Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 249

FIGURE 5.7 (A) Sorted stone polygons under water. Note the increase in grain size from the interior
of the polygon to the exterior. (Photo by J. D. Vitek.) (B) An inactive but sorted stone polygon near
Lily Lake in the Sangre de Cristo Mountains, Colorado. The vegetation in the interior indicates
stability. (Photo by J. D. Vitek.) (C) The internal structure of a stone polygon in Rocky Mountain
National Park shows finer grain size with depth, but horizontal sorting is not apparent. (Photo by
J. R. Janke.) (D) Elongated stone stripes in the Sangre de Cristo Mountains, Colorado. (Photo by
J. D. Vitek.) (E) Regularly spaced earth hummocks in the Colorado Rockies. (Photo by J. R. Janke.)
(F) An active stone polygon from August 1978. (Photo by J. D. Vitek.) (G) The same stone polygon
in July 1987. Note the displacements that have occurred. (Photo by J. D. Vitek.)
FIGURE 5.8 Schematic model for free convection of soil within the active layer. (From Hallet et al.
1988.)
ORIGIN OF PATTERNED GROUND
Patterned ground can be created by a number of different processes in a wide variety
of environments and materials (Washburn 1980). Patterned ground features have been
identified on Mars and may indicate possible sources of ice or may aid in climatological
reconstruction (Mangold 2005). The initiating process in polygonal patterns is generally
cracking of the surface because of desiccation or frost cracking, while circular patterns
are probably the result of frost heave. Sorting of materials is caused by various pro-
cesses, but in cold climates it is mainly because of frost action.
Frost heave, frost thrust, and needle-ice growth all contribute to the segregation of
rocks and fines. The precise mechanisms are still unknown, but the sorting can be ex-
plained in a general way. The surface of the ground typically consists of a heterogen-
eous mixture of coarse and fine particles. The fines hold more water than the coarse ma-
terial and will expand more when they freeze. The fines also cohere and contract more
when they thaw, compared to coarse material. With each expansion, particles move out-
ward from the freezing nucleus; when they settle back, they fail to return completely to
their original positions. The fines tend to congregate in these cycles of expansion and
contraction, leaving the coarser materials in the intervening areas. The process contin-
ues until the centers of fine material begin to impinge on each other, with the larger
particles forming the perimeters of the polygons or circles. In summary, vertical heav-
ing selectively moves coarse particles up, and lateral sorting moves fine particles away
from the advancing (top or sides) freezing front. Mechanical sorting occurs as frost
mounds force coarser rocks to roll to the base of a mound. In another model based on
convectional movement, a rock can migrate upward through denser overlying soil or
rock, driven by buoyancy forces developed seasonally in the active layer (Gleason et al.
1986; Anderson 1988; Hallet and Waddington 1992; Hallet 1998; Harris 1998a, 1998b)
(Fig. 5.8). Eventually, vegetation begins to establish at the edges of patterned ground