Sandstone Dike
Most of the rock along these cliffs is miocene era Santa Cruz mudstone (about 7-9 ma). The vertical band is a later intrusion, or "dike," of bituminous sandstone. Dikes like these were actually mined to make asphalt to pave the streets of San Francisco in the late 19th century. The honey colored mudstone is fascinating in itself. It's incredibly light. You can lift a watermelon sized hunk of it with one hand.
Sandstone Dike
Most of the rock along these cliffs is miocene era Santa Cruz mudstone (about 7-9 ma). The vertical band is a later intrusion, or "dike," of bituminous sandstone. Dikes like these were actually mined to make asphalt to pave the streets of San Francisco in the late 19th century. The honey colored mudstone is fascinating in itself. It's incredibly light. You can lift a watermelon sized hunk of it with one hand.