South Shore of theLake Tahoe Basin 2012 Geology Field Assignment By: Kate Albrecht July 10, 2012
Table of Contents• Background information• Geologic History• Cave Rock• Upper Truckee River• Rock samples• Plants and animals• Resources
Basin Formation• Although many people believe that it was volcanoes, the Lake Tahoe Basin was actually originally formed by faulting. (Hunt, 1998)• Fractures in the crust caused chunks of land to rise and sink. (Hunt, 1998)• Over a span of over several million years, this created the Sierra Nevada mountains. (Hunt, 1998)• The highest peaks in the Basin, Freel Peak, Monument Peak, Pyramid Peak, and Mount Tallac all formed on fault blocks around a large basin. (Hunt, 1998)• Over time snow, rain, creeks, and rivers all began to fill the basin to form a lake. (Hunt, 1998)• For a while, there was no outlet, so the lake kept getting deeper and deeper until it eventually found one, near the present-day town of Truckee. This is when the volcanoes come in. (Hunt, 1998)• A few volcanoes began putting lava into the basin, which eventually dammed the outlet, and the lake rose to a few hundred feet above today’s level, until it cut another outlet near present day Tahoe City (Hunt, 1998)• After that, many of the volcanoes went dormant, and the lake level slowly receded to the level we know today (Hunt, 1998)
Locations Upper Truckee River Cave Rock*Images attained using Google Earth.
Cave Rock• On the east shore of Lake Tahoe, with Highway 50 running right through it, is what looks like one massive eroded rock called Cave Rock. (Hunt, 1998)• Cave Rock was once an active volcano that poured lava into the Lake Tahoe Basin thousands of years ago. (Hunt, 1998)• After it went dormant, however, the lake level was hundreds of feet higher than it is now, which caused some serious erosion. (Hunt, 1998)• Over time, the waves slowly eroded not only the outside, but they left a catacomb of tunnels on the inside also, until they finally receded, leaving behind what we see, and drive through, today. (Hunt, 1998)
Upper Truckee River• This is a bank of clay that I found on the Upper Truckee River near the airport.• You can easily see where the river eroded away the bank when it flowed at higher levels, leaving behind the dirt overhang.• You can also see, just under the erosion overhang, the lines in the clay that show the level that the river has flowed at over the years.• For a long time, not far below this spot the river had been re-routed into a straight, unnatural line. However, a recent project this fall has re-routed the river again in a natural, meandering course.
Rock Sample: Granite• This is a specimen of what appears to be white granite. (Pellant, 2002)• White granite is an igneous rock that occurs in plutonic environments. (Pellant, 2002)• This rock is classified as an acid rock due to its high silica content, over 65 percent. (Pellant, 2002)• It is embedded with euhedral crystals of feldspar, mica, and usually quartz as well. (Pellant, 2002)• Because volcanic activity was one of the main factors in the creation of the Lake Tahoe Group Igneous basin, this rock is quite common in the area today. (Hunt, 1998) Origin Intrusive Classification Acid Occurrence Plutonic Environment
Rock Sample: Clay• This is a specimen of what appears to be clay. (Pellant, 2002)• Clay is a sedimentary rock that occurs in both marine and freshwater environments. (Pellant, 2002)• Fossils are often well-preserved in clay due to its very fine grain size. (Pellant, 2002)• This specific sample is a dried piece from a small, maybe 10 foot-long clay bed on part of the Upper Truckee Group Sedimentary River. Origin Mainly Marine & Freshwater Classification Detrital
Manzanita Bush• The manzanita bush is mainly found in the mountains of the Sierra Nevada and northern Coast Ranges. (Wilson, 2012)• Their scientific name, and genus in their scientific classification, is Arctostaphylos. (Wilson, 2012)• I believe this particular specimen to be a Mama Bear manzanita. (Wilson, 2012)• They mostly thrive in drier areas without a lot of water, and can die from too much of it, especially as they get older. (Wilson, 2012)• However in the correct conditions these plants can grow to be over 100 years old. (Wilson, 2012)
American Robin• The scientific name of this bird is Turdus migratorius. (“Mid-week reptilian #15: American Robin,” 2010)• They belong to the order Passeriformes the “perching birds” order. (“Mid-week reptilian #15, American Robin,” 2010)• However they also are related to the order of songbirds, oscines, which, combined with the Passeriformes, places them in the suborder Passeri. (“Mid-week reptilian #15: American Robin,” 2010)• These birds are common all across North America and in most of Canada and Alaska. (“American Robin,” 2011)• Their diet consists of insects; they can often be seen pulling worms and other bugs out of damp soil in the morning and evening hours of the day. (“American Robin,” 2011)
Steller’s Jay• Scientific classification: Class: Aves Order: Passeriformes Family: Corvidae Genus: Cyanocitta Species: Cyanocitta Steller (Andrijev, n.d.)• The steller’s jay belongs to the Cordivae family in the order Passeriformes, which is the order of perching birds. (Andrijev, n.d.)• They are commonly found in the evergreen forests of the western mountains of the U.S. and Canada. (“Steller’s jay,” 2011)• They are omnivores, with their diet consisting mainly of the insects and other things that they forage off of the ground. (“Steller’s jay,” 2011)
ResourcesAmerican robin. (2011). In The cornell lab of ornithology: All about birds. Retrieved July 13, 2012Andrijev, I. (n.d.). Steller’s jay. In Canisius ambassadors for conservation. Retrieved July 13, 2012Hunt, J. (1998). Lake tahoe facts and info: The geological history of the lake tahoe basin. In Lake tahoe cam.Retrieved July 13, 2012Mid-week reptilian #15: American robin. (2010, March 30). In The obligate scientist. Retrieved July 13, 2012Pellant, C. (2002). Rocks and minerals (2nd ed.). New York, NY: Dorling Kindersley, INC.Steller’s jay. (2011). In The cornell lab of ornithology: All about birds. Retrieved July 13, 2012Wilson, B. (2012, April 12). Manzanitas of california. In Las pilitas nursery. Retrieved July 13, 2012