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 hilly island between the LA.. Basin and the San Fernando Valley, a brief look at the general subject of geology will be helpful. The following are a few: "take-home" concepts and a little "instant geology".

1. Earth is very old; about 4.6 billion years, which is plenty of time for slow earth processes to produce very large effects.
2. Earth is a dynamic planet undergoing constant change. The landscape you see today is what you see today. Tomorrow it will be a little different.
3. Earth is layered like an onion. There is a hot solid inner core of iron and other heavy metals, a molten moving outer core, a thick solid hot and dense mantle, and a thin broken heterogeneous crust.
4. The fractured pieces of crust, called "Tectonic Plates", about 60-100 miles thick, are moving around on a molten layer in the upper mantle. Some coming together, some separating from each other, and some sliding past one another. Most of the major geologic events, such as earthquakes and volcanoes, take place at the plate boundaries.
5. California, and most of the west coast of North America, is on the boundary between the westward- moving North American Plate and the north- westward moving Pacific Plate. The great San Andreas Fault is this active boundary that influences much of the geology of California. Life on a plate boundary is a very moving experience!
6. The clues to the history of geologic change in the Santa Monica Mountains are seen in the rock exposures on the slopes and in the road cuts. the picture is complex with examples of all three great rock families evident.
7. There is evidence of great vertical change, as well as horizontal movement. In these mountains we see opening and closing of ocean basins, of ancient mountains thrust up from the sea, only to be eroded down to sea level by streams, winds, and waves. The sands and muds that carpeted the old ocean floors now stand high in the mountains as sandstones and shales, in places with fossils that tell us of life at that time. The many faults, large and small, in the Santa Monicas, remind us of the active history and on-going crustal movements in Southern California.
8. Let us take a brief look at the three great rock families as seen in the Santa Monicas, and try to figure out what they all mean.

A. IGNEOUS ROCKS are "fire formed" of molten material that is either extruded on to the surface from volcanoes, in the form of lava, or cools deep within the crust, intruded over millions of years, to be slowly uplifted to the surface. The overlying surface rocks are eventually eroded away and the ancient granites are revealed, as the most common intruded igneous rocks on Earth's surface.

Igneous rocks can be seen in many places in the Santa Monicas. In the rocks generally west of Las Virgenes Canyon, there are many exposures of extrusive igneous rocks, such as dark basalt lavas, or light colored volcanic breccias, broken rock and ash from ancient volcanoes. Goat Buttes in Malibu Creek State Park, are dramatic examples of a period of explosive volcanic activity that occurred from about 14 to 13 million years ago.

An interesting exposure of underwater sea-floor volcanism can be seen on Mulholland drive near Stunt Canyon Road. The black rounded forms are "pillow lavas", great extrusions that took place under the oceans that tell of both volcanic activity and an old sea floor.

Granites are exposed in Franklin Canyon, in Coldwater Canyon, in Griffith Park near the Observatory, and other sites and can be recognized by their light color and general "salt and pepper" appearance. When you see granites, be impressed by the long slow period of cooling into a solid (perhaps during a million years) of miles of uplift, and eons of deep erosion. (The scenic Sierras are almost entirely granite, with evidence of many intrusions over long periods of time.)

B. SEDIMENTARY ROCKS are formed of secondary deposits of bits of pre-existing rocks. These rocks form on the surface of Earth, at low temperatures and pressures, and the rock particles 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 the Santa Monicas and are thick bedded and light tan colored. Sandstones can be cliff formers. Shales are thin- bedded, formed from compacted and cemented seafloor muds and clays. Shales are seen in many places especially along Mulholland Drive near Las Virgenes Road. There they are contorted and twisted, evidence of the great forces that created the Santa Monicas. (All water-laid sedimentary rocks are originally horizontal. Good to remember!) Shales called the "Modelo Formation" blanket the north slopes of the range, and in places, contain nicely preserved small fish fossils.

Another frequently encountered sedimentary rock is conglomerate sometimes called "puddingstone". Conglomerates are composed of pebbles, sands, sometimes large rounded rocks, that are compacted and naturally cemented. Conglomerates can be seen in the cliffs at the corner of Pacific Coast Hwy. and Topanga Road and many other sites.

The age of sedimentary rocks in the Santa Monicas ranges from about 150 million years ago to almost the present.

The third group of rocks are the METAMORPHIC ROCKS, which can be originally sedimentary, igneous or older metamorphic rocks that were altered or changed by heat, pressure or deep burial. Sandstones become quartzite; shales become slates; limestones (a sedimentary rock rare in the Santa Monicas) become marbles; granite becomes gneiss. You can see a fine exposure of slates. thin- bedded dark rock in the heart of the range as you drive through Sepulveda Pass or in Franklin Canyon, Benedict Canyon, etc.