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Field Trip to the
California Coast

Field Trip Stop #2:

The Lighthouse at Drake's Head
Read the sign
click on the sign to zoom in
Stopping at the Lighthouse, we see the Pt. Reyes Conglomerate, which is Paleocene in age (approximately 60-50 million years old) and is uniquely characterized by its very coarse-grained texture. The grains were clearly derived from the underlying Cretaceous granites and consist of gravel to boulder-sized chunks (clasts) in a sandy matrix, rich in feldspar.
Teachers spent some time observing the Pt. Reyes Conglomerate and then shared their observations and interpretations with the group.

Geologic History:
1. Erosion:

The coarseness of the Pt. Reyes conglomerate suggests that it is a proximal deposit, a unit deposited close to the source. The source was a granitic rock that eroded by mechanical and chemical weathering processes. Mechanical weathering physically fractures exposed rocks creating angular fragments that can then be rounded during transport in rivers and high energy water systems.
2. Sedimentation:
Many sedimentary structures are visible in the Pt. Reyes conglomerate.
Thick sandstone beds are commonly interlayered with the coarse conglomerates beds that show grading, channeling, cross-bedding, and faulting.

The interlayering between the coarse and fine units reflects changes in energy levels associated with deposition in a shallow marine environment along a steep continental margin and an active plate margin setting.

3. Uplift and Submergence:

The conglomerates were derived from erosion of the basement rocks in the area, the Cretaceous granites that were transported northward along the San Andreas fault beginning in the Paleocene. This fragment of continental crust lying west of the San Andreas fault is now riding on the Pacific Plate. Called the Salinian block, it includes the area west of the San Andreas fault that is underlain by granite rather than Franciscan Complex rocks and was transported from areas further south of their present locations near Salinas and the Santa Cruz Mountains.

During their movement, these continental rocks were uplifted and then shed into shallow marine environments where they were further transported by waves and currents. After further uplift, we see outcrops of this granitic block on the west side of the fault.

4. Continued Weathering and Erosion:
The Cretaceous granites exposed on the trail to the lighthouse show signs of extensive weathering. Their reddish-brown color is from the oxidation of iron contained within the minerals of the granite. Cross-cutting fractures are common in the granite and differential weathering occurs along these joints and fractures.

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