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The North American West Coast illustrates some of the complex geology that develops along a plate boundary. The rocks we see in the Santa Monicas did not form where we see them today. Geologists believe the original sources were island arcs, similar to perhaps Japan or the Philippines; deep sea sedimentary deposits; ancient igneous lavas extruded on the sea floor, and bits and pieces of displaced continental fragments! The whole area, including the Santa Monicas, is extensively faulted, folded, uplifted and undergoing intermittent but strong erosion.

The Santa Monicas, as part of the transverse Range, are indeed unusual in their east-west orientation, as most of the mountain ranges in North America are oriented north-south. This east-west alignment is part of their long geologic history, the slow movement north form unknown locations, and the accretion of strange and complex terranes that we see today.

Geologic history is always recounted starting with the oldest rocks in the area. This would be the Santa Monica slates, metamorphic rocks formed as deep sea muds on some ancient sea, about 150 million years ago, some place, probably to the south and west. Watch for these rocks as you drive through Sepulveda Pass.

What caused the metamorphism? Intrusion of molten material that baked the old sea floor shales into slates. The intrusion, which became the Hollywood granites, took place probably between 110 and 80 million years ago.

This event was followed by millions of years of slow uplift and erosion of the overlying formations that exposed part of the granites and the slates. The eroded sediments consisting of rocks, pebbles, and sands became a conglomerate that covered much of what has become the Santa Monica mountains. Over the next millions of years (remember geologic time and the time scale of 4.6 billion years: the age of Earth!) the land subsided, the sea flowed over the old land surfaces and deposited various marine sandstones and shales.

One of the most widespread and important formations, easy to recognize, is the "Topanga Formation", a group of light colored, thick-bedded sandstones, shales and assorted pebbly formations. The Topanga group was probably deposited about 12 to 25 million years ago. The sandstones represent deposits in shallow, biologically productive seas, During this period, there was also extensive volcanic activity, seen west of Las Virgenes Canyon. The pillow lavas, basalt flows, volcanic breccia are evidence of this volcanic period, with some of the flow having occurred in the sea. A ride on Mulholland Highway will give you a fine introduction to volcanism in that time.

The last great incursion of the sea was about 10 million years ago, as seen in the Modelo Formation of light colored tin-bedded shales that are well exposed in Coldwater Canyon, on the north face of the Santa Monicas.

Today, the mountains seem to be slowly rising, perhaps slowly rotating clockwise, as the range apparently turned 90 degrees after deposition of the Modelo formation. The Channel Islands apparently rotated with the Mountains, as seen in their present east-west orientation, similar to that of the Santa Monicas.

The rocks of the transverse mountains are among the oldest in California. Some of the metamorphic and igneous rock in the San Gabriel mountains is over 1 billion years old. the oldest is a darkly banded gneiss that date back 1.7 billion years ago. You can see this rock on the Angeles Highway near Mill Creek near the Singing Spring tunnel. There is an anthorisite another igneous rock on the road to Baughman Spring. While common on the moon anthorisite is relatively rare on earth. A green schist known as the Pelona Schist forms the eastern side of the San Gabriels. Many rocks from the Paleozoic can be viewed in road cuts throughout the transverse and Peninsular ranges. These include schists, shales, sandstones and limestones. These rocks are between 700 million and 245 million years old. Rocks from the Mesozoic are best representsed in the granitics and gabbros that intruded into older rocks during subduction. Examples of Cenozoic rock include the Vasquez Rocks of the Cespe formation found in the northern San Gabriel mountains. It is composed of red siltstone. Marine sediments found throughout the basin show that this area was once under water.

The rocks of Topanga Canyon date back to the Jurassic period. Some of the metamorphic and igneous rock in the San Gabriel mountains is over 1 billion years old. the oldest is a darkly banded gneiss that date back 1.7 billion years ago. You can see this rock on the Angeles Highway near Mill Creek near the Singing Spring tunnel. There is an anthorisite another igneous rock on the road to Baughman Spring. While common on the moon anthorisite is relatively rare on earth. A green schist known as the Pelona Schist forms the eastern side of the San Gabriels. Many rocks from the Paleozoic can be viewed in road cuts throughout the transverse and Peninsular ranges. These include schists, shales, sandstones and limestones. These rocks are between 700 million and 245 million years old. Rocks from the Mesozoic are best representsed in the granitics and gabbros that intruded into older rocks during subduction. Examples of Cenozoic rock include the Vasquez Rocks of the Cespe formation found in the northern San Gabriel mountains. It is composed of red siltstone. Marine sediments found throughout the basin show that this area was once under water.

The rocks of Topanga Canyon date back to the Jurassic period.

Geologic Period	Age	Type	Location
Oligocene	25-40 million	pebbly sandstone	east Topanga fire road
Cretaceous	65-135 million	granite	stream beds east of the park
Jurassic	135-190 million	Santa Monica slate	Rustic and Sullivan canyons east of Topanga