Glacial till composed of unsorted boulders, cobbles, gravel, sand, and silt.
Figure 5: Well cemented volcanic mudflow, or lahar; also called a volcanic debris flow.
Figure 6: Fine-grained volcanic ash that weathers to clay, which is impermeable.
Walk along the roadway to both ends of the debris flow – the full extent. Make observations above and below the road. What do you see? Is there any till anywhere below the road? Try to follow the contact between the two types of rock. Some will be covered by soil and will be hard to see, but you should be able to determine if there is till below the road or not. Record your observations.
If human land use (i.e. logging, road building) in any way contributed to this event, implications would include responsibility for financial cost of repairing damaged infrastructure on public land downslope. The Board’s Crossing bridge replacement cost (across the Stanislaus River at the bottom of the canyon) was approximately $1.1 million. It is notable that a similar debris flow occurred in the Carson Iceberg Wilderness (without any human influences) during the same 1997 El Nino event…
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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 continuing east for another 8 or 9 miles. Turn left on USFS Forest Route 6N80, then after 150 yards, right on 6N59 and continue on to the EarthCache location. Please note that the US Forest Service road is unpaved and will be closed during portions of the year in winter months due to snow and mud. You will likely need a high clearance vehicle on dirt Forest Service roads.
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. Does the "slip surface", or geological contact, between the glacial till and the volcanic mudflow deposit daylight above the road? A) yes; B) no.
4. Which Big Ideas (1-9) are connected (list)?
5. 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. DeGraff, Jerome V., Forest Geologist, Stanislaus, Sierra, and Sequoia National Forests. 1997. “Geologic Investigation of the Sourgrass Debris Flow, Calaveras Ranger District, Stanislaus National Forest”. January 27, 1997. http://www.scenic4.org/documents/sourgrass_slide_brochure_47.pdf
4. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
5. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
6. Petersen, Richard, et al, 1997, "The Sourgrass Slide of 1997", Calaveras Ranger District, Stanislaus National Forest, 5519 Highway 4, P. O. Box 500, Hathaway Pines, CA, 95233, (209) 795-1381, www.r5.fs.fed.us/stanislaus.
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. Tolhurst, J., 2012, “Sword Lake Debris Flow” Field Trip Guide, 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.
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