Alluvial fans form at the base of steep mountains that are rising over geologic time. In catastrophic storms, huge slugs of rocky debris flow out of the mountain canyons to form alluvial fans. Some of the fans in our area are up to 300 feet thick. They form high, sloping plateaus strewn with large boulders in sandy soil.

The San Marcos Foothills area contains the eastern part of an alluvial fan. It is dissected by San Antonio Creek and Highway 154 on the west. Several small lobes and springs line the bottom of the fan, where the land drops abruptly to the underlying Vaqueros and Rincon formations. The tectonic story of the Fanglomerate is a dramatic part of the geology of the Foothills. See Rotation of the Western Transverse Ranges and Fanglomerate in the History section.

Low Slopes and Landslides Shale is sometimes called a "slope-former". It is made of hardened mud and clay, which weather readily. The Rincon Shale is notorious for its capacity to weather and erode. It breaks easily into loose, small pieces. Then water seeps into its fractures and breaks it down further into clay. For this reason many areas of Rincon Shale are covered with a thick layer of clay or adobe soil.

The Rincon Shale forms topographic lows, gently rolling hills and relatively open, wide drainages. In addition, it contains an ingredient called bentonite that swells up and becomes very slippery when it gets wet. As a result, the Rincon collapses and forms landslides readily.

Steep Slopes and Outcrops Sandstone is more resistant to weathering. It is made of sand grains that have become buried, compacted, and cemented together. The cement makes sandstone strong, so it is left in high ridges after the nearby shale breaks down around it. The Vaqueros Sandstone shows resistant outcrops of hard, gray rock. It forms topographic highs with steep slopes and deep cuts in the creek drainages. Where it breaks apart, it forms a sandy soil.

A band of Vaqueros Sandstone crosses Atascadero Creek at the southern boundary of the Foothills area. Here the creek bank is resistant and rises steeply some 40 meters on either side. Both upstream and downstream, the shale in the Sespe and Rincon formations have eroded to form wider valleys.

The Sespe redbeds contain both shales and sandstones as well as other intermediate rocks (silt and claystone). Its shale layers tend to weather and erode between its more resistant sandy layers. This erosion causes undercutting, where the more resistant sandy layer protrudes outward. Eventually the protrusion collapses, and a landslide occurs