In late 2018, while on a leisurely hike with my son and dog among the waterfalls in Uvas Canyon Park, I stumbled across an intriguing geologic fault locality. Here, some sort of recent (last few millenia) wrenching of the land ― the ground surface ― seemed to be on obvious display. It was only my second trip to Uvas Canyon Park, and I didn’t know where I was in relation to the geology, and I assumed that this quirky, spectacular landform twisting was due to the passing of the well-known, well-documented, very active San Andreas fault through this spot. I was wrong.
Back at home, I did some online research. The San Andreas fault is nowhere near here.
Instead, the Sargent fault had been mapped passing somewhere through this locality, but buried under gravel, passing somewhere off to the side. I discovered that if anyone anytime earlier had noticed this fault exposure and the associated spectacular geomorphic features, then their findings had not made it into any published geologic maps. Apparently, no geologist had appreciated what’s on display here: a locality where recent movement along the Sargent fault, an active branch of the San Andreas fault system, is on vivid display. The surface expression of neotectonic movement along the Sargent fault over the last few tens of millenia shows up here as a quirky set of tiny hollows, bent streamflows, and warped ground, along with a semispectacular waterfall thrown in for good measure. I revisited the site a couple times and concluded that the Sargent fault is exposed here, and there are numerous neotectonic geomorphic features showing latest Quaternary (including Holocene, likely) dextral surface fault offset totaling about 75 m.
I reached out to the U.S. Geological Survey. Two geologists from the USGS’s Menlo Park branch, Carol Prentice and Belle Philibosian, expressed interest in my findings and accompanied me on a field trip to this locality on April 2, 2019. There, in the field, they confirmed my shutter ridge interpretation, although they may have been nearly as perplexed by some of the tiny localized late Pleistocene and/or earliest Holocene deposits as I was. Whether these geologists with the USGS will follow up on these findings and have the resources to perform any paleoseismologic studies at this site remains to be seen.
In the interim, there’s this writeup that I prepared in spring 2019: