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Geology of the Big Chico Creek Ecological Reserve
Topography
The Reserve is located in the foothill portion of the Big Chico Creek watershed.
The property is irregular in outline and extends
a maximum of 4.5 miles in a North/South axis and 2.0 miles East/West. The narrowest part (where Big Chico
Creek first enters the property) is only 0.5 mile East/West. Elevations range from 700 feet where Big Chico
Creek exits the property to 2160 feet at the northeastern property line. The eastern approximately half of
the property lies within Big Chico Creek Canyon with a fairly narrow floodplain and terraces, then walls
rising 1000 feet in about 0.5 mile, often including vertical cliffs. The western half can be described
as occupying the top of a gently tilted plain dissected by small headwater streams. The south (lower)
end of this plain is about 1700 feet, dissected in spots to 1000 feet. The north (high) end is about
2000 feet, dissected to 1800 feet. Soils are relatively shallow, bedrock is exposed in many places,
and surface boulders are scattered over much of the area.
Geomorphology
Geologic processes in the Reserve are dominated by the stream dissection of
gently sloping layers of rock to form steep canyons. The upper portions of the Reserve are in the Tuscan
Formation, about 4 million years old, composed of layered ash, volcanic
mud flows (lahars) and patches of alluvium. Some of the lahars are very hard and form
cliffs along the sides of the canyon. Beneath
the Tuscan Formation lies the Lovejoy Basalt, about 15 million years old, very resistant to
erosion and often forming cliffs.
Below the Lovejoy is the Chico Formation, about 75 million years old. It is
sedimentary rock composed primarily of sand and contains numerous marine
fossils. Relative hardness and permeability of the rock layers have a major influence on hydrology
and vegetation within the Reserve. Mass wasting, landslides, and slumps have been major contributors
to the current morphology of the canyon. Since the Chico Formation is much softer than the Lovejoy
Basalt, as the stream carved its canyon it undercut the Lovejoy, allowing huge slabs and boulders of
basalt to tumble down into the creek to be left on the eroded surface of the sandstone after the
creek meandered away. As the canyon deepened, the hard Lovejoy formed cliffs high up the walls.
Pieces of the basalt, ranging from small cobbles to large boulders, have fallen off the cliff
faces to form talus slopes on the gentler surface of the Chico Formation. Occasional huge slabs
have calved off the Lovejoy cliffs to generate debris flows that fanned out across lower terraces.
Between the Lovejoy and the Tuscan lies a layer about a foot thick of tuff from a volcanic ash fall.
Water seeping through the tuff weathers it to clay, forming a slip surface that lets masses of Tuscan
slump off the top of the Lovejoy. Consequently, the floor and lower walls of the canyon are often
covered with deposits of material from above, and the Chico Formation is rarely visible except in
parts of the active stream channel. The Lovejoy basalt
appears to be intermittent so that in some parts of the reserve the Tuscan Formation rests directly
on the Chico Formation. In areas where the Lovejoy Formation is present, Chico Canyon is narrower
and the creek bed is steeper.
Reserve lands from the top of the Musty Buck Ridge west consist of gently sloping ridge
tops of exposed Tuscan dissected by headwater streams from Sycamore and Mud Creeks. Some cliffs are present
in the deeper canyons, but landslides have been much less important than in Chico Canyon.
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Marine Fossils in a rock from the Chico Formation
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