Each year, thousands of people hike to the highest point in the Santa Monica Mountains—Sandstone Peak (3111’). Sandstone, however, is a sedimentary rock made of marine deposits and other sediments. This peak has very different origins.

Imagine standing on the shores of Malibu 24 million years ago. The dinosaurs are long gone, but the Santa Monica Mountains are yet to be born. Broad plains extend from the middle of California to the coast. Much of the land that makes up the Los Angeles area is underwater.

For millions of years, the ground slowly moves, churns, and presses until the Earth begins to crack and molten lava oozes from underwater vents. Thus, only 15 million years ago, the Santa Monica Mountains and Channel Islands began to glimmer in the ocean depths.

In time, the eruptions became explosive and burst through the water’s surface. For another 3 million years, the mountains continued to grow to over 10,000 feet high. Three times taller than they are now! The elements of wind and rain peal away this surface creating the beaches and valleys we know today.

You can download a map of the Geological History Overview of the Santa Monica Mountains area. (PDF)

– Judy Lively, NPS Park Ranger and Mike Theune, Geology Intern