San Marcos Foothills Geology Features

Features

	Rock Types All rocks in the San Marcos Foothills are sedimentary rocks. Sedimentary rocks form from pieces of pre-existing rocks that have broken down into loose material called sediment. The sediment is usually carried downwards from some high source - a mountain, hill, or cliff - and is deposited in flat layers in a low area - a lake, basin, or beach. Over geological time, each layer of sediment gets buried deeper and deeper as more sediment covers it. And each layer is compressed from the weight of the material on top. The grains in the sediments - gravels, sands, and clays - get coated by chemicals in the ground water. After a long time, the chemical coating acts as a cement that makes the grains stick together. These processes - burial, compression, and cementation - eventually change the loose sediment into hard rock. We call this lithification, and it results in sedimentary rock. The Foothills contain two good examples of sedimentary rocks: the Vaqueros Sandstone and the Rincon Shale. Sandstone is made of sand-sized grains cemented together. The Vaqueros Sandstone has gray or brown sand grains and is quite hard because the cement is strong. Shale is made of clay-sized grains, which are too small to see. For this reason, the Rincon Shale looks smoother than the Vaqueros Sandstone.
	Weathering Rocks weather away as they disintegrate and break down. Weathering can be caused by the action of water, ice, or wind. It can also result from changes in chemistry, temperature, or burial depth in the environment of the rocks. Even the actions of animals and plants can cause rocks to break up. When rocks containing iron are exposed to air and water, their colors change to rusty browns, reds, or mustard-yellows. This is called oxidation, and is the same as the rusting of iron. When water seeps into cracks in the rocks, it often changes the color of the rock in the cracks.	Cracks and landslides in the Rincon Shale cause color changes from oxidation.
	Water can dissolve the cement that holds the grains of a rock together. When this happens, the grains are loosened and the rock falls to pieces. Sandstone without cement becomes loose sand. Sometimes rocks will expand and contract as their surface heats and cools across the seasons. When this happens, the surface of some rocks may split off in thin sheets. An example can be seen in the sandstone boulders of the Fanglomerate.	Weathered surface in Fanglomerate boulder
	In general, rocks weather fastest at their corners, since several surfaces weather in that area at once. This causes the rock to become rounded. In some rocks, this pattern of rounding produces a beautiful "onion-peel" shape to the rock, and is called spheroidal weathering.	Spheroidal weathering in Rincon Shale
	When animals like worms or squirrels or gophers dig their homes in the ground, their burrowing can break up the rocks. And the roots of plants often find cracks in the rocks and grow into them, looking for water that has seeped in. After awhile, the roots grow larger and break up the surrounding rock.	Ground squirrel burrow breaks up the Rincon Shale
	Erosion and Deposition Erosion is the moving of weathered material by wind, water, ice, or gravity. When rocks and sediments have eroded away, they usually leave their mark on the landscape. In the San Marcos Foothills, stream erosion carves out deep canyons where it cuts through hard rock. It leaves wider gullies where it cuts through softer material. Landslides leave scars as steep bare faces where the earth has slid away.	Red scar in Sespe rocks from recent landslide
	Eventually, loose eroding rocks come to rest. This is called deposition. Landslides, alluvial fans, sand bars in streams, avalanches, and river deltas are some examples of materials - rocks, sand, snow, and mud -- that have been deposited after erosion. The high plateau of the Fanglomerate is part of a 3-mile-long alluvial fan. It was created during a series of great storms, when huge masses of boulders and mud came tumbling out of San Antonio Canyon and were deposited on the gentler slopes of the Sespe redbeds below. Alluvial fans cover the foothills in the Santa Barbara region. Some of the sandstone boulders in the Fanglomerate have beautiful, whimsical shapes. These occur where the cement that binds the rock dissolves more quickly in some areas than in others. In the weakly cemented areas, the rock erodes away faster by the action of wind or water. But where the cement is stronger, the rock resists erosion and holds itself together. This process is called differential erosion.	Differential erosion causes whimsical shapes in the Fanglomerate boulders.
	Fossils Two kinds of fossils can be seen in the San Marcos Foothills. One is the actual remains of the ancient plant or animal still embedded in the rock, and the other is the impression or cavity left in the rock after the organism has disintegrated. Fossil remains of wood sticks are found in the Fanglomerate boulders. Their color - a deep rusty brown to black - results from oxidation of the organic material. Their hardness is caused by permineralization, where the wood grain has been filled by a mineral called calcium. The wood fossils are only seen where the boulder has split open, showing us a fresh surface. If too much time goes by, these fossils disintegrate and are lost. They are beautiful, and very fragile.	Wood fossils in Fanglomerate boulder
	Other fossil remains are found in the Vaqueros Sandstone. Pieces of shells - clams and scallops - are cemented into the rock. They tell us that this was once a tidal area near a beach, where the wave action broke up shells and left them in the coarse sand of a tidal channel.	Fossil shells in Vaqueros Sandstone
	Trace fossils are the impressions left in the rock by living things. Examples are footprints and burrows, or excavations left by animals or roots. Traces of burrows can be found in the sandstone boulders of the Fanglomerate, where shellfish dug into the sand. Today we see the gaps in the sandstone where the shellfish once lay, over 35 million years ago. Their bodies disintegrated long ago, but their shapes remain as rounded openings.	Trace fossils in this boulder show the impressions left by ancient shellfish.
	Tectonics: Tilted Rock Layers and Incised Alluvial Fans Tectonics is force that moves huge pieces of the earth's crust over geologic time. Because it happens so slowly, we aren't usually aware of it. But sometimes tectonic forces cause sudden results, like earthquakes, landslides, and volcanic eruptions. Normally, sedimentary rocks lie in flat layers. But all around our area, the rocks are being slowly squeezed together and uplifted by huge tectonic compression. So we see them folded upwards, tilted on end, and sometimes even overturned. In the Santa Barbara region, we are living in the middle of a mountain-building event! In the San Marcos Foothills, we can see clear evidence of tectonic uplift. The tilted layers of the Vaqueros Sandstone poke up through the high ground to the west of Cieneguitas Creek. And on the mountains we can see the cliffs of other, older sandstones tilting almost vertically. It took a gigantic tectonic force to push these layers up to such heights, and they are still rising.	Layers of Vaqueros Sandstone are tilted upwards from tectonic forces
	As the land rises, rocks that were once found near sea level are now found much higher. The alluvial fans of the Fanglomerate were originally deposited low, on the flat coastal plain. But the land has risen since then, so we see those fans as high mesas 600 feet above sea level. As the alluvial fans rise, water from the mountains is flowing downhill, and cutting into the fans. In the San Marcos Foothills, and all along the mountain front, we see the Fanglomerate mesas carved, or incised, with gullies and canyons. Incised alluvial fans are clear evidence of tectonic uplift.