This Earthcache highlights the San Francisquito Formation.
Map courtesy of T.W. Dibblee, Jr.
About 60 million years ago a submarine basin formed along a
shallow continental shelf. This basin located in the
area now known as the Devil's Punchbowl. North of the basin is a
granitic body of rock now known as Pinyon Ridge. To
the South, is a body of rock know as the Pleasant View Complex.
Sand, silt, and rounded clast
conglomerates from the Pleasant View Complex
washed into this shallow oceanic basin as Abyssal fans. These fans created the San Francisquito
formation.
East of Devil's Punchbowl is a large exposure of the the San
Francisquito formation. This exposure is known as the "lower unit,"
and it consists primarily of dark shale. An
occasional interbedding of Sandstone is visible from the listed coordinates. This
portion of the formation was created during the Paleocene
age (60 million years ago). The shale unit is considered the
oldest portion of the formation.
San Francisquito Formation Turbidites and
Conglomerates
Northeast of Devil's Punchbowl along Big Rock Creek Road, you'll
see the San Francisquito Turbidite
Deposit. This deposit was the last incursion of widespread
marine conditions on this part of the continent. The majority of
this formation is tan colored, extra hard, arkosic
sandstone. An occasional layer of shale, and
conglomerate layer parts the predominant sandstone layers. The most
amazing part about this portion of the "San Fran formation" is the
orientation of the rock layers. In general this portion of the
formation tilts Southwestward like the older shale portion. Some
portions of this formation are oriented vertically or even
overturned. This was caused by three near vertical faults!
Photographs of steeply oriented sandstone layers that were
once horizontal.
Examples of coarse cobble and pebble conglomerate
lenses.
Requirements to log this
Earthcache.
- Park at N 34° 24.298, W 117° 49.449, then hike to the shale
formation located at N 34° 24.306, W 117° 49.535 and look for the
formation found in this picture. Give a rough count of how many
different layers you see.
- Head to N 34° 25.098, W 117° 50.334, (ample parking is found
South of this point) and look at the conglomerate formation at the
stream level, and above the road. Examine this formation closely.
Email me privately and describe the binding material (the material
used to hold the cobble together.) Does the material resemble
cement, dried mud, etc?
- A Forest Adventure Pass is needed for parking.
