Along the vertical walls of the Split Mountain Gorge is
the Split Mountain Formation. A geologic formation is an
easily recognizable rock or group of rocks that can be
identified across a large area. Usually the formation is named
after a geographic feature in the area where it was first
described or where the best example of the formation can be
found. This formation has been further subdivided into the Red
Rock Canyon Member and the Elephant Trees Member (also named
after a geographic feature).
The Red Rock Canyon Member
As you enter the gorge you can see reddish colored sandstone at the
base of the cliffs. The sandstone is said to be coarse massive
sandstone. Coarse describes the size of each individual sand grain
in the sandstone and massive indicates that there are no distinct
features (bedding planes, ripple marks, etc) within it, just a big
lump of sand.
It is thought that the sand was deposited in a
braided stream that eroded sand from the surrounding Fish
Creek and Vallecito Mountains
Elephant Trees Member
Above the Red Rock Canyon Member is the slightly younger Elephant
Trees Member is comprised of a reddish boulder fanglomerate. This
member can be recognized by the layers of conglomerates and
sandstone. The layers range from the massive reddish sandstone of
the Red Rock Canyon Member to massive, matrix supported boulder to
cobble conglomerates (the individual boulders don’t hold each other
up), sheet-flood deposits. These rocks probably were deposited in
an alluvial fan setting along the Vallecito Mountains during a
period of uplift about 15 to 20 million years ago. It is also
likely that the area was arid, similar to the way it is now.
Although not visible at this location, on top of the Split
Mountain Formation would be the younger Fish Creek Gypsum
(earthcache). This transition marks a change
from the non-marine Split Mountain Formation to the marine Fish
Creek Gypsum.
The gorge itself is an impressive geomorphologic feature that is
typical of ephemeral desert rivers. For short periods of time, this
gorge will be filled with rushing water. Typically this water is
moving quickly and transporting a lot of sediment, mud, sand,
boulders, anything you see on the floor of the gorge was moved by
water.
This sporadic flow erodes quickly down through the mountain
creating the tall vertical cliffs. Where the gorge makes turns or
if there are an unusual number of floods, the floor of the gorge
widens a little, creating undercut cliffs.
Logging requirements:
Send me a note with :
- The text "GCZ5WK Split Mountain Gorge – Anza Borrego SP" on the
first line
- The number of people in your group.
- Send me a note the approximate size of the largest rock you can
see in the conglomerate.
- Include a guess at the kind of flood that would be needed to
move that size rock.
The following sources were used to generate this
cache:
- Paul Remeika and Lowell Linsay, Geology of
Anza-Borrego: Edge of Creation, Kendall/Hunt Publishing Company,
Dubuque, Iowa, 1992
- Rebecca Dorsey, 2005, A Summary of Late
Cenozoic Stratigraphy, Tectonics, and Basin Evolution in the
Anza-Borrego Desert Region, Ver. 14 July 2005