Lake Tahoe was formed 2-3 million years ago when geological faults caused land masses to rise and fall, creating a large valley surrounded by mountains. A volcanic eruption later blocked the only outlet, and snow and rain runoff began filling the basin to create Lake Tahoe. The surrounding geology includes granite, limestone, and quartz, and the area is home to animals like black bears and plants such as the snow plant and butterflies like the pale swallowtail. Relative dating of the rocks shows intrusions cutting across older formations, and unconformities represent periods of erosion.

1. Lake Tahoe
Mollie Whittingham
Professor Lawler
GEL 103
June 2016

2. Index
● Overview
● Lake Tahoe
● Granite
● Limestone
● Quartz
● American Black Bear
● Snow Plant
● Pale Swallowtail Butterfly
● Relative Dating
● Unconformity

3. Overview
Located in the Western United States just East of the Sierra Nevada Mountains lies the Lake Tahoe Basin.
This area has an incredibly rich geologic history, which has resulted in the creation of the “...highest lake of its size
in the United States”(Gould, 2012), with an average elevation of 6,225 feet above sea level.Here we will discuss a
few aspects of Tahoe’s geology, including the lakes formation, rock identifications, and different flora and fauna.

4. Formation of Lake Tahoe
● 2 to 3 million years ago three major geologic faults
resulted in land masses rising and falling in different
areas, creating a large valley surrounded by mountains.
○ “The Tahoe landscape was formed, and continues to
be formed, by powerful tectonic forces”(UC Davis,
2011).
● A volcanic eruption from Mt. Pluto led to a large lava
flow that eventually closed off the remaining outlet in
the basin.
● Snow and rain run-off from the ring of mountains
surrounding the basin began to fill the area with water.
○ “The enclosed basin has contained a lake for the last
2 million years”(UC Davis, 2011).
Figure 1 Lake Tahoe Faults

5. Formation of Lake Tahoe
● Eventually a new outlet for water flow,
the Truckee River, was formed.
However, areas such as Cave Rock
are proof that the lake was once
drastically deeper.
● The Tahoe Basin later entered an Ice
Age resulting in the creation of
massive glaciers, which would later
cut through the mountain regions,
creating many of the canyons,
moraines and lakes that exist in the
region today.

6. Granite
● Rock Type : Igneous
○ “Igneous rocks result when magma or lava crystallizes, or
when volcanic ejecta...accumulate and consolidate”
(Monroe & Wicander, 2016, p.18).
● Texture: Phaneritic
○ Visibly coarse, granulated
● Composition: Variation of multiple minerals
○ Most common:
■ Quartz (Milky, Grey, Shiny)
■ Feldspar (Most commonly: White, Pink)
■ Biotate (Black, Shiny)

7. Limestone
● Rock Type: Sedimentary
○ “Any rock composed of sediment that forms at
or near Earth’s surface” (Monroe & Wicander,
2016, p.686)
● Texture: Non-Clastic (Varies)
○ “[Not] composed of particles or fragments”
(Monroe & Wicander, 2016, p.162)
● Composition: Primarily composed of Calcium
Carbonate (CaCO3)
● “Most limestones form in shallow, calm, warm
marine waters” (King, 2016)

8. Quartz
Quartz
● Type: Mineral
● Luster: Nonmetallic
● Silicate: Nonferromagnesian Silicate
○ “White, cream, colorless and shades of pink
and pale green are typical” (Monroe &
Wicander, 2016, p.162)
● Hardness: 7
○ Based on Mohs Hardness Scale
● Quartz is a common rock-forming mineral
which can be found in all rock types.

9. American Black Bear
● The American Black Bear can be traced back to ancestors living over 20 million
years ago, however, it has seen very little evolution in its anatomical makeup.
○ 20 million years ago - Dawn Bear (Ursavus Elmensis)
■ Small, size of a “Fox Terrier” (Black Bears, 2007)
■ Spent majority of life in trees
■ Ate vegetation and insects
■ Became extinct due to climate change and loss of forest habitat
○ 5.3-1.8 million years ago - Auvergne Bear (Ursus Minimus)
■ Began small, grew in size throughout existence
■ “..anatomically the same as today’s black bears” (Black Bears, 2007)
■ Teeth evolved to incorporate a more carnivorous diet

10. American Black Bear
○ 5 million-11,000 years ago - Etruscan Bear (Ursus Etruscus)
■ Lived forrest adapted lifestyle, however, the Ice Age and glaciation led
to need for evolution
■ “..gave rise to three evolutionary lines” (Herrero, 2007)
● Asia: Black Bear, Brown Bear
● Europe: Cave Bear
■ Bering Land Bridge allowed Black Bears to migrate from Asia into N.
America
○ 500,000 years ago-Present - American Black Bear (Ursus Americanus)
■ Adaptive to multiple environments, still prefer forest habitat
■ Capable of climbing trees (for finding food and escaping predators)
■ Strong sense of smell (for finding food)
■ Omnivorous (can survive off vegetation and meat)
■ Average Size between 100-300 lbs

