================================================================================
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. Determine the depth of the debris flow at the trail crossing.
4. Estimate the size of the largest particle observed in this area.
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples (1-7) 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.
===============================================================================
GEOLOGY OF THE SWORD LAKE DEBRIS FLOW
The winter of 1996-97 was an El Nino year that caused extensive flooding in the Sierra Nevada and 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), that created a rain-on-snow event lasting several days (see Figure 1 and Table 1).
The Sword Lake debris flow presumably occurred during the this storm (on or near January 1, 1997), though it went undiscovered until later that spring since it was in a wilderness area, inaccessible due to winter road closure on Highway 108. There were no mechanical manipulations of the slope by human activity – no logging, no road building in the wilderness – so destabilizing the slope due to oversteepening, or lowering cohesion in the soil due to tree removal, was not a trigger for this debris flow. The length of the debris flow is approximately 6,500 feet (1.25 miles), average width is approximately 300 feet wide, and it covers an area of around 56 acres.
Figure 1: NOAA National Weather Service Total Precipitation Map for Northern California for December 29, 1996 through January 3, 1997.
Table 1: Rainfall data in Northern California for 6 and 9 day periods beginning in December 1996 and ending in January 1997. The weather station elevation at Gianelli (Stanislaus River Basin) is 8,350 feet.
This EarthCache is located at the base of the Dardanelles, a locally well-known landmark composed of Tertiary volcanic rocks underlain unconformably by Mesozoic granitic rocks. The Tertiary volcanic sequence includes the Valley Springs Formation and the Stanislaus Group (which is composed of the Relief Peak Formation, the Dardanelles Formation, and the Disaster Peak Formation). The exposure in the head scarp reveals several rock types including: 1) glacial till; 2) river cobbles; 3) columnar jointed lava; 4) volcanic ash; and 5) soil. Tertiary volcanic rocks underlie approximately the first 1,000 feet of the debris flow, while the remainder of the slide path exposes granitic rock (see Figure 2).
Figure 2: Google Earth aerial photo dated 09-15-10 with map overlay of approximate areal extent of the debris flow; length is approximately 6,600 feet (Head to North Toe) to 6,400 feet (Head to South Toe) and widths range from approximately 50 to 650 feet (mean ~300 feet); Area is approximately 56 acres (calculation from ArcGIS).
Observations of the head scarp (top of the feature) show glacial till (loosely consolidated glacial deposits of boulders, cobbles, gravel, sand, and silt) overlying less permeable rock, including a lava flow deposited on fluvial (river) cobbles, unconformably overlying volcanic ash. Tension cracks are evident to the north of the scarp, indicating the slope could be reactivated with an addition of abundant water. A spring was observed at the base of the head scarp. Cobbles, ash, and till are all capable of flowing, when saturated, due to an increase in pore water pressure and a decrease in shear strength of the material. The lava is only observed in outcrop at the very top of the scarp. A large block of material composed mostly of volcanic ash covered with a very small amount of till (rapidly eroding away) that dips back into the slope, is a back-tilted block that did not continue to move downslope with the rest of the evacuated material. There is no trail to the head scarp, but it is worth a look.
Rough estimates of the upper portion of the debris flow suggest that approximately 65,000-75,000 cubic yards of material may have been transported downslope from the head scarp and deposited at the base of the slide in two lobes. If a typical dump truck holds 10 yards, then this would be approximately 6,500-7,500 truckloads of material! The lower slide area is underlain by granitic rock resistant to downcutting and exposed by glacial processes. The debris flow scoured down to bedrock where granitic rock crops out (~1,200 feet below the head scarp). Abrasion and vegetation damage can be seen on the upslope sides of some of the trees left standing. Some aggradation (raising of the stream bed) is also visible where the gradient shallows downslope and debris flow material has been deposited. Some boulders deposited by the debris flow are > more than 10 feet in diameter. Additionally, lag deposits, or debris flow “erratics” can be seen overlying granitic bedrock toward the South Toe (see Figure 3).
Figure 3: A: Lag deposits or debris flow “erratics” were observed deposited onto granitic bedrock upstream of the South Toe (Latitude = 38.391903; Longitude = -119.933010); B: Evidence of abrasion on the upslope side of a tree (on the left bank of the unnamed stream that flows to the South Toe) that survived the debris flow; the top of the abrasion marks are approximately 12 feet high and the streambed is ~15 feet below the base of the tree; the debris flow was approximately 25-30 feet deep as it passed by this location (Latitude = 38.393439; Longitude = -119.932295, which is ~1,500 feet downstream and ~250 feet in elevation below the EarthCache site); C: Debris flow deposit ~3,400 feet downstream of head scarp (Latitude = 38.393738; Longitude = -119.930685; card showing scale is 3 X 5 inches).
Most likely the initial movement was a debris slide on a 35-40% slope that disaggregated into a debris flow, leaving a few blocks behind near the head scarp. The rest of the material flowed downslope, bulking up slightly until it reached the underlying granitic rocks. It continued downslope, splitting into 2 flows, following topography around a granitic hill.
