Walking South-East from the Wilmar staircase, you will see the
Wilmar Street Fault from N35° 8.760 W120° 38.885; this fault is
associated with the
San Luis range (North/West endpoint W120° 79 x N35° 18 and
South/East endpoint W120° 24 x N34° 85; slip rate of about
0.2mm/year).
Walking North-West and looking North-East (the direction and
viewing attitude for this walk), notice a brilliant white
Tuff formation at N35°
8.781 W120° 38.913, with another formation at N35° 8.820 W120°
38.997.
Tuff is formed from volcanic ash which was compressed over the
millennia and ultimately uplifted with erosion and/or seismic
activity. Inspection of the material shows that it is made up of
very fine individual particles that look like miniature angular
crystals.
Continue in this direction – up the beach – and you will come
across a point which is quite passable at low tide;
at the point, there is a stairway, and if you climb up
approximately 30 steps to the spot between a healthy pine and
one that has seen better days, to your right you can see some
sedimentary rock that has Pholad markings, many with the shells
forming an outline.
Now you will need to go around the point – prepare to get your
feet wet depending on the tide, or climb up and down as there are
other stairways for access to the coast.
A fascinating location is at N35° 8.858 W120° 39.028! Here, you
can see a volcanic remnant – an igneous dike
with Gabbro and Xenoliths in the middle
and hornfels on the
side!
There is quite a bit to talk about in this nice little cove. The
igneous dike is the result of hot magma (or lava) squeezing between
sheets of material – in this case, basalt. As it squeezes through,
it often gets pressed into dark, coarse-grained individual rocks
within the dike, and when it carries “alien” material along with
it, well…those are called xenoliths. Hornfels are on the edge of
this: they are the result of hot magma or lava baking the material
next to it. In summary, this structure is the magma (dike), the
rocks as it extruded (gabbro), the odd material carried along with
it (xenoliths), and the result of the heat on the neighboring
material (hornfels). A sandwich!
A break in the hornfels shows a small cove to the North, with
more of the white Tuff.
Moving South, as you end this walk, you will find another fault
at N35° 8.850 W120° 39.011 with the fault showing a division
between two types of Tuff – one which is yellow with mordenite and the
other white (which you saw earlier). The mineral mordenite is in
the classification group called “zeolites”, which are such - that
by the openness of their structure - they may easily include alien
particles (tuff) with this silicate; mordenite can be of many
different colors, ranging from colorless, to yellow, to pink and
red. This yellow Tuff is a bit more transparent than the white as
the included mordenite is much more transparent. Mordenite is
Hydrated Calcium Sodium Potassium Aluminum Silicate, or
(Ca,Na2,K2)AL2Si10O
24. Unfortunately, this mordenite has merged with the tuff,
so you won’t find any large pieces of pure mordenite which can have
spectacular patterns owing to the size of its chemical
channels.
The southern end of the dike can be found at N35° 8.838 W120°
39.008.
To log this earthcache, send an email to me with the answer to the
question:
Which Tuff is the tough tuff? The Yellow Tuff with mordenite, or
the really white Tuff stuff? By “tough” I mean strength and
hardness – can you squish one between your fingers, are they as
hard as they look? If you rubbed one type against the other, which
would win?
For your post: As this is a very scenic spot, please think about
bringing a camera and posting a picture of yourselves. In the log,
please indicate the number of people in your party and include any
links you might find that add value to another visitors’ experience
of this earthcache.