WHITE SANDS NATIONAL MONUMENT NEW MEXICO
White Sands National Monument is famous for the white (gypsum) sands that produce a variety of dune types. All of the pictures presented here can be viewed as larger files by following the link in the left hand margin.
Here are a few images from this fascinating place.
The
dunes are actively migrating across the desert floor. In this photo the
dunes are moving left to right. What looks like tire tracks among the
vegetation (interdune area) are actually remnants of an older dune structure.
The lines are sedimentary bedding structures know as cross bedding.
Driving winds cause the sand grains to move along in a bouncing type of motion
called saltation. The sand sized gypsum particles are not lifted
very far into the air (typically only a few inches) and fall back to the surface
of the dune. This same wind that bounces the sand grains along can also
erode older dunes. In this picture an older cross bedding structure of a
dune has been exposed in the area between the dunes.
Smaller scale features are seen on this dune. Ripple marks are produced
when the wind moves the sand along in waves... up one side of the ripple and
down the other. This bouncing movement of the sand (saltation) tends to
pack the grains of sand together to produce a firm surface. A person
walking along this part of the dune would leave only a slight foot print.
The process of compaction is like have a jar full on pennies. If
you have a few more you want to add to the jar there is no room unless you shake
the jar. This shaking causes the pennies to pack together and take up less
space. Along the surface of the dune the multitude of saltating grains of
sand produce tiny impacts that jostle the surface grains into a tighter
alignment.
The
sand on this dune is migrating from left to right. The side that is in the
shadow is called the "slip face". This is where the sand slips down the
side of the dune as it migrates. The saltating sand grains are driven by
the wind. But the wind blows horizontally across the surface of the dune.
The sand grains are too heavy to be carried with the wind so they are deposited
at the edge of the dune. More sand is deposited until the slope becomes
over steepened and slips. The sand grains along this side of the dune are
randomly deposited producing a much "softer" surface. Stepping on the
surface of an active slip face will result in a person sinking in sand to ankle
depth or a little more.
This image is
of an active slip face. The central band (where the ripple marks are
nearly absent) is the steepest part of the slip face. Here the sand has
become unstable and slipped down.
This is another slip face. This image was take at sunset (Sun is to the
right). The shadows nicely show the flow of sand down the slip face.
These three images show progressively closer views of the windward side of one of the dunes. The wind in each case is blowing from the west (right side of the picture). The dune itself has sand that is firmly packed, perhaps even slightly cemented together. The looser surface sand has been blown away to expose the cross bedded nature of the dune. Along this exposed surface more sand in the form of ripples are now being deposited as a new layer on top of the existing dune. The shape of the ripples is similar to aerial photos of dune fields - even dune fields in the polar regions of Mars. These "mini dunes" have many similarities to full sized dunes. The mini dune in the third image is a mini barchan dune. Where these mini barchan dunes merge they produce mini barchanoid dunes. (The small "rocks" seen in the second and third pictures are actually pieces of gypsum crystals that grew in the interdune area.
A
view looking west across White Sands to the San Andres Mountains in the
distance.
