Geology of the San Marcos Foothills

By Susie Bartz, Tanya Atwater and Ken Owen

Geologic Setting

The San Marcos Foothills Preserve lies at the dramatic interface of mountain front and coastal plain, where the rocks, geologic structures, topography, and vegetation all reflect the big dynamic processes that are at work shaping our landscape. 

Unlike elsewhere in California, the landforms and geologic structures of this region are mostly oriented in an east-west direction.  This includes the Santa Ynez Mountain ridge rising above local communities.

All the rocks in this landscape are sedimentary. This means that they began as loose sediments – rocks, gravel, sand, or clays – and got carried from a high place to a low place and deposited in flat horizontal layers. Over time, the sedimentary layers got buried, compacted, and cemented into rock. Much later, those rocky layers were folded, tilted on end, pushed up high and partially eroded away, leaving the broken edges of the stronger layers sticking up as ridges. In places like the Preserve, the tilted rocks were then draped with huge deposits­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ of loose, boulder-strewn material that spread out toward the coast to form alluvial fans. In turn, the fans have been cut through by creeks bringing water from the mountain canyons to the ocean.

How did the landscape get such a dramatic shape? The answer lies in the geologic story of our land, and how it has responded to the gigantic forces that are continually acting upon the earth.        

Geologic History in a Nutshell

Our geologic story begins some 35 million years ago, and about 200 miles south of the Santa Barbara region we know today.  Back then, much of present-day California was under water, and the coast ran north-south. For millions of years, sediments  (gravel, sand, silt and clays) were washed off the land and deposited either on the coastal floodplain or farther out onto the continental shelf. There they accumulated in layer upon layer, eventually being compacted and cemented into rock.

Then, about 25 million years ago, a big area of ocean crust, the Pacific plate, began scraping against the continental crust. The San Andreas Fault zone developed where these huge pieces of crust grind against each other. Under all that stress, some pieces of the continental edge broke off and were dragged up the coast. One piece – ours --got snagged at one end, so was rotated more than 90 degrees until it was oriented east-west. About 5 million years ago our piece began colliding with others and its layers got wrinkled and lifted thousands of feet up out of the water, forming the mountain ridges known as the Western Transverse Ranges. The mountain backdrop of our region is one side of one wrinkle -- the Santa Ynez Mountain ridge!  The rocky layers of our region are tilted toward the ocean on the flank of that wrinkle.

About 3 million years ago, the ice ages began in the north and our local climate turned cold and wet. Torrential rainstorms pounded the region, eroding the land away even as compression was pushing it upwards. Huge sandstone boulders broke off the cliffs and were swept out of the canyons in raging flows of muddy debris. When these debris flows reached the flat coastal plain, they came to rest, forming a series of huge alluvial fans. The West Mesa of the Preserve is part of one of those fans.

Even though we may not see it on a day-to-day basis, GPS measurements and earthquakes tell us that the San Andreas deformation is ongoing in our region, compression continues to lift the land.  The high West Mesa of the Preserve, once near sea level, now sits about 500 feet above the shoreline.  At the same time, the creeks are cutting their channels through this rising mesa, eroding it away. As long as the San Andreas action continues, our landscape will be stressed by uplift and deformation.  And the ensuing erosion will continue to carve and shape the San Marcos Foothills.

Rock Formations of the SMFP

A “formation” is a rock group distinct from surrounding rock and occurs on a scale large enough to map. The Preserve contains five major formations:

Sespe, Vaqueros, and Rincon – Oldest, alternately hard/soft, uplifted and tilted south

Fanglomerate -- much younger, drapes over some tilted layers as huge alluvial fan

Alluvium – youngest, fills valley floors and creeks that cut through land toward ocean

Common Rock Types of the San Marcos Foothills

There are five surface geological formations that visitors to the San Marcos Foothills will encounter. These are depicted in the cross-sectional diagram and the geology maps below. Photos of the formations, together with descriptions are presented farther down the page.

Stratigraphy: A Vertical Picture of Rock Layers Across Time

Stratigraphy is a vertical representation of rock layers shown across time. Formations on the Preserve are listed below stratigraphically with the youngest at the top and oldest at the bottom. The chart below includes the abbreviations (Qa, Qog, Tr, Tvq,Tsp) identify the geologic age and name of the formation and are used below and on the geology map in Figure __. Ma means million years old, Ka means thousand years old. “T” means Tertiary period (65 to 1.8 Ma).  “Q” means Quaternary period (1.8 Ma to present). 

Alluvium (Qa)

Age: Holocene (about 11,000 years to present)
Origin: Non-marine: Stream bed debris and valley fill from rocks upslope
Composition: Loose mud, silt, sand, and gravel
Features: Sand/mud bars in streams, undercut creek banks, flat surfaces in wider valleys
Vegetation: Willows and oaks downstream, brush upstream
Where Seen: Creekbeds: Atascadero and Cieneguitas Creeks and their tributaries

Fanglomerate (Qog)

Age: Pleistocene (about 2.6 million years to 10,000 thousand years ago)
Origin: Non-marine: catastrophic debris flows eroded from sandstone cliffs upslope
Composition: Yellowish-tan sandstone boulders and cobbles in loose sandy matrix
Topography: Alluvial fan 3 miles long, uplifted 500 ft to high mesa, incised by downcutting creeks
Vegetation: Grasslands dotted with occasional oaks; pepper tree invasives
Where Seen: Highway 154 roadcut; West Mesa; east edge of the Preserve at Debra Drive entrance Characterized by large boulders scattered around on the surface

Rincon Shale (Tr)

Age: Early Miocene (about 23 to 20 million years ago)
Origin: Marine: clay deposited as mud on an offshore continental shelf
Composition: Gray-tan shale, poorly bedded, weak, easily broken
Features: Swells when wet, shrinks and cracks when dry; spheroidal weathering where exposed Low porosity; excludes water (aquaclude), poor drainage
Topography: Erodes to low rolling hills; collapses as landslides; forms gullies and arroyos
Vegetation: Grasses where open, scrub in gullies, some invasive plants in disturbed areas
Where Seen: Southeastern quadrant of the Preserve at both sides of Cieneguitas Creek Forms smooth clay pathways and roadways underfoot; slippery when wet

Vaqueros Sandstone (Tvq)

Age: Early Miocene (about 23 million years ago)
Origin: Marine: nearshore sands of a shallow, transgressing sea
Composition: Sandstone, gray-tan, bedded, in places coarse, subrounded, moderately cemented
Features: Relatively hard, resists erosion, porous, aquifer, some fossil shell hash, good drainage
Topography: Forms topographic highs: hills, steep creek banks, south-tilted outcrops
Vegetation: Scrubby brush and chaparral common; grass and oak in steeper areas
Where Seen: Diagonally exposed from NW corner trending SW through central the Preserve; also at base of Atascadero Creek and contacting Fanglomerate at base of West Mesa

Sespe Formation (Tsp)

Sespe Formation (Tsp)
Age: Oligocene (about 34 to 23 million years ago)
Origin: Non-marine: gravels, sands, and overbank muds of creeks, deposited on a coastal floodplain
Composition: Red/maroon and gray siltstone and shale with interbeds of tan or gray sandstone
Features: Varies from hard scarps to soft beds with reddish muddy arroyos and slumped areas
Topography: Forms sloped or subdued topography, erodes to valley bowls
Vegetation: Heavy brush and chaparral, some grasses where more sand
Where seen: Hwy 154 roadcut; upper slopes in northeast quadrant of the Preserve. Easily identified by its distinct red sandstone and clay layers.