The geology in and around Chico is unusually diverse. Chico sits
right at the beginning of the Cascade range and near the end of
the Sierra Nevadas. Within a short distance, you can see all
sorts of interesting formations.  Abundant Boulders  A Very
Generalized Geologic Map of Butte County by RyanMikulovsky using
data from US Geological Survey and US Census Bureau. See below
for PDF.  Someone driving north towards Chico on Highway 99 may
be struck by the number and size of boulders strewn about. To
the sharp observer, they appear to occur in strips, alternating
between zones of rocks and grassland. Why?  These strips are
part of the Tuscan Formation, which were formed approximately
2-4 million years ago1 by successive volcanic mud flows or
lahars. Lahars typically occur when a volcano erupts causing
snow to melt. This mud flow accumulates volcanic rocks along the
way, eventually depositing onto the valley floor (typically
following river valleys). The mud then hardens around the even
harder volcanic (igneous)