Craggy or rounded, green or purple, the mountains that ring the San Fernando Valley have many faces. Although their geologic histories are similar, the Santa Susanas and the Santa Monicas are youngsters compared to the granddaddy of our mountains, the San Gabriels.
Before there was a San Fernando Valley, seas covered the area and dried up, leaving sediment that turned into rock. Underwater volcanoes erupted to the west, spewing lava. Then the tremendous forces that move continents caused land to fold and uplift, creating the mountains. The mountains we see today began pushing up about 4 million years ago, eroding into the ranges that Valley residents know. Geologists attribute the existence of these mountains of the Transverse Range to the same forces that cause earthquakes on "low-angle thrust defaults" similar to the one that caused the Northridge quake. Quakes in these areas can create mountains or raise ranges that already exist. The Santa Susanas rose about 15 inches in the Jan. 17 quake. The 1971 Sylmar quake, also on a thrust fault, lifted the San Gabriels three feet.
The mountains are still rising and eroding, and evidence of earth-moving activity miles below the ground is visible in many areas. The San Andreas fault can be seen in the Antelope Valley, and other faults are visible in road cuts around Chatsworth. Layers of ancient sedimentary rock, often tilted from past quakes, are noticeable at many places throughout the mountains.
What Caused Our Mountains
When plates collide rather than slide horizontally past each other, rocks will either fault (crack), or fold upward, forming mountains. In our region, earthquakes on low-angle thrust faults have caused rocks to move vertically, as well as horizontally. This results in the horizontal layers of sedimentary rock, or strata, at the surface to be pushed up. The buckling forms an arch called an anticline, visible as the upswept mountains that surround the Valley.
Geologists categorize folds and, according to some geologists, the mountains here are the result of "fault propagation folds." Similar to a rug on a floor being pushed together, rock layers that were once flat have been pushed up when one block rises over another at a thrust fault.
What They Are Made Of
The mountains are made up of sedimentary, igneous and metamorphic rock. Some sedimentary layers are horizontal but most are tilted and twisted due to folding and uplift from quakes. Sedimentary rock is youngest and metamorphic rock, which is the core of the mountains, is oldest.
The result of weathering and erosion, these rocks formed from materials that were once part of older rocks, plants and animals. They accumulated in layers called strata and hardened. Most in our mountains are less than 18 million years old.
* Sandstone: Natural chemical substances cement grains of sand together.
* Shale: Clay is compressed into thin, hard layers, which often split apart.
* Conglomerate: Naturally cemented gravel.
Rock that has solidified from a molten or nearly molten state.
* Granite: Hard, coarsely crystalline rock that makes up most of the Earth's continental crust.
Sedimentary and igneous rock that changed due to heat or mountain-building movements deep in the Earth's crust, geologists call this "basement" rock. It makes up the core of our mountains.
* Gneiss: Formed from metamorphism of granite, causing feldspar and quartz crystals to form layers between which mica crystals often lie in wavy bands.
Where They Are Headed
Geologists divide California into "geomorphic" regions. The Transverse Ranges east-west, the opposite of most other western ranges, which run north-south. Many geologists believe the Transverse Ranges used to run north-south before rotating 90 degrees starting about 14 million years ago. They believe this was caused by isolated blocks of rock caught between the southeastward-moving North American plate and northwestward-moving Pacific plate. According to this theory, the twisting opened the space now occupied by the Los Angeles Basin.
Santa Susana Mountains
* Size: 30 miles long, 16 miles wide at longest point.
* Highest point: Oat Mountains, 3,747 feet.
* Appearance: Brushy slopes made up of young, sedimentary rock. The oldest exposed rock is 15 million years old. Very little metamorphic rock is exposed. Vegetation is abundant because sedimentary rock allows good soil to build up quickly and good chaparral cover to develop.
* History: About 3 million years ago, a large river created a delta in what is now the Santa Susanas and Simi Valley. The river existed until about 700,000 years ago, when seismic forces squeezed the area up, forming the Santa Susanas. The mountains lifted so quickly the river couldn't cut through the range and was forced to the north, forming the Santa Clara River.
San Gabriel Mountains
* Size: 64 miles long, 24 miles at widest point.
* Highest point: Mt. San Antonio, 10,064 feet.
* Appearance: Erosion is deeper than other ranges, exposing older metamorphic and igneous rock such as granite and gneiss, giving the mountains their rocky, craggy look. Vegetation is sparse since there is little soil in which seeds can germinate.
* History: Igneous rocks in the range area age 80 million to 1.2 billion years, and some metamorphic rocks are older than 1.5 billion years. Lifting began 45 million years ago; by 25 million years ago, the mountains were fairly large. Rapid uplift has taken place in the last 5 million years. Since at least 80 million years ago, about 12 miles of rock has been eroded and deposited into the Santa Monica Bay.
* Size: 14 miles long, 5 miles at widest point.
* Highest point: Verdugo Peak, 3,126 feet.
* Appearance: Exposed granite that formed 200 million years ago is visible. The Verdugos are considered the foothills of the San Gabriels, so their rock content is similar. They have the same geologic history of erosion of sedimentary layers.
Santa Monica Mountains
* Size: 50 miles long, 10 miles at widest point
* Highest point: Sandstone Peak, 3,111 feet.
* Appearance: Tilted sedimentary rock, predominantly sandstone and shale, is rounded because it is young and less eroded. Underlying granite, the oldest rock, can be seen where they have been exposed by erosion in Griffith Park. Dark Santa Monica slates are visible in Topanga State Park and in road cuts along the San Diego Freeway. Brown or reddish volcanic rock can be seen in the western part of the range. Modelo shale, which forms a mantle over the older rock layers on the north flank, is the youngest formation, containing fish and seaweed fossiles. Chaparral thrives in sedimentary soils of the steep hillsides.
Formation of the Santa Monicas
This geologic timeline highlights some of the events that formed the Santa Monica Mountains. Histories of other ranges parallel this one, although some rock is much older. Years on the timeline are approximate.
140 million years ago: Slates in area that is now Santa Monicas are formed when mud or shale is deposited beneath a shallow sea.
140 million years ago: Oldest known bird, Archaeopteryx, the size of a crow, flew the earth.
120 million years ago: Molten rock bakes sedimentary shales into slates and cools into granite.
90 million years ago: Period of uplift and erosion, with seas covering the land. Marine rocks that formed the cliffs in lower Topanga Canyon are deposited.
80 million years ago: Eroded area begins to be covered by oceanic sedimentary rock. Area begins long period under water.
63 million years ago: Mammals became the dominant animals on Earth.
60 million years ago: Another cycle of erosion begins and then another return of the sea, laying down sedimentary rocks of marine origin, which can be seen from Topanga Canyon.
25 million years ago: Western part of range remains under water for another 10 million years.
15 million years ago: Underwater volcanoes near Conejo Mountain erupt, spewing lava, now visible as rock in Conejo Valley and Griffith Park.
13 million years ago: Volcano becomes extinct. Area again drops under water and remains that way for about 10 million years
5 million years ago to present: Seas recede. Compression pushes land up, causing the folded and faulted rocks we see today.
2 million years ago: Modern human beings develop and the Ice Age begins.
Sources: California Division of Mines and Geology; Dr. Peter Weigand, Dr. Eugene Fritche, Cal State Northridge; Davis-Namson Consulting Geologists; "California Landscape"; "Flowering Plants of the Santa Monica Mountains"; World Book Encyclopedia