Paleocene Point Reyes Conglomerate,
Point Reyes National Seashore, California


ID 2987

The Paleocene Point Reyes Conglomerate is composed of granodiorite, varicolored volcanic rocks, quartzite pebbles, and chert. The granite boulders are derived from the underlying bedrock, but the origin of the volcanic rocks is uncertain. In nearby outcrops, some of the subangular granodiorite boulders in the conglomerate are 6 to 9 feet long. The dark volcanic clast in the center is about 6-inches long.  The conglomerate and sandstone beds were probably deposited in a submarine canyon or channel at the head of a submarine fan. 

The Point Reyes Conglomerate is similar in age and composition to the Carmelo Formation exposed at Point Lobos State Reserve, 100 miles to the south. It is likely that the Point Reyes Conglomerate was carried northward from the Monterey Peninsula by the San Gregorio fault, a member of the San Andreas fault system.

The Point Reyes Conglomerate is well exposed along the 0.5-mile trail leading to the lighthouse. Between December and April, this scenic headland is the best place in the park to see gray whales on their southward migration to Baja California or northward migration to Alaska.


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