The first outcrop you come to after leaving the parking area is a dark,
fine-grained sheeted igneous rock with no visible quartz. This is
probably a basalt. This formation lasts only a
short distance.
From there down, the rock formations are made up of layers of rhyolite flows which solidified from silica (SiO2) rich
lava, tuffs which are compacted volcanic ash, and volcanic sediments. A
large portion of the tuff and volcanic sediments are made up of material which
settled out of the air after being blown out by volcanic explosions and
eruptions.
At first, the canyon walls are made up of unbedded
horizontal flows of rhyolite which are weathered to
rounded shapes. Some flow breccia is also seen
here. This breccia is formed when a layer of
cooled lava that has solified at the top of the flow
is broken up by the movement of the still fluid, taffy-like mass, and
incorporated into the top layer as large angular pieces. Below the breccia the flow is fine-grained and may show
banding. The banding is caused by minerals which were able to segregate
during cooling. However, large crystals are not present since the cooling
was too fast to allow crystal growth.
On the trail you will see vugs (a cavity or hole in
the rock) filled with a bluish-white mineral called chalcedony which is
microcrystalline quartz. Also you will see spherical cavities filled with
needle-like quartz and feldspar crystals growing outward from the center. They
are called spherulites and were formed by gases and
liquids trapped within the cooling lava.
About one mile from the start, a flow breccia forms a
dam across the stream. In the right season you can enjoy a cool swim in
the pool below the dam. The breccia that forms
the dam is a lot harder than the rock that was eroded away to form the
downstream pool.
About another mile down the canyon there is a drastic narrowing and the floor
of the canyon and the walls are swept clean of all debris by the fast flowing
water. You will see areas of well-bedded steeply dipping formations
containing pebbles in some places. There are tuffaceous
sediments and the fine-grained areas are air-fall tuffs. The air-fall
tuft layers are formed by volcanic ash settling out of the sky during eruptions
and explosions. You can see the difference between these well-bedded
pebbly layers and the very fine-grained layers of rhyolite
lava flow which solidified from lava flowing out of a volcanic crater.
This hike turned around at this point due to the difficult terrain below this
very interesting area. Somewhat below this point,
Notes:
Igneous rocks, as the name indicates, are those that have been
formed by the cooling and subsequent solidification of a once hot and fluid
mass of rock material known as magma.
Basalt. The basalts are dense
fine-grained rocks that are of very dark color, green or black. They are
composed of a soda-lime feldspar with pyroxene, iron
ore, often more or less olivine, and at times biotite,
mica, or hornblende.
Rhyolite. Rhyolite consists chiefly
of alkaline feldspars and quartz. This is the same composition as a lot
of granites, but are very fine-grained and do not show the large crystals of
feldspar and quartz as in the granites.
Tuffs. Tuff is a layered rock
that was formed by the settling of volcanic material blown out of a volcano and
then settling out of the air. The coarser pieces lie on the bottom, while
the smaller, slower falling pieces lie on top, grading down to fine dust.
A breccia is a rock mass with angular
pieces of rock of all sizes. The pieces are sharp-edged and they differ
from a conglomerate whose rocks are rounded by erosion. Breccia is formed by solidified rock at the top of a lava
flow being mixed into the still molten, moving mass of magma.
Prepared in January 2004 by Nello
Modesto. Prepared for the Web Site by Jean Husemann.
Additional Material: GVHC Library File 48