The Mogul Landslide EarthCache
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Landslide – for some the word has connotations of the song by the Dixie Chicks or Fleetwood Mac, but at this Earthcache site just west of Reno you can see a real-life example of the geologic formation for which the song was titled.
This cache is part of the 1st Annual International EarthCache Day event, October 8, 2006.
To get to an easily accessible viewing point for the landslide, take the Mogul exit #7 off Interstate 80 eastbound, a few miles west of Reno, and turn west on the frontage road at the foot of the off-ramp. Pull off the road in a large gravel turn-out (location of coordinates). From this spot, look due south across the river. If you scan the area just below the crest of the hill, you will see an arcuate cut of layered gravel deposits overlying soft, unstable sediments of what is called the Sandstone of Hunter Creek. This scar is the headwall of a historic landslide that occurred sometime between 1880 and 1906. The crumpled-looking lobe of ground below the scar is the material that moved down slope in the landslide, extending into the river. (The feature can also be seen less distinctly from the Scenic View overlook area of eastbound Interstate 80 between exits #6 and #7 west of Reno.)
Now look east and west along the face of the hillsides adjacent to this most obvious recent landslide. The hummocky ground extending from the steeper slopes down to the Truckee River is all material that has broken away and moved downhill as landslides in the past 100,000 years or so. The toe of each of these landslides has forced the river to make a wider curve to the north, and some landslides may actually have blocked the river flow for a while. The deposits are now being eroded away by the river.
A landslide is a geological phenomenon of ground movement down a slope. Although gravity acting on an over-steepened slope is the primary reason for a landslide, there are other contributing factors affecting slope stability that may trigger a landslide:
• erosion by rivers, glaciers, or ocean waves create over-steepened slopes;
• rock and soil slopes are weakened by water saturation (from snowmelt, heavy rains, or man-caused events);
• earthquakes create stresses that weaken slopes and volcanic eruptions produce loose ash deposits and sediments on steep slopes;
• vibrations from machinery, traffic, blasting, and even thunder may trigger failure of weak slopes;
• excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may cause weak slopes to fail;
• groundwater pressure;
• in shallow soils, the removal of deep-rooted vegetation that binds the colluvium to bed rock.
On the slopes south of the Truckee River at Mogul, several of these factors combined have contributed to multiple landslides over the past 100,000 years, resulting in the hummocky topography characteristic of landslide deposits. Erosional cutting away the toe of the slopes by the Truckee River, steepening of slopes by uplift of the Sierra Nevada along faults, saturation by rainfall of a past wetter climate, and earthquakes have all played a part in the past landslide history of the area.
The most recent landslide that left the most obvious and prominent scar on the slope to the south was perpetrated by the inhabitants of the Truckee Meadows. This slide was the direct result of water leakage from the Steamboat Ditch saturating the soil on an already steep slope. The ditch at that time passed around this point of land in a combination of short tunnels and a flume. Following the landslide, the ditch was re-routed through a longer more leak-proof tunnel, behind the hill, avoiding conflict with the less stable ground on the face of the slope. The line of green trees and bushes that crosses the slopes above the small, perched meadow to the right of the slide is the present trace of the Steamboat Ditch. The Mogul landslide is still considered to be active.
Question to be answered to log this site: What general direction (north, south, east, or west) did the landslide block move in the most recent landslide at this site?
E-mail your answer to this question to the site developer, along with a photograph of your group and the number of people in your group.
CAUTION IN USING GPS UNITS AND TOPOGRAPHIC MAPS
(Note that GPS coordinates are relative to a particular datum used to describe the nearly spherical shape of the Earth’s surface. Most topographic maps published by the U.S. Geological Survey use the North American Datum of 1927 (NAD 27), but most GPS units are set for either the North American Datum of 1983 (NAD 83) or the World Geodetic System of 1984 (WGS 84). NAD 83 and WGS 84 give nearly identical locations with hand-held GPS instruments, but NAD 27 can be off considerably. In this part of Nevada, there is little change in latitude between the 1927 and 1983 data, but for longitude the datum of 1983 is shifted relative to the datum of 1927, such that, if you use your GPS unit to measure a location using WGS 84, the point will plot on the U.S. Geological Survey topographic map approximately 100 meters farther east than its true location on the map. If your GPS unit is set to NAD 27, you need to look for the WGS 84 waypoint approximately 100 meters farther east from your location.)
BASIC VISITOR-USE AND PUBLIC SAFETY INFORMATION FOR PUBLIC LANDS IN NEVADA:
All visitors need to plan ahead and prepare for outings in Nevada’s public lands 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 (Maps are available for purchase at all BLM offices and from the Nevada Bureau of Mines and Geology, www.nbmg.unr.edu.);
• Staying on existing roads and trails;
• Staying away from all mine shafts and adits;
• Planning for extreme weather, hazards, and emergencies;
• Carrying a full-size spare tire, extra food, water, and warm clothing;
• 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;
• Linking to “Tread Lightly and Leave No Trace” websites (www.treadlightly.org and www.lnt.org).
Please see www.nbmg.unr.edu/EarthCache.htm for more information about this and other Nevada EarthCache and GeoCache sites of geologic interest. Thank you
References:
U.S. Geological Survey Fact Sheet 0071-00, Landslide Hazards.
Gates, William Chester Bruce, 1994, Regional slope stability of the Truckee River Canyon (drainage basin) from Tahoe City, California to Reno, Nevada; Ph. D. dissertation, University of Nevada Reno.
Tingley, J.V., Pizarro, K.A., Ross, C., Purkey, B.W., and Garside, L.J., 2005, Geologic and Natural History Tours in the Reno Area, Nevada Bureau of Mines and Geology Special Publication 19, expanded edition.
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