7.62 to 20.32 cm (3 to 8 in.) wide. These often
the ground." Of particular interest is that, according to
Hunt (1975), surface water is associated with polygo-
have another set of smaller polygons that may in
nally cracked crusts on hard surfaces, while capillary
turn be overtopped with mud curls. The primary
fringe input of moisture is associated with net patterns
cracks are generally less than 0.635 cm (0.25 in.)
on soft surfaces.
and many times deeper than wide, but the width
Giant desiccation polygons, stripes, or fissures (e.g.,
may be as much as 2.54 cm (1 in.), thus giving the
Neal 1965, 1972; Neal and Motts 1967) are thought to
surface a characteristic blocky appearance. Rep-
be due in part to subsurface drying (due to decreasing
resentative examples are found at Rosamond
Playa.
Shallow mud cracks from 0.32 to 0.64 cm (0.13
10 to 300 m (33 to 984 ft) but are more typically 1575 m
to 0.25 in.) deep with thin polygons of a varying
(49246 ft) across. The fissures, which are often found
number of sides that typically turn up on the
in association with the polygons, may be several hun-
edges. These are usually less than 15.24 cm (6 in.)
dred meters long and arranged in parallel, in concentric
rings around the polygons, or around the edges of pla-
in diameter and may have a second generation of
yas. Neal and Motts (1967) state "...polygon dimensions
cracks on their surface. This type is common on
are related to...the ephemeral or long-term nature of the
hard playas, but may occur on a few soft playas.
desiccation." Examples occur at Rosamond Playa, where
Representative examples are found at Red Pass
they have been extensively studied because of their
Dry Lake.
Mud cracks that are usually less than 2.54 cm (1
hazards for aircraft operations (e.g., Dinehart and
in.) deep with polygons whose edges turn down-
McPherson 1998); they have been observed recently
ward. The polygons have a varying number of
at El Mirage Dry Lake, California (Brostoff, personal
observation) and Salt Creek Playa, New Mexico (Lich-
sides, generally five, and are commonly 7.6215.24
var and Sprecher, personal observation).
cm (36 in.) in diameter. These are common on soft
Several other smaller-scale formations, related more
playas, but may occur on hard ones as well. Rep-
to surface than subsurface hydrology, are also common
resentative examples are found at North Panamint
and provide varying degrees of inferential evidence for
Dry Lake, California.
Thin, sinuous mud cracks with resulting irregu-
assessing previous inundation. These include crusts,
lar polygons.
The mud cracks are less than 1.27
surface cracks, and polygons. A crust is a surface layer
that is generally more compact, harder, and more brittle
cm (0.5 in.) deep and do not have a second gen-
than the soil beneath (Souirji 1991). Crusts can form from
eration on top. These are typical of salt-clay
a range of causal factors: (1) rearrangement of the soil
encrustations on soft playas. Representative
fabric as a result of wetting and drying, (2) biological
examples are found at Troy Dry Lake and Danby
factors (discussed later), or (3) externally applied
Dry Lake, California.
mechanical pressures (e.g., rain drop impacts). The
The extent to which these five types overlap or pos-
occurrence of crusts, cracks, and polygons on playas
sess unique causal factors has not been rigorously
was reviewed by Stone (1956). Mud cracks form when
investigated. However, the various types are readily
fine-grained sediments lose their contained water; the
apparent in the field and often do not appear to inter-
surfaces between the cracks are known as polygons.
grade on a small spatial scale.
Stone (1956) identified five types of mud cracks/poly-
Patterns associated with evaporites
gons for playas in the southwestern United States:
Large five- to eight-sided polygons up to 61 cm
Some of the cracks and polygons common on hard
(24 in.) in diameter and with cracks from 2.54 to
playas have counterparts on soft playas where evapor-
20.32 cm (1 to 8 in.) deep. These occur around
ites can exert an influence on surface patterns.
the edges of small depressions on the playa sur-
Salt pavements or crusts are conspicuous accumu-
face at Coyote, El Mirage, Ivanpah, and Roach Pla-
lations of salt. These can be intermixed with fine silt and
yas, generally immediately after rains, or around
form surface layers ranging from a few centimeters to
isolated inundated areas. These have also been
more than a meter in thickness. Microrelief of salt pave-
referred to as "mud-crack polygons" (Neal 1965,
ments ranges from a few centimeters to 30.48 cm (1 ft)
Neal and Motts 1967), which are reported to be
(Neal 1965). Salt crusts may be seasonal, being dis-
about 10.16 cm (4 in.) in diameter, a ubiquitous fea-
solved by winter rains (Malek et al. 1990). These are
ture of hard playas, and the result of desiccation
common at Devils Golf Course, Death Valley, California
of fine-grained sediments.
(Neal 1965).
Mud cracks from 0.64 to 5.08 cm (0.25 to 2 in.)
Salt polygons (Stone 1956, Neal 1972) (Fig. 4c) are
deep and four- to five-sided polygons that are from
ubiquitous features on soft playas that contain brine
8
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