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
Teachers spent some time observing the Pt. Reyes Conglomerate
and then shared their observations and interpretations with
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.
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.