11. Snow Plant
● Latin Name: Sarcodes sanguinea
○ Meaning: “the bloody flesh-like thing” (USDA, 2016)
● First recorded in the early 19th century
● Mycotrophic Plant - fungus eating
● Native to California, Washington and Nevada
● Snow Plants are actually “...parasites on soil fungi” (Muir, 2007, p.106)
○ These “parasites” adapted and evolved to “freeload” off of other
plants that contain chlorophyll and are capable of photosynthesis,
which allows the snow plant to receive nutrients that it did not have to
create itself.
● “It will be found growing out of pine needles and other forest litter on
the surface of the ground, near a conifer.” (USDA, 2016)

12. Pale Swallowtail Butterfly
● Latin Name: Papilio Eurymedon
● “Butterflies and moths originated when flowering plants were beginning to
proliferate and have developed in close association with them” (Butterfly
Garden)
○ Fossil records date back approximately 40 million years
○ Due to their fragility, fossils are rare
● Developed defense mechanisms throughout each stage of their
metamorphosis:
○ Egg - Resembles “bird droppings” (Butterfly Garden)
○ Larva - Pattern resembles large eyes, emits bad smell if threatened
○ Chrysalis - Camouflaged to resemble leaves or twigs
○ Adult - Long, colorful tail which distracts predators from the more vital parts,
also poisonous to the majority of birds

13. Pale Swallowtail Butterfly
● Male Swallowtails have developed the ability to extract
sap from plants which allows them to produce
pheromones that attract females and assist in
reproduction
● Butterflies have developed their own methods of
hibernation and migration to increase survival
○ Life span typically between several weeks to several
months
● Biggest threats to existence today: humans, habitat loss,
insecticides/herbicides

14. Relative Dating
● Principle of Intrusive Relationships
○ “..when an igneous intrusion cuts across a formation of
sedimentary rock, it can be determined that the igneous
intrusion is younger than the sedimentary rock” (Geology,
2013)
● In other words, the sedimentary rock had to exist before the
intrusion was able to cut across it.
● Here we can see a large boulder covered in moss and
lichen
○ Cutting diagonally across the rock is an igneous intrusion,
known as a dike, possibly comprised of the igneous rock
basalt or the igneous rock gabbro

15. Unconformity
● Unconformities “...represent times of
nondeposition, erosion, or both” (Monroe &
WIcander, 2016, p. 422)
● There are three types of unconformities:
○ Disconformity
○ Angular unconformity
○ Nonconformity
● Here we can see the beginning of a
nonconformity, where the erosion of an igneous
rock is being covered by sedimentary rock.

16. Resources
Black Bears. (2007). Retrieved June, 2016, from http://www.bearsoftheworld.net/american_black_bears.asp
Butterfly Garden. (2016). Association of Zoos & Aquariums. Retrieved June, 2016, from http://www.tnaqua.
org/images/uploads/subpages/Butterfly_Garden.pdf
Cazarez, S. (2012). Lake Tahoe Faults [Digital image]. Retrieved June, 2016, from http://skywalker.cochise.
edu/wellerr/students/lake-tahoe/project.htm
Gauna, F. J. (n.d.). Plant of the Week:Snow Plant. Retrieved June, 2016, from http://www.fs.fed.us/wildflowers/plant-of-the-
week/sarcodes_sanguinea.shtml
Geology. (2013, December 10). Retrieved June, 2016, from http://www.newworldencyclopedia.org/entry/Geology
Gould, M. (2012, December 13).Geography and History Overview. Retrieved June, 2016, from http://tahoe.usgs.
gov/geography.html

17. Resources
Herrero, S. (2007). Aspects of Evolution and Adaptation in American Black Bears. Retrieved June, 2016, from http://www.
bearbiology.com/fileadmin/tpl/Downloads/URSUS/Vol_2/Herrero.pdf
King, H. (2016). Limestone, What is limestone? Retrieved June, 2016, from http://geology.com/rocks/limestone.shtml
Monroe, J.S., & Wicander, R. (2001) The changing earth: Exploring geology and evolution (7th ed.) Pacific Grove, CA:
Brooks/Cole
Muir, J. (2007). The Laws Field Guide to the Sierra Nevada. San Francisco, CA: California Academy of Sciences.
UC Davis Tahoe Environmental Research Center. [Steven McQ]. (2011, December 11). Lake Tahoe In Depth 2D. [Video
File]. Retrieved from https://www.youtube.com/watch?v=q_mzGm-g9LI