The San Andreas Fault is a defining feature of the geology of the
Santa Clara Valley. We live in a zone where two massive plates of
the Earth's crust collide and fracture, occasionally releasing
great amounts of energy in the form of earthquakes.
The Pacific Plate slides north past the North American Plate. In
the past, its eastern edge slipped beneath into the Earth's mantle
(a process known as Subduction). Because of this, older stone
slipped under the crust and became magma that that fueled the
volcanic ranges to the east. The rocks we see now near the edge of
the plate are relatively young. On the other side of the fault, the
rocks along the western edge of the North American plate are much
older. This edge has been pushed upwards, and overlying newer rock
is eroded away, exposing the older layers.
In the area in and around Sanborn County Park, you can walk
across time by passing from the North American Plate to the Pacific
Plate. For this cache you will visit two very similar creek
canyons. To get to the first, park at the coordinates posted above,
and walk to N37º 14.830 W122º 03.976. Here the stream has
eroded the upper layers of rock, exposing an outcropping of shale
and chert (the dark stone with white streaks running through it).
This is part of the Franciscan Complex, and dates to around the
start of the Cretaceous Period, about 140 million years ago.
This rock was formed from sediments in an ancient sea. At this
time there were dinosaurs on the mainland, about 150 miles east,
where we now find the Sierra Foothills. The coast was very
different then, more closely resembling the Andes of South America.
These high mountains eroded and deposited sediments on the sea
floor. Over millions of years, as the plates collided, pressure and
heat transformed these sedimentary layers into the metamorphic
forms you see here. About 25 million years ago, the edge of the
plate fractured, forming this fault system, and the coastal
mountains began to rise. The upper layers have eroded away,
exposing these rocks.
Now drive up the road and park at N37º 14.322 W122º
04.270. On the way, you will be crossing over the San Andreas
Fault (look for evidence of the fault rift zone at N37º 14.214
W122º 04.042 as you drive up) You are now standing on the
Pacific Plate. Walk to N37º 14.304 W122º 04.430. Here the
more recent seafloor has been pushed up, and what you see is the
Vaqueros Marine Deposit. This is the primary rock you see in
outcroppings in Castle Rock State Park to the west of this site.
This sandstone dates from the early part of the Oligocene period,
about 36 million years ago. In these sedimentary rocks you may find
fossil shells and shark teeth.
Over time the Pacific Plate will continue northward, and these
soft rock formations will eventually wash away. The coast ranges
will continue to rise while the stones on the eastern side wash
into the valley floor, exposing older rock formations. The
landscape of 100 million years in the future will look as different
from this , as this does from the land where these rocks were
formed.
Logging Requirements:
To log this cache, you must perform a geological excersize by
making some observations of the different types of rock formation
found at the cache sites.
1) Examine the chert (darker stone near the water) at site one.
Describe it's texture. Scratch it with a pocket knife. Is it hard
or soft? Write down your observations.
2) Examine the sandstone at site two. Describe it's texture,
scratch it as well. Over time and pressure, sediments are
compressed and change form. Which rock was harder, the older or
newer stone?.
Email your observations.
