|By Ruth Lebow|
The Geology of the Santa Monica Mountains is still a topic of lively interest, although geologists, botanists, zoologists, ecologists and developers have tramped over its scenic surface for many decades.
Before we try to unravel the geologic complexities of this hillyisland between the LA.. Basin and the San Fernando Valley, a brieflook at the general subject of geology will be helpful. The followingare a few: "take-home" concepts and a little "instant geology".
- Earth is very old; about 4.6 billion years, which is plenty oftime for slow earth processes to produce very large effects.
- Earth is a dynamic planet undergoing constant change. Thelandscape you see today is what you see today. Tomorrow it will be alittle different.
- Earth is layered like an onion. There is a hot solid inner coreof iron and other heavy metals, a molten moving outer core, a thicksolid hot and dense mantle, and a thin broken heterogeneous crust.
- The fractured pieces of crust, called "Tectonic Plates", about60-100 miles thick, are moving around on a molten layer in the uppermantle. Some coming together, some separating from each other, andsome sliding past one another. Most of the major geologic events,such as earthquakes and volcanoes, take place at the plateboundaries.
- California, and most of the west coast of North America, is onthe boundary between the westward- moving North American Plate andthe north- westward moving Pacific Plate. The great San Andreas Faultis this active boundary that influences much of the geology ofCalifornia. Life on a plate boundary is a very moving experience!
- The clues to the history of geologic change in the Santa MonicaMountains are seen in the rock exposures on the slopes and in theroad cuts. the picture is complex with examples of all three greatrock families evident.
- There is evidence of great vertical change, as well ashorizontal movement. In these mountains we see opening and closing ofocean basins, of ancient mountains thrust up from the sea, only to beeroded down to sea level by streams, winds, and waves. The sands andmuds that carpeted the old ocean floors now stand high in themountains as sandstones and shales, in places with fossils that tellus of life at that time. The many faults, large and small, in theSanta Monicas, remind us of the active history and on-going crustalmovements in Southern California.
- Let us take a brief look at the three great rock families asseen in the Santa Monicas, and try to figure out what they all mean.
A. IGNEOUS ROCKS are "fire formed" of moltenmaterial that is either extruded on to the surface from volcanoes, inthe form of lava, or cools deep within the crust, intruded overmillions of years, to be slowly uplifted to the surface. Theoverlying surface rocks are eventually eroded away and the ancientgranites are revealed, as the most common intruded igneous rocks onEarth's surface.
Igneous rocks can be seen in many places in the Santa Monicas. Inthe rocks generally west of Las Virgenes Canyon, there are manyexposures of extrusive igneous rocks, such as dark basalt lavas, orlight colored volcanic breccias, broken rock and ash from ancientvolcanoes. Goat Buttes in Malibu Creek State Park, are dramaticexamples of a period of explosive volcanic activity that occurredfrom about 14 to 13 million years ago.
An interesting exposure of underwater sea-floor volcanism can beseen on Mulholland drive near Stunt Canyon Road. The black roundedforms are "pillow lavas", great extrusions that took place under theoceans that tell of both volcanic activity and an old sea floor.
Granites are exposed in Franklin Canyon, in Coldwater Canyon, inGriffith Park near the Observatory, and other sites and can berecognized by their light color and general "salt and pepper"appearance. When you see granites, be impressed by the long slowperiod of cooling into a solid (perhaps during a million years) ofmiles of uplift, and eons of deep erosion. (The scenic Sierras arealmost entirely granite, with evidence of many intrusions over longperiods of time.)
B. SEDIMENTARY ROCKS are formed of secondarydeposits of bits of pre-existing rocks. These rocks form on thesurface of Earth, at low temperatures and pressures, and the rockparticles are naturally cemented together. Sandstones, obviously,form from sand grains usually deposited in shallow seas or beaches.Sandstones such as the "Topanga Formation" are widely seen in theSanta Monicas and are thick bedded and light tan colored. Sandstonescan be cliff formers. Shales are thin- bedded, formed from compactedand cemented seafloor muds and clays. Shales are seen in many placesespecially along Mulholland Drive near Las Virgenes Road. There theyare contorted and twisted, evidence of the great forces that createdthe Santa Monicas. (All water-laid sedimentary rocks are originallyhorizontal. Good to remember!) Shales called the "Modelo Formation"blanket the north slopes of the range, and in places, contain nicelypreserved small fish fossils.
Another frequently encountered sedimentary rock is conglomeratesometimes called "puddingstone". Conglomerates are composed ofpebbles, sands, sometimes large rounded rocks, that are compacted andnaturally cemented. Conglomerates can be seen in the cliffs at thecorner of Pacific Coast Hwy. and Topanga Road and many other sites.
The age of sedimentary rocks in the Santa Monicas ranges fromabout 150 million years ago to almost the present.
The third group of rocks are the METAMORPHIC ROCKS, which can beoriginally sedimentary, igneous or older metamorphic rocks that werealtered or changed by heat, pressure or deep burial. Sandstonesbecome quartzite; shales become slates; limestones (a sedimentaryrock rare in the Santa Monicas) become marbles; granite becomesgneiss. You can see a fine exposure of slates. thin- bedded dark rockin the heart of the range as you drive through Sepulveda Pass or inFranklin Canyon, Benedict Canyon, etc.