After crossing the Boney Mountain fault just east of Big Sycamore Canyon, we spent the rest of the day in the Lower Topanga Formation, which is about 20,000,000 years old. Most of the Lower Topanga Formation in this west end of the Santa Monica Mountains is dark gray shale, like shown on the right side of this photo. Additionally, the shale has been intruded by abundant dikes of diabase, like the brown rock that cuts across the shale on the left side of the photo.
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