Point Reyes Conglomerate

The intriguing rock exposure in front of you is a part of a formation that caps the highest hills in this area. The Point Reyes Conglomerate is a formation consisting of a sandy matrix embedded with pebbles, cobblestones and boulders. Geologists estimate that the formation may be over 50 million years old.

Here you can see layers intersecting at different angles (cross bedding), and rounded cavities caused by the erosion of poorly cemented materials. Notice also that in any one layer the larger sand grains which sank first are at the bottom, while the finer ones lie on top (graded bedding).

1. This unique rock formation is exposed only on the Point Reyes peninsula.
2. Most of the colorful stones or “clasts” embedded in the conglomerate are about the size of grapes, but some measure several feet in diameter
3. The sands, pebbles, and boulders that make up the conglomerate were transported by fast-flowing streams, currents, and underwater landslides to the marine environment where the rock formed.

Return to Stop #2: The Lighthouse at Drake's Head.

Updated January 5, 2003

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