Unless you are camping or have a handicap placard you
must take the shuttle bus down into the valley from Mammoth
Mountain. A fee is required. There is no access during the
winter months. Following the bus ride, there short hike to the
top of the Postpile from the ranger station.
Collecting is prohibitied.
The last of the glaciers flowed through this
valley approximately 10,000 years ago. While the glaciers
flowed through the valley, it eroded much of the lava flow
that creates the postpile. Pressure of ice moving across the
lava flow gradually ground it down and polished the surface.
Rocks embedded in the ice were scraped across the bedrock
creating lines in the bedrock called
striations. These lines indicate the
direction that the glacier was moving.
Initially the entire top of the Postpile was
covered with
glacial polish. Natural erosion has removed
some of the polish. Unfortunately, collecting has also removed a
significant portion of the glacial polish.
Glaciers can also rip off large pieces of bedrock. As glaciers
move over bedrock outcroppings, the ice generally slides up the
upstream side of the outcrop. The pressure of the ice behind it
actually melts a little of the ice lubricating its movement up the
outcrop. Then on the downstream side, the pressure is released,
allowing the water to freeze in the cracks of the bedrock wedging
pieces of rock off. This process is called plucking and it
typically creates steep cliffs on the downstream side of glacier
movement.
Logging requirements:
Send me a note with :
- The text "GC1FE07 Devil’s Postpile Glacial Polish" on the first
line
- The number of people in your group.
- Using the striations in the glacial polish and the location of
the plucked side of the bedrock outcropping, what direction was the
glacier moving at this location?
- See if you can find another outcrop with glacial polish and
plucking.
The following sources were used to generate this
cache:
- USGS, Devils Postpile National Park Geologic
Story; The Postpile is Exposed
http://geomaps.wr.usgs.gov/parks/depo/dpgeol6.html
- drwater.ru
http://www.drwater.ru/index.php?section_num=215
- Thomas Juon and Dak Helentjaris, Last modified
May 17, 1999,
http://gemini.oscs.montana.edu/~geol445/hyperglac/eroproc1/
Placement approved by permit from the
Devils Postpile National Monument