The
coordinates bring you to a large pullout and NPS informational
sign. Two dirt roads head off to the north and northeast.
Alternately, you can go to an overlook along the Mastodon Peak (N33
44.047 W115 47.917). Personally I think the Mastodon Mine view
provides a better view because of its height, but it adds a couple
of stars to the terrain rating and does not allow dogs or
strollers. At both locations, the pediment is located to the east
toward the Eagle Mountains.
Pediments are wide gently sloped regions at the
base of more steeply sloped desert mountain ranges. Often there is
little or no soil on top of the bedrock. While the mechanisms that
form pediments has not been determined with certainty, the
following sequence of events has been proposed as at least
plausible given the geologic history of the area.
During the last glacial period, a thicker layer of soil covered
the area supported by a cooler and wetter climate. However, higher
elevations had a thinner layer of soil than lowlands. Subsoil
erosional processes worked faster on the bedrock core of the
mountainsides eroding back the core, leaving a gently sloped
bedrock base protected by a thicker layer of soil. Then as the
glacial period ended and hotter dryer conditions prevailed, the
vegetation that stabilized the soils died out. The soils were
eroded away leaving the gently sloped bedrock base called a
pediment.
In this theory of pediment formation, bajadas (Bajada
– Joshua Tree NP)are visually similar to pediments,
however pediments are formed from erosion rather than deposition.
There are two features that can help differentiate between a
pediment and bajada. First, pediments will likely have common
exposures of bedrock sticking up through a thin veneer of sediment
while bajadas will not have these exposures. Second, the major
drainages of bajadas can be traced back to a few steep narrow
canyons that are the source of the material deposited on the
surface of the bajada. Pediments will not have these source canyons
and erosion will be removing sediment from the entire surface.
In an alternate theory of pediment formation, a
bajada forms along the mountain front first. Then as the mountain
range erodes so much that it no longer supplies sediment to the
bajada, the bajada begins to erode down relatively evenly. This
results in the broad gently sloping incline up to the bedrock
remnants of mountain.
Logging requirements:
Send me a note with the answers to the following questions:
- What features can you see that indicate the area is a pediment?
Include a quick description of them such as size, color, and
number.
- Does the pediment continue to the west?
- Take a picture of you and/or your gps in front of the sign at
the given coordinates.
Please Note:
1. You must answer all the questions in your email to me and post
all picture(s) with your log.
2. Your logging this cache and sending me the email should happen
at around the same time.
3. Begin your email with the name and geocaching code of this
Earthcache, your name(s), and the number of people in your
group.
Failure to comply with the above will result in
your log being deleted!
The following sources were
used to generate this cache:
- Trent, D.D. & Richard W.
Hazlett, Joshua Tree National Park Geology, Joshua Tree National
Park Association, 2002
- Faulted Structures -- Geology
101
http://www.slackpacker.com/faultedstructures.html
- Encyclopedia Britanica,
pediment,
http://www.britannica.com/EBchecked/topic/448534/pediment#default
Placement approved by the
Joshua Tree National Park