From the Calavera
Hills Trailhead to Stop 1
This shows the
location of the stop #1, just southwest of the volcanic dome. Note the light
colored cement on the right just before you get to the dam levy.
From
the Calavera Hills Trailhead to Stop 1
As
you walk down you will notice the cement culvert on your right, this was
emplaced years ago over the top of excellent exposures of the Cretaceous Green
Valley Tonalite. The remaining pictures were taken before the culvert was
emplaced.The freshly broken surfaces are gray and its coarse grained/phaneritic
as the crystals can be seen easily with the naked eye. Using a hand lens on a
fresh surface you will notice it consists of three minerals. The primary
mineral is grayish-brown and has two distinct cleavage planes. You may note
some striations on the cleaved faces. Most of us are used to seeing a
lighter variety of this mineral in hand specimen. This is a darker variety
of plagioclase feldspar. There are also smaller amounts of amphibole and quartz
in this rock. This unit is mapped as a tonalite which is between a diorite and
a granite in composition.
Photo 1 The new
culvert. Photo 2 Past exposure of the Green Valley Tonalite. Note the fresh
outcrop in the bottom of the ditch, this represents the early stages of
spheroidal weathering and has the characteristic redish/orange
hue.
Fresh tonalite
surfaces, note the gray color and phaneritic (coarse grained) texture. The
greenish hues are not olivine but a byproduct of the photograph.
Stop
1
Stop
1 is on the levee. The lake and levee were built in 1940 using the rocks from
the Calavera Hills Volcanic Dome.
Looking back at
Stop 1 on the levee and lake with the culvert in the background.
At
this stop, there are exposures of the Green Valley Tonalite and volcanic unit.
The tonalite exhibits “spheroidal weathering”, which is common in rocks that
contain cooling joints/fractures. Note the pinkish red orange hues. On the left
lower corner of photo 2 is some volcanic unit, the color is gray with lichen on
it.
1st photo Green
Valley Tonalite with spheroidal weathering and pink red
orange hues. 2nd photo shows the volcanic unit in the lower left corner.
Walking
from Stop 1 to Stop 2 you see eroded pieces of the volcanic unit on the trail
going up the hill. It’s fine grained/aphanitic and gray in color.
Volcanic unit
rubble on the trail to Stop 2.