Our geological story begins in Oligocene times, about 35 million years ago, some 200 miles south of the Santa Barbara region. The dinosaurs had disappeared 30 million years earlier. And much of present-day southern California was under water.

River Flood Plain Rocks of the Sespe Formation
A time came when the sea level dropped and a broad section of land was uncovered. Rocks eroding from hills near present-day northern Mexico and the Salton Sea were carried downstream in gurgling creeks. On the way, these rocks broke down into sand, gravel, and mud. Finally they were deposited as the Sespe sediments on a big flood plain by the coast. Oxidation from air bubbles in the creeks rusted the streambed rocks to a dark red color. Many animals, including the ancestors of today's camels, birds, and horses, came to drink from the pools and streams.

Today, the characteristic redbeds of the Sespe are easy to spot on the way up to San Marcos Pass. The fossils of many animals have also been found in the Sespe rocks nearby. In the Foothills area we can see rust-colored soils on the dirt roads, maroon creek banks, and a big red landslide scar along the eastern fork of Atascadero Creek.

The Coastal Sandstones of the Vaqueros Formation
In the earliest Miocene age, some 24 million years ago, the Sespe lands subsided and a warm, shallow sea flowed over them as the shoreline moved inland. Sand layers and tidal life accumulated over the old Sespe redbeds and compacted their sandy mud into silty shale. The overlying sea sands became the Vaqueros Formation. Shells from this tidal zone were embedded in the sand.

Today, the Vaqueros Sandstone appears as gray or tan rocky outcrops at the edges of the steeper slopes. This sandstone is stronger than the other rocks next to it, which erode away easily. So the Vaqueros rocks are left as higher ridges or steeper areas. Beds of broken fossil shells can be seen in some places.

The Muddy Ocean Bottom of the Rincon Formation
Further into the Miocene age, the sea level continued to rise. The shoreline moved inland, and the San Marcos Foothills area became an offshore continental shelf under 100-200 feet of water. Only fine grains of clay were carried out this far from the shore. Here, away from the turbulence of the surf, they dropped to the sea floor and formed the smooth gray muds of the Rincon Formation And after millions of years of burial, the muds became compacted as the Rincon shale.

Today, when a rainstorm soaks the Rincon shale, its clay minerals swell up with water and become slick and mobile. It erodes easily, forming gentle slopes and frequent landslides.

More Mud on the Ocean Bottom
Time passed, and more sediment accumulated over the Rincon muds at the ocean floor. When fish and tiny plankton died, their remains sank to the bottom. Occasionally, clay and some sand would be carried out from shore and deposited on the bottom also. Across the next few million years, these sediments formed many layers of soft, gooey mud which hardened slowly into rock.

Today these rocks form the coastal plains, mesas, and cliffs by the beach. They can be seen from the San Marcos Foothills looking down toward the ocean. But how did all these rocks get pushed up above the ocean, so that we can see them at all?

The Rotation of the Western Transverse Ranges
Ever since the Miocene age, a huge section of earth crust, known as the Pacific tectonic plate, has been sliding alongside of another piece called the North American Plate. The zone of cracks where these plates meet is known as the San Andreas Fault System.

Starting about 18 million years ago, some pieces of southern California's rim tore loose as the tectonic plates ground against each other. One loose piece rotated clockwise more than 90 degrees as it was rolled northward between the two huge tectonic plates. The Santa Barbara region was part of this piece. Eventually it collided with other pieces to the north. In the process, the crust was compressed and wrinkled up into a series of mountain ridges known as the Western Transverse Ranges. The sedimentary rock layers in the San Marcos Foothills are on one side of one of these wrinkles.
For an animation of the Rotation of the Western Transverse Ranges, see
http://www.geol.ucsb.edu/~atwater

The Fanglomerate
As the mountains rose, streams carved steep canyons into their sides. About 2 million years ago, during the Pleistocene ice ages, the climate turned cooler and wetter. Great rainstorms pounded against our region. The action of this water was part of the process of erosion that is continually wearing away the land. Even as tectonic forces folded our land upwards, the rain and floods eroded it down.

In the worst upheavals and storms, huge sandstone boulders broke off the rising mountain cliffs and were swept out of the canyons in a raging mass of rocky debris. When this debris hit the flat area at the open coastal plain, it came to rest, forming a series of alluvial fans. These are known as the Older Fanglomerate Alluvium.

Meanwhile, tectonic compression continued to fold the land upwards. So today we see the Fanglomerate, not at sea level, but as uplifted, sloping mesas. One of them is the boulder-strewn field of the western San Marcos Foothills.

The Drama of Uplift and Erosion
Today tectonic uplift continues in our area, along with erosion from storms and floods. These two forces are counteracting each other, even though we may not see it on a day-to-day basis. But the mountains, canyons, and landslides are all around us, telling the story.

The Sespe, Vaqueros, and Rincon rocks -- and many others -- have been folded and wrinkled up. In the San Marcos Foothills these rocks tilt to the southeast. In other places they have been pushed almost vertical or even overturned. We can see these tilted rocks as we look toward the mountains, or when we hike in the canyons where the creeks are cutting through them and slowly eroding them away.

As long as there is water on the earth and the San Andreas Fault is active, we will have storms and flooding as well as earthquakes and landslides from time to time. And we will be reminded of the two great forces that are at work shaping our land.