Blake Murphy observed the geology around Secret Beach in Lake Tahoe. Lake Tahoe formed over millions of years as the Sierra Nevada mountains uplifted. Glaciers eroded the granite and other igneous and metamorphic rocks that make up the mountains and basin. Sedimentary rocks like sandstone also formed around the lake. Today, human impacts are decreasing water quality and clarity in Lake Tahoe as well as reducing habitats for black bears and other wildlife.

1. GEOGRAPHY OF LAKE
TAHOE
BY BLAKE MURPHY, LAKE TAHOE COMMUNITY COLLEGE

2. SECRET BEACH, CA
Secret Beach is a small lakefront area at the center of South Lake
Tahoe next to a residential area. It is next to a marsh with hiking trails.
For this presentation, I will be using my observations of this area to
draw some conclusions about the geology of Lake Tahoe. This beach is
very close to my house, so for my field work I just walked down the
trail, through the marsh, to the beach. It was a very nice day! On my
walk I saw hummingbirds, bluebirds, ducks, lots of chipmunks and
many little fish in the Lake. After some research, I can only assume the
fish are some sort of trout. I observed the rock formations around the
Lake, and will be telling you more about them as well as how the Lake
formed.

3. FORMATION OF LAKE TAHOE
• Back in the days of the supercontinent, Pangea, the area that
is now Lake Tahoe was under a shallow sea
• 210 million years ago, the North America Continental Plate
broke off from Pangaea
• After that, the Pacific Ocean Continental Plate broke off as
well and drifted East
• The Pacific Plate was then pushed under the North America
Continental Plate
• Tectonic pressure and pressure from the magma chamber
under the Earths surface over the next 130 million years
caused the rise of the Sierra Nevada mountain range
• The uplifting of the mountain range caused parallel faults to
form
• In the West, the Sierra Crest rose
• The Carson Ridge dropped in the East, and when lava
dammed the Carson Valley, the Basin was formed
• Eventually, glaciers carved and polished the rocks around the
basin and snow run off eventually filled the Lake

4. ROCKS OF THE LAKE TAHOE
BASIN
• Igneous rocks (granite)- these rocks are formed from magma
cooling and then solidifying and are very hard in texture.
These are very abundant in the greater Lake Tahoe area
• Sedimentary rocks (sandstone)- these rocks are formed by the
accumulation or deposition of small particles which cement
together, typically on the ocean floor or under another body
of water
• Metamorphic rocks- these rocks are the product of an already
existing rock changing (or metamorphizing). Sedimentary and
Igneous rocks turn into Metamorphic rocks when subjected to
high temperatures and pressure
• To the right is a picture of some of the mountains around the
Lake Tahoe Basin, which are mostly compromised of igneous
rocks such as granite, diorite and monzonite

5. ROCKS I FOUND:
GRANODIORITE
On my walk to the beach I found a few different kinds of rocks.
The first is what I believe to be a granodiorite rock. These are
intrusive igneous rocks, similar to granite but with more
calcium and sodium.

6. SANDSTONE
Another rock commonly found in Tahoe is sandstone. Sandstone
is a sedimentary rock made of small pieces of rock, minerals and
other organic materials. Sandstone is created underwater,
whether it be the ocean or a Lake, so it’s no surprise we have
them here in Tahoe.

7. METAMORPHIC (MARBLE)
Metamorphic rocks arise from the transformation of already
existing rocks. Marble is composed of recrystalized carbonate
minerals such as calcite or dolomite.

8. EXAMPLES OF CHANGE THROUGHOUT TIME
• One unfortunate change Lake Tahoe is currently experiencing
is the decrease in water quality and clarity. Every year, more
and more tourists visit Lake Tahoe, many with little to no
regard for the ecosystem and wildlife. Because of this. The
Lake loses about one foot of water clarity per year. When it
rains or snows, the storm water causes anthropogenic
disturbances to enter the Lake, such as fertilizer, car exhaust
and other urban runoff.
• Humans have greatly impacted the animals and plants in the
Tahoe Basin as well. A prime example of this is the black
bears, descendants of fauna. We have encroached on so
much of their natural habitat here that they have less and
less space to roam and call their own. Human development
has also greatly lessened the number of animals that the
bears naturally prey on in the area.
• The Tahoe Basin has a very high number of sugar pine trees
(depicted to the right) which encompass the greater area.
Some of these trees have been alive for hundreds of years!
Though they are still very common here, there numbers have
decreased significantly due to the building of homes, resorts,
casinos, etc. Populations of bird species such as woodpeckers
have been influenced negatively by this.

9. PRINCIPLES OF
RELATIVE DATING
The Law of Superposition- states that the oldest
layer of rock in a formation is at the bottom and
the youngest layer is at the top. The youngest
layers of sediment along the basin are the ones
we are lucky to be able to explore and hike!

10. TYPES OF UNCONFORMITIES
• There are three types of unconformities- disconformities,
angular unconformities and nonconformities
• An unconformity is a contact between two rock formations,
one older and one younger. The younger will be on top and
the older on the bottom.
• Disconformities- erosional contacts which are parallel to the
bedding planes of the upper and lower rock units.
Disconformities can be hard to recognize.
• Angular unconformity- contact that separates a younger,
dipping rock formation from an underlying rock formation
that is either tilted or otherwise deformed or
metamorphosized.
• Nonconformities- contact that separates a younger
sedimentary rock from either an intrusive igneous rock or a
metamorphic rock formation. Nonconformities suggest that
uplifting and erosion occurred.
• To the right is an example of an angular unconformity right
here in South Lake Tahoe.

11. REFERENCE PAGE
• N. (2020). Lake Tahoe. Retrieved June 15, 2020, from
http://www.ndow.org/Bodies_Of_Water/Lake_Tahoe/
• Hopson, F. R. (2018, July 31). Travels in Geology: Lake Tahoe Jewel of the Sierra Nevada. Retrieved June
15, 2020, from https://www.earthmagazine.org/article/travels-geology-lake-tahoe-jewel-sierra-nevada
• Service, U. (n.d.). Geology of the lake tahoe basin. Retrieved June 15, 2020, from
https://www.fs.usda.gov/detail/ltbmu/learning/?cid=stelprdb5109570
• Monroe, J. S., & Wicander, R. (2011). The Changing Earth: Exploring Geology and Evolution, 6th
Edition (6th ed.). mount pleasant, michigan: Cengage Learning.