Conejo Volcanic Breccia, Ventura County, California

ID 275

Miocene dacitic breccias of Conejo volcanics near Sandstone Peak (misnamed) in the Santa Monica Mountains National Recreation Area. During Miocene time, extensive volcanic activity took place in the western Santa Monica Mountains and Channel Islands. Many of the rocks are believed to be of submarine origin, based on the recognition of pillow structures in lava flows. The volcanic rocks attain a thickness of approximately 10,000 feet in this region. The east-west trending Santa Ynez Mountains are present in the distant background.

A moderately strenuous trail (3-miles round trip) leads to the summit of Sandstone Peak (3,111 feet), the highest point in the Santa Monica Mountains. On a clear winter day, I saw the Channel Islands as well as snow-capped peaks in the Santa Ynez and San Gabriel Mountains, north of Los Angeles.

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