Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 60

the sorting by differential hop-length and rolling of different
particle sizes or compositions. Dune slip faces can result in
distinctively layered sand deposits (Fig. 5.19 ), some layering
of which may represent the advancing slip face of an active
dune. Traditionally, the layered structure of dunes was
explored by sectioning or trenching the dune, sometimes
with a bulldozer, although ground-penetrating radar can
sometimes reveal such layering with less effort (see Chap. 18 ).
In some circumstances the positions of former slip faces are
exposed in the top surface of the dune, revealed either by
cementation (Fig. 5.20 ) or by sorting (Fig. 5.21 ).
David Rubin of the U.S. Geological Survey has com-
prehensively mapped out via simulation how a growing
layer of sand might record the style of deposition. In par-
ticular, when a repeating sequence of winds generates
alternating episodes of slip face avalanching and ripple
movement, a 3-dimensional record of layers is left in the
resultant sandstone (Fig. 5.22 ). Usually only one, but
sometimes more than one, section of this 3-dimensional
pattern is exposed, in which case the interpretation can be
non-unique. Rubin and Carter's (2006) animations of the
growing pattern are a mesmerizing introduction to the
richness of the patterns that can be found. The striking
cross-bedded sandstones in Southern Utah (Fig. 5.23 ) are
perhaps among the best-known examples of this aeolian
deposition and lithification process. Some magnificent
cross-bedding can also be seen in the sandstone into which
the remarkable ancient city of Petra in modern Jordan was
hewn: planetary geologists have been known to get more
excited by its sedimentology than its archaeology!
The advent of very high resolution imaging from Mars
rovers has now enabled similar cross-bedding textures to be
found on that planet. The Opportunity rover studied
Victoria Crater, a 750 m-diameter crater in Meridiani
Planum, from 2004 to 2008 and observed stratified sedi-
mentary rocks in the crater walls. As explored by Hayes
et al. (2011) these layers chronicle the paleo-environments
of Terra Meridiani and provide glimpses into the broader
history of early Mars. The stratigraphy at Victoria Crater
(Fig. 5.25 ) includes the best examples of meter-scale cross-
bedding observed on Mars to date (Fig. 5.24 ). The Cape St.
Mary promontory is characterized by meter-scale trough-
style cross bedding, suggesting sinuous-crested dunes with
scour pits migrating perpendicular to the outcrop face.
Finally, another deposit type can form in certain cases,
where the sand is easily cemented. This is particularly the
case for gypsum. Rain can cement gypsum sands to form a
resistant layer: this will prevent movement for some time
until the layer is broken up. However, where the layer
intersects the substrate surface it may be preferentially
preserved, leaving a surface scar that marks the perimeter of
the dune. These can be seen in some instances at White

Fig. 5.13 Ubiquitous TARs seen on Mars. From the image alone, it is
possible to infer that these are ridges and they are transverse (strictly
the 'aeolian' in TAR is an assumption, though it seems safe to expect
that this valley system has not seen water flowing for some
considerable time). In all probability these are ripples in genetic
terms, but TAR is a safer term. They are appreciably larger than
typical ripples on Earth. MOC image (E02-02561) Credit NASA/JPL/
MSSS

particles migrating via creep can accumulate within a sand
deposit at locations that represent a former bedform,
although stringers of coarse particles preserved within
aeolian sandstone (in contrast to sandstone derived from
sediment deposited by a river or other fluvial process) are
surprisingly rare in the terrestrial geologic record (Sullivan
et al. 2012). However, saltation and avalanching in dunes
and ripples can lead to magnificent subsurface structures.
On the stoss side of a dune, grains are impacted onto the
upward-sloping surface, and the grains tend to be packed
(making it easier to walk on the stoss side). The wind on the
sheltered lee side of the dune is weak, and the sand grains
tend to be dropped by the airflow. The lee side of a dune
under sand-driving winds experiences almost continuous
slope failure in the form of numerous avalanches as dis-
cussed earlier.
Different episodes of deposition (e.g., seasonally-depen-
dent winds) can result in distinct layers—usually distinct by
means of different compaction and/or particle size (see pic-
tures earlier in this section). Makse (2000) shows with a
numerical model how cross-bedded layers can form due to