The Geology of the Santa Monica Mountains: Seafloor on the Ridgetops, Mountains Turned Sideways

Santa Monica Mountains National Recreation Area lies near greater Los Angeles, in Los Angeles and Ventura counties on the coast of southern California. It takes in rugged mountains, chaparral-covered canyons, abundant wildlife and 64 km (40 miles) of coast with sandy beaches and rocky shores.

The northern Channel Islands offshore belong to a largely submerged part of the same province — once its southern end, before it rotated — and share its history (see the Channel Islands atlas page).

A complicated place

The mountains hold active faults, fossils, remnants of volcanoes, and rock layers laid down both on land and under the sea — the marine ones now often found on the tops of ridges. Most of the rock is Miocene, but the oldest dates back about 162 to 143 million years, to the Late Jurassic.

The rocks record a turbulent history on the edge of the continent:

1. Subduction. The oceanic Farallon Plate dove beneath the western edge of the North American Plate — and broke up.
2. Stretching. Part of the East Pacific Rise was partly subducted; the margin of North America then stretched, sank and faulted, and some pieces were captured by the Pacific Plate.
3. Rotation. Blocks of crust along the margin — including the Western Transverse Ranges — rotated.
4. Transform. A sliding boundary between the North American and Pacific plates developed along today's San Andreas Fault System.

Rising and falling sea levels, driven by the growing and shrinking of the Antarctic ice cap, left alternating marine and land-formed layers across the mountains.

Why these mountains run east–west

In most of California, faults, folds and the mountains and valleys they produce run northwest–southeast. In the Transverse Ranges they run east–west. They once ran roughly north–south — but complex interactions between neighboring plate edges rotated them about 70° to 110° clockwise away from the northern Peninsular Ranges.

Boney Mountain, elevation 3,000 feet.

Maps and data

The mountains belong to the Pacific Border physiographic province. The NPS Geologic Resources Inventory, which produces digital geologic maps and reports for more than 270 natural resource parks, has products for the park in the NPS DataStore, and a Soil Resources Inventory is complete (DataStore).

More from the park: natural features and ecosystems, geologic formations and fossils. Elsewhere: the California Geological Survey.

Sources

Based on "NPS Geodiversity Atlas—Santa Monica Mountains National Recreation Area, California," National Park Service, a product of the Geologic Resources Inventory; a work of the United States government in the public domain. The imported copy stopped mid-paragraph; the rest of the geologic setting comes from the live page. The image of the GRI report's cover is left out; rewritten in hubnx's own words.