The document provides information on the geology and natural history of the San Francisco Peninsula region. It discusses the formation of the region from the Franciscan Complex rocks approximately 200 million years ago during the subduction of the Farallon Plate. It also describes the local climate, faults, and various native plants and animals that have evolved to adapt to the unique environmental conditions of the region, including mule deer, California buckwheat, black-tailed jackrabbits, and local rock types such as limestone, granodiorite, and serpentine. Examples are provided of field identification of these rock types based on their properties. Photographs illustrate examples of rock layers, erosion features, and local flora and fauna.

1. Historical The
Bay Area
Peninsula
Region
J A C K I E C R U Z
G E L 1 0 3
P R O F E S S O R L A W L E R

2. Peninsula Region of the Bay Area
∙ The bedrock of a most of the peninsula region belongs to the
Franciscan Complex, which is essentially a heap of
metamorphosed sedimentary rocks, including shale, sandstone,
and chert, volcanic rocks, serpentine
∙ The formation of the Bay are was a multi-stage process. The
Franciscan complex Rocks formed during the Mesozoic Era
(Late Jurassic to Early Cretaceous periods), which makes them
approx. 200 million years old.
∙ These rocks originated from a subduction zone when the
Farallon Plate (oceanic) slid underneath the North American
Plate (Continental)
∙ The San Franciscan Bay began to form approx., 10,000 years
ago during the last Ice Age: the ice melted causing sea levels to
rise and flood the lower valleys

3. Conditions of the Peninsula Bay
∙ The Peninsula region has a “Mediterranean Climate”,
meaning the winters are mildly wet, and the summers are
dry. The temperature is moderated by the Pacific Ocean
It typically rains 20-25 inches per year
∙ There are microclimates in the areas closer to the ocean,
where there’s more fog and temperatures are cooler.
∙ The San Andreas Fault runs through the Western side of
the Peninsula region, making this region very seismically
active.

4. Mule Deer
Pictured: Mother and baby Mule Deer
in my backyard
∙ Mule Deer are native mammals of North America
∙ How? Deer are in apart of the Cervidae family, and it’s said that
they originated from Europe and Asia.
• The earliest ancestors of Mule Deer are ungulates (During the Eocene),
the 1st “true” deer evolved during the Miocene, and these deer began to
adapt to different environments, forming many deer species.
• Mule Deer directly evolved from female white-tailed and male black-
tailed deer
• The arrival of humans to North America played a huge role in the
evolution of Mule deer, as human hunters were predators to them.
• When? Mule deer began to evolve about 1-2 million years
ago.

5. Mule Deer
∙ Why? Some of the reasons they evolved are
predation, environment, and dietary reasons.
• Predation: Mule deer evolved to have larger,
mule-like ears in order to hear faint sounds of
predators. They also evolved to have powerful
legs for speed to escape predators.
• Environment: They evolved to have a grey-
brown fur coat to help them blend into the rocky
environments of the Peninsula region.
Additionally, their hooves are adapted to sustain
the rockier areas and the more desert areas.
• Dietary: Their dental structure evolved to be
able to break down tough, fibrous plants. They
also have diet variation based on the season;
they have adapted to eat less and conserve
energy in the winter when food is harder to find.
Amsel. S, (2005). Mule Deer [Diagram]

6. California Buckwheat: Eriogonum
fasciculatum Benth.
Pictured: California Buckwheat among
shrubs along the Bay in the Peninsula
region

7. California Buckwheat
∙ California Buckwheat is apart of the
Polygonaceae family, originating back to more
than 100 million years ago (Cretaceous period)
∙ California Buckwheat is native to Southwest U.S.
and (all throughout California)
∙ It’s typically found in coastal areas; However, the
California Buckwheat has evolved to adapt to a
variety of climates such as alpine regions, desert
regions, and coastal regions
∙ The small leather-like leaves of California
Buckwheat help them conserve water in more arid
environments, such as the Peninsula region
during the summer. They have also evolved to be
salt- tolerant due to their environment.
∙ They co-evolved with peninsula-native bees to
help promote reproductive success. They are a
major source of nectar for these bees.
Ethington. K, (2030). Honey bee visiting buckwheat
[Photograph]

8. Black-tailed Jack
Rabbit
∙ The Black-tailed Jack Rabbit is a native
species of Western United States,
predominately California
∙ They originated in Asia from the genus
Lepus, and migrated to North America
during the Miocene Epoch
∙ Because Western America is warmer and
more arid these rabbits evolved to have
large ears. Their large ears helps with
thermoregulation in this environment.
∙ Their coats evolved to match a color that
blends in more in shrubland and desert
environments
Pictured: Black tailed jack rabbit in
Belmont along the bay trail

9. Limestone Rock
∙ Pictured: Limestone rock along the Bay Trail in San
Mateo I took during a run.
∙ Limestone is a sedimentary rock
∙ It’s made up of calcium carbonate, and can even
have shells, and marine fossils
∙ Limestone was formed in the Bay Area from a
variety of processes
• Subduction of the Western edge if the North
American Continent, resulting in suitable conditions
for calcium-rich materials
• Accretion: gradual increase in limestone size.
Fragments from the ocean floor accumulate
overtime to form a larger rock.

