Traveling west on RR1233 just past the intersection with FM1053,
one will suddenly find oneself in the middle of a beautiful field
of sand dunes. The dunes are part of a vast set of dunes that lie
east of the Pecos river between Fort Stockton and the New Mexico
border. Other famous fields that are part of this ensemble are the
Monahans sandhills and the Kermit dunes.
The establishment of these dunes probably started 1 million
years ago during the Pleistocene ice age. The sand is believed to
be coming from the erosion of the dry flats of the Pecos river.
Predominantly westerly winds push the sand towards the east.
Dune land is on convex, undulating to rolling ridges that are low
compared to the high plains of the Llano Estaco on the west. As the
wind raises to go above the escarpment, it loses speed and drops
the sand.
The sands have formed active dunes that are constantly shifting
due to the wind. Sometimes vegetation helps stabilize the
dunes.
Dunes usually have a ridge that is perpendicular to the wind
direction. The grade on the opposite side from where the wind is
coming from is usually much steeper.
Grains of sand overtime travel by bouncing up the winward side
(this is called saltation) and then fall down the steep side.
Overtime the dune moves in the direction of the wind. Grains of
sand also form wind ripples, a kind of miniature dunes on the
surface of the dune.
Dunes can have many shapes, but there are two predominant types
in this area:
- Traverse: They have long ridges that are perpendicular
to wind direction. They require a continuous big supply of
sand.
- Barchan: C or crescentic shape dunes. They tend to
happen where the sand supply is lower. The “horns” of
the crescent point downwind.
In fields of sand dunes, Barchan dunes tend to be on the edge and
traverse dunes in the middle.
Tasks
Looking at the Dune that is south east of the coordinates, answer
the following:
- Is it more of a Barchan or a traverse dune?
- Can you estimate the height of the Dune in feet?
- Based in the orientation of the Dune, what approximate angle
are the predominent winds coming from?
- Measure the distance in inches between the smaller ripples on
the surface of the dunes at the bottom of the dune on the winward
side. Repeat the same measurement at top of the dune on the winward
side.
- Can you speculate on why there is a difference?
Please email the answers to the owner of the cache.
Optional: Take a picture of your GPS device or yourself
with the dunes in the background.
Please attach the picture to the log.
References
- Roadside Geology of Texas, Darwin Spearing, Mountain Press
Publishing Company 1991.
- Soil Survey of Ector and Crane counties Texas:
http://soildatamart.nrcs.usda.gov/Manuscripts/TX606/0/Ector_Crane.pdf
- Wikipedia Dunes: http://en.wikipedia.org/wiki/Dune
