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(a) Formation of surface salt blisters
(b) Formation of polygonal salt plates by
crystallisation from subsurface brines
(c) Lateral growth of polygons and vertical salt
pinnacles
Dry
wind
Point of intense
evaporation and
continuous salt
crystal growth
Forms plate margin ramparts.
May develop into salt
pinnacles as preferential
crystallisation site
Broken blister
Salt polygon
Plates c.
1 cm thick
Mud
Void
Cracks extending to
subsurface mud
(d) Thickening and marginal trimming
(e) Development of thrust polygons
(f) Extrusion of mud pinnacles
Periodic
rain
Plate margins
trimmed by
solution
Brine on plate
crystallises causing
thickening
Mud squeezed through
void by loading of
thick plates
Plates now up
to 30 cm thick
Thermal expansion
of mud
Flow of
plastic
wet mud
Figure 15.11
Idealised sequential development of a playa salt crust (after Krinsley, 1970).
15.3.2 Surface dynamics: mapping pan surface
morphologies using remote sensing
(Figure 15.12(b)), and tufa deposits, partly organic in
origin, as in many of the playas of the southwestern United
States (Neal, 1969). Algae may also be preserved as cal-
careous or siliceous stromatolites towards pan margins,
as at Urwi Pan, southern Kalahari (Lancaster, 1977) and
the East African salt lakes (Casanova and Hillaire-Marcel,
1992).
Because of the difficulties of field investigation of pans
and playas (Millington et al. , 1989), remote sensing data
have been applied to the study of playa basins in a number
of instances in order to overcome these issues and gain
(a)
(b)
Figure 15.12
Playa surfaces: (a) salt thrust polygons, Soda Lake, USA; (b) salt karst chimney (locally termed aioun ), Chott el
 
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