Look at the rock exposure next to the road at this location. How
do these rocks differ from other sites you have observed while
walking along the gravel road? Geologists must note changes like
this and interpret the significance.
Rocks can be classified as one of three types: igneous,
sedimentary, or metamorphic. These three categories reflect the
genetic origins of rock; igneous form from crystallization of
molten rock, sedimentary from lithified (compacted and cemented)
particles, and metamorphic from the high pressure/temperature
alteration of other rocks.
The rocks exposed by this road cut are not really one rock type,
but are rather a loose collection of debris derived from some other
rock source. Observe the clearly visible layering. Look at it
closely, but be careful as it is not the most stable road cut!
Notice the shape and average size of the particles within the bulk
of the exposure.
Now step back and study the deposit in total. If you walk a
short distance to either side, you should be able to see where this
particular deposit ends. 
At Reference Waypoint REF1 you can see on the west
side of the exposure where the gravelly layers contact their
bedrock source. Notice how the layering of the bedrock is
significantly different from the layering in the deposit. A similar
boundary exists at the end of the embayment to the east. Clearly
the deposit is confined to this location and therefore must
represent a process that is controlled or enhanced by the steepness
of the terrain.
What might have happened to produce this exposure? A reasonable
explanation is that it represents a deposit known as colluvium.
Colluvium is the name for loose bodies of sediment that have been
deposited or built up at the bottom of a slope or against a barrier
on that slope, transported by gravity. Avalanches, mudslides, and
landslides are processes that deposit colluvium. It is worth noting
that this was clearly not the result of one event, but rather the
accumulation of many events over a long span of time. No doubt,
this process is not over yet.
To log this Earthcache, send me a message with the following
information:
- The text "GC25RVK: Colluvial Deposits" on the first line.
- The number of people in your group.
- The number of past mass-wasting "events" you can identify
within the exposure.
- The average thickness of each layer in the deposit.
- Describe how the layers differ from each other (such as color,
size of fragments, etc).
A limited amount of parking (Waypoint PARK1) is available on a
turnout off of Highway 101, accessible from the northbound lanes
only. There is a graded road leading from the turnout to the top of
the East Cuesta Ridge. You will need to climb over a locked gate.
The Earthcache is about a 2 mile hike or bike ride from the
gate.
Image credits: The Dynamic Earth, by Skinner/Porter,
John Wiley and Sons.
Sources: Special thanks to instructor Jeff Grover of the Cuesta
College Geology Department for providing the content.
