This Earthcache is placed along the San Andreas fault trail in the
Los Trancos Open Space Preserve. Park at N37º 19.567 W122º 10.786.
Here you will find trail maps and an interpretive guide to this and
other geological features of this fault zone. Allow at least an
hour to explore this trail. The terrain is moderately sloped and
mostly shaded.
In these coastal mountain ranges, you will only find natural
ponds in landslide or fault zones. They don't last long,
geologically speaking, as they will fill with sediments and become
marshes, and later disappear as the surrounding bushes and trees
gain a foothold.
Sag ponds are formed when fault movement stretches the crust,
causing the ground to sink. In several places along this trail, and
the nearby Monte Bello trail, you will see a variety of sag ponds.
These were caused by recent fault movement, most likely the 1906
earthquake.
The coordinates take you to a good example of this. At this
point on the trail, you can see the fault marked by posts with a
yellow stripe. This is the San Andreas, and this section of the
fault opened during the 1906 quake, shifting 2-3 feet. The fault
shifted laterally, but also spread apart, opening cracks in the
ground. The cracks have since filled, forming natural benches in
the surrounding terrain. Just beyond the fence, you will see a
large shallow depression. In winter months this fills with water,
becoming a small pond. In the spring and summer it becomes a marsh,
then dries out until the next rain.
Other examples of this can be found in the area:
N37º 19.577 N122º 10.813- look to the north of the parking
lot for a large seasonal marsh.
N37º 19.473 N122º 10.509- Just past the entrance to the
Monte Bello trail, a large year-round marsh that continues along
the fault zone for nearly 1/2 mile.
In order to log this cache, you will need to participate
in the following study and include your findings in your log as
well as answer a simple question to ensure that you physically
visited the location.
A. Measure the length of the pond (bring a tapemeasure) from
East to West. Has the pond grown or shrank since the last geocacher
visited? Is this what you would expect for this time of year?
(For logs after 10-19-07) Has it grown or shrank since this time
last year? Is this what you would expect?
If you visit the pond when it is dried up, describe the dirt in the
low area? Is it hard and packed or soft and loose? Is this what you
would expect in a sedimentary area? Describe any plants growing in
the low area. Are they different from the other plants nearby? Is
this would you would expect for a meadow-to-be?
B. E-mail me the number from a bolt at the posted
coordinates. It is located below and to the left of marker post #6,
on the fence. The number is in the form ###X. Please don't post
this number in your log.
Pictures are always appreciated.
