Geologically Speaking

Handsome beds of the San Onofre Breccia formation. These beds record a time of debris flow events, stacked one on top of another.

The angular shape of the clasts (bits of rock) within the beds indicate that they weren’t transported too far from their source rock, as the further clasts are transported the rounder they become.

Hammer at lower-right for scale.
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Spheroidal weathering processes are sculpting this monzonite wonderfully.
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Subtle slickenlines spotted within the Orocopia schist.

Slickenlines indicate fault movement. These grooves or striations show the direction of motion between two rock surfaces as they slid past each other.

From this, geologists can infer the direction of past fault movement to help them gain insight into the tectonic forces that shaped an area. Essentially, slickenlines are a record of past fault activity.
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If you were standing at this site five million years ago, you would be floating in a shallow sea, the ancestral Gulf of California. To the west, you would see the ancestral Vallecito Mountains standing high and shedding a large cone of eroded sediment that extended into the sea toward you as an alluvial fan/fan delta. Looking east you would see the ancestral Fish Creek Mountains.

About five million years ago, a great earthquake shook loose 300,000,000 cubic yards of rock from the steep face of the Fish Creek Mountains, the mass fell, shattered on impact, and this sturzstrom (or rock avalanche) flowed at high speeds for seven miles to the northwest toward the Vallecito Mountains.

Lobes of rocky debris detached from the fast-moving mass and injected downward into the sediments on the sea floor, pushing and buckling the sand layers into the anticline in front of you — today, some of the once fast-moving rocky debris that caused the anticline is exposed above and alongside it.

-Dr. Pat Abbott

👉🏽 Swipe for interpretive help.
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Beautiful alternating layers of relatively high-energy and lower-energy alluvial deposits.

High-energy = rocky debris from mud flows.
Lower-energy = sandy layers with small pebbles.
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https://youtu.be/SHigfNuh2xI?si=BQ4LwX95stqWjlIK

Southern California Geology | Deposition in the Salton Trough

Epic tafoni weathering patterns within the Chatsworth Formation, marine sediments from the Late-Cretaceous.
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Delicate orbicules within this orbicular gabbro showcase the stunning intricacy of mineral growth.

Formed by semi-concentric layers of plagioclase and pyroxene, these orb-like structures are the result of a unique cooling process that requires precise conditions within the magma chamber.

The fine, rounded shapes highlight the rock’s complex crystallization and add an unexpected touch of elegance to an otherwise rather boring texture of gabbro.
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Spotted massive sprays of biotite mica within a pegmatite!

Biotite is rich in iron and magnesium, giving it that contrasting dark color compared to the host rock here (primarily quartz and perthite).

Bonus: Did you see the small patch of black tourmaline? Bottom-left, shaped like a butterfly. 🦋
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The wonderful vitreous (mirror-like) luster of muscovite bathed in the golden-hour light. Muscovite has perfect cleavage in one direction, which means it splits easily into thin, elastic sheets and also reflects light very well, easily catching your eye in the field.

Fun fact: the name muscovite comes from the term “Muscovy glass”, which refers to its historical use as a glass substitute in Russia, particularly in the region of Muscovy (now Moscow).

In medieval Russia, muscovite was widely used for windows, particularly in stoves and lanterns, because of its transparency and heat resistance.
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Beautifully reflective sheets of biotite within an amalgam of quartz and perthite (intergrowth of sodic and potassic feldspar).
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