Access this Earthcache by taking the loop trail from the
paved parking lot near the park headquarters. While this is
only a 1-mile long trail, it does involve quite a bit of
elevation change and a few steps. Be prepared with food and
water. The features described in this earthcache can be seen
from the trail.
As you walk along the trail, you can see that the Devil’s
Punchbowl is made up of layered rock that curves around in a bowl
shape. This shape is the result of a plunging syncline that is
described in
The Devil’s Punchbowl Earthcache. From this trail all the
layered rock that makes up the punchbowl is the called Punchbowl
Formation. This formation is found only in this area, an area
bounded by the Punchbowl and San Andreas Faults. An examination of
the composition and features in this Formation provides clues about
where it was formed and how the
San Andreas and Punchbowl Fault have transported it from where
it formed.
A quick examination of the rock outcrops at the coordinates show
that the Punchbowl Formation is a sedimentary rock, a rock made up
of eroded pieces of other rock. It is mostly made up of light gray
to white sand sized particles making it a sandstone. But there are
distinct layers with slightly rounded cobbles, making these layers
a conglomerate. The conglomerate layers are more common in the
older, deeper sections of the formation. You will also find some
layers of grayish-green silt and mudstone, a stone comprised of
silt and mud sized particles. Looking at the patterns in the rock
and layers within rock you will find many parallel layers and some
layers with cross-bedding. The cross-bedding occurs in all three
rock types of the formation. The crossbedding is identified by the
angled layers of grains truncated on the top and bottom by a
horizontal layer.
This is enough information to conclude that the Punchbowl
formation was deposited in a steep sided basin that quickly filled
with sediment transported into the basin by water. The large
cobbles require a lot of energy to move. That much energy is
usually only naturally achieved by fast moving water. Each cobble
is not very round, so they must not have travel very far. Silt and
mud settle out of very slow moving water so they likely were
deposited as the basin filled, decreasing the slope of the sides of
the basin or at the end of episodic rainfalls. The cross-bedding
also supports the conclusion that these sediments were transported
by water. It is thought that this basin formed as blocks of rock
pulled away from each other as the Punchbowl Fault began
moving.
To piece together where the Punchbowl Formation formed requires
examination of the individual particles that make up the rock. The
individual rock types were determined from the large cobbles in the
conglomerate layers and correlated with the granitic and gneissic
rocks in the Cajon Pass area or Pinyon Ridge to the east. In some
areas, there is also a distinct layer of angular cobbles of
San Francisquito Formation, the rock formation that the
Punchbowl Formation covered.
Logging requirements:
Send me a note with :
- The text "GC1ATAZ What the Devil Makes A Punchbowl From " on
the first line
- The number of people in your group.
- An estimate of the percentage of sand, conglomerate and
mudstone in the outcrop near the coordinates, then estimate the
relative frequency of very fast, fast and slow moving water based
how much of each type of rock there is.
- How many layers of conglomerate show cross-bedding at the
coordinates? Can you find a layer of silt/mudstone that shows
cross-bedding?
The above information was compiled from the
following sources:
- Chester, Fredrick M. 1999, Field Guide to the
Punchbowl Fault Zone at Devils Punchbowl Los Angels County Park,
California, V. 2.1, January 1999 Center for Tectonophysics, Dept.
of Geology and Geophysics, Texas A& M University, College
Station, TX
- Dibblee, T.W Jr., Geology of the Devil’s
Punchbowl, Los Angeles County, California, Geological Society of
America Centennial Field Guide—Cordilleran Section,
1987