Blue Cut and the geology you see around you

The Santa Clara River watershed is located in a geologically active area within the San Andreas Fault system which forms the ever-changing boundary between the Pacific and North America tectonic plates.

Relative motion of the plates as they spin and drift and seek the same destination creates what is called strike-slip displacement (rocks moving laterally along the trend of the fault zone) as well as "convergence" which, as it sounds, are rocks smashing into one another. Convergence has led to rapid uplift in coastal and interior mountain ranges throughout the region. California west of the Sierra Nevada range might be thought of as a mix of rocks brought on a pacific plate conveyor belt over the millenia to smash into our continental plate and become a jumbled mix of ages. Geologists call this a "franciscan melange" and what had baffled centuries of early practicers of the science became explained by plate tectonics and continental drift theories of the 1960's up the road a bit at UCSB and a man named Diblee.

Anyway, back to the blue cut promontory and its metamorphic - (think baked and compressed and chemically changed) - rock. Due to the same compression that forced up the mountains north and south of your drive, the blue cut strata was made very dense and difficult to erode. It formed a pinch in the valley where the looser alluvium (think runoff of sands and pebbles from uphill canyons) was ushered by in the stream as it ran in pulses over the millenia. It was this promontory of very hard rock that re-routed the flood waters into a whirlpool that cold March morning.

Persistent regional geologic instability since about 28 million years ago has exposed a wide variety of highly deformed, fractured, and faulted rock types in the Santa Clara River watershed (Yeats and Rockwell 1991; Rockwell et al. 1984; Rockwell 1988). Igneous and metamorphic rocks, including gneiss, schist, and granite, dominate in the upper watershed to the east, while younger sedimentary and volcanic rocks are more prevalent in the lower watershed, west of the San Gabriel Fault. Fractures, deformation, and faulting contribute to high bedrock erodiblity throughout the watershed.

Intense seismic activity in the region is reflected in frequent ruptures along faults. Seven of the roughly 30 high-magnitude (MW ≥6) earthquakes that have shaken southern California over the past 80 years have occurred in the Transverse Ranges. Seismic shaking during the magnitude 6.7 Northridge event in 1994 triggered nearly 7,400 landslides in the watershed highlighting the importance of geologic factors in the production of sediment, which ultimately affects the dynamic geologic processes in the lower river corridor.