Fortunately, no damage to humans or structures resulted since the debris flow occurred entirely within the Carson-Iceberg Wilderness area. The trail to Sword Lake did suffer damage and was rebuilt the following summer. Debris flows will continue in the Sierra Nevada Mountains. Another, smaller, debris flow occurred recently in the Disaster Creek drainage (~10 miles ENE) during July, 2011 according to Glen White, professor of earth sciences at Columbia College. In general, “sediment delivery from mountainous terrain is extremely episodic, sporadically subjecting mountain stream ecosystems to extensive disturbance” (Kirchner, et al).
To earn credit for this EarthCache, you will determine: 1) the depth of the debris flow near the trail crossing at the GPS location (see below); and 2) the size (longest side) of the largest particle observed in this area. First, find the following GPS location: Latitude = 38.391830; Longitude = -119.929240. There are several trees close to this spot that survived the debris flow (which would have been like a fast moving concrete slurry containing very large boulders) as it rumbled downhill through the location where you are standing. It may be a bit difficult to get much of a view since some of the vegetation has grown back since 1997, but move around a bit to get the best vantage point you can. Also, in 2018, a wildfire burned through here - the Donnell Fire - and killed some of the trees that survived the debris flow. Observe the upstream side of one of the largest remaining trees (dead or alive) and note the highest point of the abrasion marks (where the bark was eroded off of the tree by the debris flow). Now walk down the trail a bit further and find where it intersects the small stream flowing across it. Try to estimate how many feet lower, in elevation, the streambed is compared to the high point of the abrasion mark on the tree you just observed. Using both pieces of information, estimate the depth of the debris flow that flowed through this area. Finally, estimate the size of the largest particle, or rock, carried by the flow and deposited in the area of the GPS location. After providing these estimates, and before leaving, take a moment to think about the size and power of this natural disaster. What if it happened again, but in a populated area? What would be the consequences?
If you want to explore this feature more, the hike to the headscarp is all off-trail and it is best to stay along the right edge of the debris flow in the trees. The hike to the bottom is also interesting and there are numerous dead trees, destroyed by the flow, deposited at the foot of the feature, that were part of the forest prior to the event. The flow splits into 2 lobes, so you will have to decide which you'll want to follow first.
Sword Lake is another 2 miles past the GPS location for this EarthCache and is a great place to swim on a hot summer day!
===============================================================================
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
===============================================================================
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
===============================================================================
LOGISTICS AND SAFETY:
This EarthCache is in a wildnerness area - the Carson-Iceberg Wilderness. Please be sure to follow the guidelines posted at the trailhead for responsible wilderness recreation use. The hike to the EarthCache is at higher elevations (above 7,000 feet) and the altitude changes along the trail, so hikers should be in good shape and take all the necessary safety precautions (see list below). The EarthCache can be accessed from Highway 108 to Clark Fork Road, then take Fence Creek Road 6 miles to the end of the road and the parking area where the trailhead is located. Be sure to take the left fork at the trailhead, toward the northwest side of the Dardanelles, into the wilderness area. The EarthCache is approximately 1.1 miles from the trailhead/parking area at an elevation of approximately 7,200 feet.
The road is closed during the winter months due to the closure of Sonora Pass/Highway 108. The best time to access this EarthCache is in the summer and early fall.
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 (lightning high winds, rain/snow, etc.), 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.
===============================================================================
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. Determine the depth of the debris flow at the trail crossing.
4. Estimate the size of the largest particle observed in this area.
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples (1-7) 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.
===============================================================================
Note: For a brief summary of the geologic history of the Central Sierra, see this EarthCache:
Dragoon Gulch EarthCache
===============================================================================
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. 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”. http://www.scenic4.org/documents/sourgrass_slide_brochure_47.pdf, January 27, 1997.
4. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
5. Kirchner, Finkel, Riebe, Clayton, King, Meghan. “Mountain Erosion Over 10; yr., 10 k.y., and 10 m.y. Time Scales”. Geology, July, 2001, v. 29, no. 7.
6. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
7. National Oceanic and Atmospheric Administration (NOAA), Department of Commerce. 2007, “Heavy Precipitation Event, Southwest Oregon, Northern California, and Western Nevada December 26, 1996 - January 3, 1997”; California Nevada River Forecast Center (CNRFC) - Storm Summaries. http://www.cnrfc.noaa.gov/storm_summaries/jan1997storms.php. Updated May 2007. Web. 30 June 2012.
8. Portland State University, “Glaciers of California”, last updated 08-04-2011, http://glaciers.research.pdx.edu/glaciers-california.
9. Putnam, Roger, (pers. comm.), May 2017, Professor of Earth Science, 11600 Columbia College Drive, Sonora, CA, 95370.
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.
11. White, Glen, M.S., R.G., Adjunct Professor of Earth Sciences at Columbia College, 11600 Columbia College Drive, Sonora, CA., 95370, pers. comm., 2012.
===============================================================================