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
||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.