The Geology of So Cal
- Tom Shankapotomous
- Jan 1
- 4 min read
Granite in Transition: Material Behavior and the Rock Cycle in Southern California
As part of The Dirt Project, during the Southern California expedition, (EXP-001) I made two side trips: one to the race site ahead of the event, and another to San Jacinto Mountain, about 45 minutes from the venue. The purpose was to collect dirt and rock samples to better understand the region’s geology. Dominated by extensive granite formations, this area offers a clear, tangible example of the rock cycle in action—illustrating how a healthy planet continually breaks down, reshapes, and renews itself over hundreds of millions of years.
The Rock Cycle as Process, Not Diagram
Granite forms deep underground, from slowly cooling magma. Its interlocking crystals of quartz, feldspar, and mica give it structure and strength. Buried, it stays protected. But tectonic uplift and erosion bring it to the surface. Once exposed, granite begins to fail.
Physical weathering fractures the rock: daily temperature swings, freeze–thaw cycles, and stress from overlying material being stripped away. Chemical weathering starts subtly, often targeting feldspar, which breaks down into clay when exposed to water and weak acids.
Over time, the rock loses integrity. It looks intact from a distance but will crumble under pressure. At this point it becomes decomposed granite—a granular, friable material that still holds the mineral signature of its parent rock, but no longer behaves like rock.
This material hasn’t moved yet. It hasn’t been sorted or abraded. Grains are still angular. There’s almost no organic input. It's loose but unmobilized.
Only when transport begins—through gravity, water, or wind—does it enter the next phase. Edges get worn. Fines separate from coarser grains. Material begins to settle and accumulate in new places. The farther it travels, the more it changes.
Most rock on the surface exists somewhere in this transition—not fully coherent, not fully restructured. These transitional materials are not the exception; they’re the norm.
A Region Built from Granite
The San Jacinto Mountains are made of granite—more specifically, they are part of the Peninsular Ranges Batholith, a massive igneous formation created roughly 100 to 90 million years ago during the subduction of an oceanic plate beneath the North American continent.
Multiple pulses of magma intruded into the crust, cooled, and crystallized. Over tens of millions of years, these granite bodies were uplifted by tectonic forces and exposed by erosion.
What makes Southern California unique is how thoroughly this granite has been laid bare. In most parts of the world, granite remains buried or masked by vegetation and soil. Here, it dominates the surface.
Why? A combination of factors:
Strong tectonic uplift tied to the nearby San Andreas Fault
Aggressive erosion on steep terrain
Arid to semi-arid climate that limits vegetation and slows soil development
Sparse biological cover, which means less organic matter and less binding of loose material
The result is landscape after landscape built almost entirely of weathered granite—loose on the slopes, accumulating in the basins.
The Basin as a Collector
Perris Lake doesn’t sit in a natural lake basin. It’s a human-made reservoir, created in the 1970s by damming Perris Creek. But it now acts as a collector for granite-derived sediment flowing downslope from the surrounding highlands.
The material that athletes run across near the race course has already traveled. It’s no longer decomposed granite in place. It’s sediment—fine, mobile, and easily disturbed.
This sediment is:
Finer grained than in situ DG
Moderately sorted, with some layering
Partially rounded, showing evidence of abrasion
Lightly enriched with organic material from surface exposure
It compacts more easily. It forms dust clouds in dry conditions and slippery surfaces when wet. It doesn’t resist erosion. Foot traffic and rainfall quickly rework it.
The course wasn’t randomly loose. It sits on a recycled granite surface, behaving exactly as expected given its material state.
What the Samples Show
I collected a total of five (5) samples from two distinct but related locations.
They aren’t opposites. They are positions along a continuum. One waiting to move. One already in motion.
Process, Not Product
The traditional rock cycle treats sedimentary rock as an endpoint. But most material never makes it that far. It stays in motion, caught between stages, responding to disturbance, weather, and time.
Decomposed granite and sediment aren’t “unfinished.” They’re active, transitional, and everywhere—especially in places like Southern California where granite is at the surface and erosion is always working.
Understanding terrain means understanding where a material sits in that cycle. Some surfaces are still breaking down. Others have already traveled. Either way, they behave in predictable ways.
For runners, this means recognizing that surface feel isn’t just a design issue—it’s a material history. Dust, slide, slop, and collapse are baked into the geology.
Looking Ahead
In future entries, The Dirt Project will continue collecting and cataloging surface material across venues. But it will also keep asking bigger questions:
How does local geology shape performance terrain?
What are the signals that material has moved?
What does it mean to train on surfaces that are mid-transition?
References
Geological Society of America. (n.d.). Peninsular Ranges batholith: Geology, tectonic evolution, and emplacement. Geological Society of America Bulletin.
Harden, J. W. (2004). Soil development and erosion in dryland environments.
A. R. Orme (Ed.), The physical geography of North America (pp. 271–290). Oxford University Press.
Nettleton, W. D., Brasher, B. R., & Fanning, D. S. (1987). Carbonate and silica cementation in arid and semiarid soils. Soil Science Society of America Journal, 51(1), 176–183. https://doi.org/10.2136/sssaj1987.03615995005100010038x
National Park Service. (n.d.). The rock cycle. U.S. Department of the Interior.
Schaetzl, R. J., & Anderson, S. (2005). Soils: Genesis and geomorphology. Cambridge University Press.
Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2020). Earth science (16th ed.). Pearson Education.
USGS. (n.d.). Weathering, erosion, and deposition. U.S. Geological Survey.
Comments