10. Process of Identification:
∙ Color: Limestone is usually light gray-tan, and
sometimes even black due to impurities.
∙ Texture: Coarse texture
∙ Hardness: I was able to scratch the rock with my
fingernail, leaving a scratch mark. Meaning this
rock is softer, which aligns with limestone
characteristics.
∙ Location: this rock was found in shallow waters
along the bay, where limestone is typically found.

11. Granodiorite Rock
∙ Pictured: Rock found on Mills Canyon Trail in San Mateo
∙ Granodiorite is an intrusive igneous rock, made up of quartz,
plagioclase feldspar, hornblende, biotite, and augite. It is
similar to Granite.
∙ Granodioritic magmas in California were formed when the
Farallon plate subduction occurred during the Mesozoic Era.
∙ The Farallon plate subducting resulted in the plate melting
under high pressure and temperature, further expelling the
magma.
∙ Over the span of millions of years, these Granodioritic
magma formations underwent erosion and weathering,
wearing away at the rocks.

12. Process of Identification:
∙ Color: Granodiorite Rock is usually grayish, with a
mix of lighter and darker minerals.
∙ Texture: Coarse texture, individual minerals are
easily distinguished
∙ Hardness: Scratched with my fingernail and there
wasn’t a scratch mark left, reflecting the quartz in
the rock.
∙ Location: this rock was found near a subduction
zone (the Farallon plate)

13. Serpentine Rock
∙ Pictured: Serpentine rock at the Pacific
Coast, in Pacifica, CA I took while on a run.
∙ Serpentine is a metamorphic rock that is
composed of many minerals, such as
antigorite, lizardite, and chrysolite
∙ It is one of the more common rock types in
the Peninsula region of the Bay Area
∙ Serpentine in this region was also formed
during the Farallon subduction
∙ Serpentine Rock can be associated with
ophiolite sequences, which essentially is
the uplifting of serpentine rock from the
ocean floor up to the surface.

14. Process of Identification:
∙ Color: Granodiorite Greenish with black or brown
tints.
∙ Texture: Serpentine rock typically looks like reptile
or snakeskin
∙ Location: this rock was found along the coast in
the ocean, near an Ophiolite, specifically the
Franciscan Complex. Additionally Serpentine is
extremely common in this location.

15. Steno’s Law example:
∙ Pictured: Rock layers near
Pacifica, CA at Devil’s slide trail
∙ Here is an example of
Superposition where you can see
the rock layers with the oldest
layers (at the bottom) and the
youngest layers (at the top)

16. Disconformity
∙ In the photo you can see the parallel layers, the
sequence of the different types of rocks
∙ There is erosion between the layers
∙ During interglacial periods sea levels here were
way higher than they are now, and over time
they’ve lowered
∙ The ocean waves have eroded the cliffs
Pictured: Sea cliffs in Half Moon
Bay

17. References
∙ Sloan, D. (2006). Geology of the San Francisco Bay Region (No. 79). Univ of California Press.
∙ Geist, Valerius, and William J. McShea. "Deer of the world: their evolution, behavior & ecology." Arctic 52, no. 4 (1999): 416.
∙ Heffelfinger, J. R., & Latch, E. K. (2023). Origin, classification, and distribution. In Ecology and Management of Black-tailed and Mule Deer
of North America (pp. 3-24). CRC Press
∙ Ohnishi, O. (1991). Discovery of the wild ancestor of common buckwheat. Fagopyrum, 11(5), 5-10.
∙ Ferreira, M. S., Thurman, T. J., Jones, M. R., Farelo, L., Kumar, A. V., Mortimer, S. M., ... & Good, J. M. (2023). The evolution of white-tailed
jackrabbit camouflage in response to past and future seasonal climates. Science, 379(6638), 1238-1242.
∙ Blake Jr, M. C., Howell, D. G., & Jayko, A. S. (1984). Tectonostratigraphic terranes of the San Francisco Bay region.
∙ Stoffer, P. W. (2002). Rocks and geology in the San Francisco Bay region. US Department of the Interior, US Geological Survey.
∙ Reid Jr, J. B., Evans, O. C., & Fates, D. G. (1983). Magma mixing in granitic rocks of the central Sierra Nevada, California. Earth and
Planetary Science Letters, 66, 243-261.
∙ Goodwin, L. B., & Wenk, H. R. (1995). Development of phyllonite from granodiorite: Mechanisms of grain-size reduction in the Santa Rosa
mylonite zone, California. Journal of Structural Geology, 17(5), 689-707.
∙ Harrison, S., & Rajakaruna, N. (Eds.). (2011). Serpentine: the evolution and ecology of a model system. Univ of California Press.