Lab Assignment for GEO 103

1. GEOLOGICAL EVIDENCE
IN SAN LUIS OBISPO
By Sara Johnsen

2. Introduction
◦ For this assignment, I decided to use the area of San Luis Obispo County because there are
many different types of natural formations. Through careful observations and research, I was
able to document many changes of the geological periods.
◦ Since San Luis Obispo is located in Central California, near the coast, and surrounded by
many hills and mountain ranges, many types of geological formations have occurred here.

3. PART 1
Observing and Documenting Changes in SLO County.

4. Morro Rock
◦ A geological formation that is pretty predominate in SLO County would have to be Morro
Rock. This formation is located in the city of Morro Bay and was formed over 23 million years
ago by plugs of long-extinct volcanoes (City of Morro Bay, n.d.).
◦ The rock consists of a mixture of the igneous rock, dacite, and bird feces. The rock was the
result of pressurized lava flow coming and hardening on the surface above sea level (Atlas
Obscura, 2012). Over time, erosion and weathering has caused the rock to evolve into the
shape it is today with. Birds of many types have also made it a nesting site, causing the rock
to have another layer of bird feces.

5. Ivory Gull
◦ An animal that can be seen in SLO County would be the Ivory Gull, a type of bird that is
considered an endangered species (Seattle Audubon Society, n.d.). The species was first
discovered in SLO in 2010.
◦ This species is normally located in the Arctic circle, following some of the harshest conditions
to survive (Hickey, 2012). Descendants of the theropod dinosaurs, many bird types have
evolved to meet the lifestyle needs to survive (UCMP, n.d.).
◦ The migration of the Ivory Gull to the central coast of California could be cause for climate
change, as some researchers have gathered (Hickey, 2012).

6. Deer Grass
◦ A native plant of SLO County that is quite commonly seen would be the Deer Grass plant. It
grows to be 4 feet tall and wide, is grassy in texture, and is drought-friendly (WILD BLOOM
SUSTAINABLE LANDSCAPING DESIGN, 2018). It is one of the easiest to grow in the county.
◦ Muhlenbergia rigens is the plants scientific name. Many animals use it to graze, especially
the animal whom it is named after, deer. Humans also used it as resources to survive. Native
Americans used Deer Grass to make weaved baskets (University of San Diego, 2018).
◦ The earliest sign of the grass on Earth was about 60 to 55 million years ago. This can be seen
by grass pollens in fossils (Kellogg, 2001).

7. PART 2
Identifying Rock Types in SLO County.

8. Igneous - Dacite
◦ As mentioned before, Morro Rock is made of the igneous rock, Dacite. Dacite is often found
in lava flows, lava domes, dikes, sills, and pyroclastic debris (Geology.com, 2018).
◦ It is normally light in color and contains 62-69% silica and moderate amounts of sodium and
potassium (Program, 2015). It commonly includes minerals like plagioclase feldspar,
pyroxene, and amphibole (Hamilton, 2008).

9. Metamorphic - Blueschist
◦ Found in parts of the Franciscan Rock, Blueschist is a type of metamorphic rock that is
formed by the metamorphism of basalt and rocks with similar composition at
high pressures and low temperatures (Elsevier, 2018).
◦ Blueschist was actually first found in SLO County in San Simeon by exposed sea cliffs. The
oldest piece that was found originated from the Neoproterozoic era and is 700 to 800
million years old (Phys.org, 2018).

10. Sedimentary - Franciscan Rocks
◦ Franciscan Rocks are a type of sedimentary rock located in the San Andres Fault. The San
Andres Fault runs right through SLO County.
◦ It is made of four types of rock: sandstone, chert, basalt, and serpentinite (National Park
Services, n.d.). Most of these rocks are probably Late Jurassic and Cretaceous in age, but
some of are as old as Early Jurassic age (NORTHERN AND CENTRAL CALIFORNIA
FRANCISCAN ROCKS, n.d.).

11. PART 3
Understanding Relative Dating

12. Law of Superposition
◦ The Law of Superposition states that in a sequence of rocks, the top layer is normally the
youngest and the bottom one is normally the oldest. An example of this can be seen in the
Franciscan Rocks. The bottom layer of Basalt is seen as the oldest rock type while the top
layer (including all the deformations and weathering) is seen as the newest.
◦ The law of superposition is used to determine a relative age of a rock. This method can only
be used on sedimentary rocks.

13. Angular Unconformity
◦ An Angular Unconformity is when the older layers of rock are deformed, tilted, or eroded
before another layer is deposited on top. An example of this would be in Pebble Beach, CA.
The layers of sedimentary rock are eroded and tilted before another layer was additionally
added.
◦ Angular Unconformity is used to also help determine the relative age of rocks and can only
be used with sedimentary rocks.

14. Work Cited
◦ Atlas Obscura. (2012, July 10). Morro Rock. Retrieved August 6, 2018, from
https://www.atlasobscura.com/places/morro-rock
◦ City of Morro Bay. (n.d.). Morro Rock Beach. Retrieved August 6, 2018, from
https://www.morro-bay.ca.us/383/Morro-Rock-Beach
◦ Elsevier B.V. (2018). Blueschist. Retrieved August 6, 2018, from
https://www.sciencedirect.com/topics/earth-and-planetary- sciences/blueschist
◦ Geology.com. (2018). Dacite. Retrieved August 6, 2018, from
https://geology.com/rocks/dacite.shtml
◦ Hamilton, C., & Hamilton, R. (2008). Dacite. Retrieved August 6, 2018, from
http://scienceviews.com/geology/dacite.html
◦ Hickey, J. (2012, November 8). Ivory gull makes first appearance in San Luis Obispo
County. Retrieved August 6, 2018, from
https://www.sanluisobispo.com/news/local/article39137178.html

15. Work Cited
◦ Kellogg, E. A. (2001, March 01). Evolutionary History of the Grasses. Retrieved August 6,
2018, from http://www.plantphysiol.org/content/125/3/1198
◦ National Park Services. (n.d.). Franciscan Rocks of the Marin Headlands Terrane. Retrieved
August 6, 2018, from https://www.nps.gov/goga/learn/education/upload/Franciscan
Rocks poster.pdf
◦ NORTHERN AND CENTRAL CALIFORNIA FRANCISCAN ROCKS. (n.d.). Retrieved August 6,
2018, from http://www.johnmartin.com/earthquakes/eqsafs/safs_321.htm
◦ Phys.org. (2018). Age of blueschist does not indicate the origin of plate tectonics.
Retrieved August 6, 2018, from https://phys.org/news/2015-12-age-blueschist-
plate-tectonics.html
◦ Program, V. H. (2015, July 14). USGS: Volcano Hazards Program Glossary - Dacite.
Retrieved August 6, 2018, from
https://volcanoes.usgs.gov/vsc/glossary/dacite.html
◦ Seattle Audubon Society. (n.d.). Ivory Gull. Retrieved August 6, 2018, from
http://www.birdweb.org/birdweb/bird/ivory_gull#

16. Work Cited
◦ UCMP. (n.d.). The origin of birds. Retrieved August 6, 2018, from
https://evolution.berkeley.edu/evolibrary/article/evograms_06
◦ University of San Diego. (2018). Summary of deer grass and its traditional use. Retrieved
August 6, 2018, from https://www.sandiego.edu/cas/biology/kumeyaay-
garden/plants/deer- grass.php
◦ WILD BLOOM SUSTAINABLE LANDSCAPING DESIGN. (2018, March 07). Top 5 Plants for
San Luis Obispo | Greener Environments. Retrieved August 6, 2018, from
https://wildbloomlandscapes.com/top-5-plants-san-luis-obispo/