San Marcos Foothills: Of Owls and Boulders

By Susan Vansant Bartz

Last weekend my husband and I went with another couple to the San Marcos Foothills Preserve to enjoy a beautiful sunset. We parked and walked, masked and distanced, into gently sloping boulder-strewn fields until we found a perfect rock, where we set up our chairs and sat together as the sun went down. We watched as the sunlight on the wide-open fields changed from sunny tan to a warm golden color, and finally to a red “alpenglow” as the sun set.  We beheld this stunning reddish glow reflected all along the cliffs of the Santa Ynez Mountains that form the scenic backdrop to our town. Dusk came, and then darkness...

And then, suddenly, as we sat in the darkening night, the owls began to emerge from some nearby oak trees on their night-time hunts. It was beyond spectacular to see how fast and silently they flew and to watch as -- one after another -- they dived straight down toward their prey, then rose within seconds to fly back to their nests. I realized, too, that "our" boulder was providing safe haven for the little creatures whose diggings around the bottom of the rock.

Short-Eared Owl in Flight, West Mesa, San Marcos Foothills (Photo: Mark Bright)

As the stars began to come out, I contemplated the shadowy shapes of the boulders lying here and there in the fields around us. And I realized that the importance of this beautiful open space consists not only in the grassland ecosystem it supports, but also in the dynamic geologic history that it records. Because the boulder fields of San Marcos Foothills are the signature feature of the huge San Antonio Alluvial Fan – an unspoiled remnant from a very different geologic time. 

This alluvial fan - a high mesa-like structure nearly 300 feet thick and some 3 miles long -- was deposited in a series of gigantic debris flows that would make the Montecito debris flows of 2018 look tame indeed. And it is only one of many others along our coast. 

Like the others, the San Antonio debris flows were powered by the torrential rainstorms of the glacial ages.  The deluge of those storms dug the cliff rocks loose and brought huge slugs of earth material roaring down the mountain in catastrophic episodes.  All that material – sand, mud, water, and boulders – came to rest on the coastal plain, where it spread out in a large fan-shaped structure thousands of years ago, and where it remains today.

Boulder Fields of West Mesa, San Marcos Foothills (Photo: Susan Bartz)

Many of our local alluvial fans have been bulldozed and developed one way or another.  Amazingly though, San Marcos Foothills' boulder fields still retain the shape and much of the intact composition of the original super-catastrophe that formed them.  Measurements of some boulders near this particular fan remnant are considered to be about 100,000 years of age by UCSB geologists.  Clearly, the San Marcos Foothills offers a precious and intact piece of our local geologic story.

You can see a nice cross-section of the San Antonio Fan deposits in the road-cut of Highway 154, just south of San Antonio Creek, but don't look unless someone else is driving! Farther east, many homes on the “Riviera” are built on the Rattlesnake Fan, and a few remnant boulders lie near the Riviera Theater’s bus stop.  

…..Finally, night had come and it was time to go. I paused, humbled by the gift of raw natural history offered by these Foothills.  I was thankful for the majestic mountains that provided the boulder where we sat, and for the little creatures whose entrance diggings led to their homes beneath.  As we left, I carried with me the silent symphony of the owl hunts. And I'm thankful to Channel Islands Restoration for leading the effort to obtain and preserve this unique open space.

If you would like to support the campaign to buy and protect the undeveloped portions of the San Marcos Foothills, please visit this site.