Geology Reference
In-Depth Information
conjunction with the overall heave that is associated with freezing of the enclosing sedi-
ments. Upon thawing, the pebble is unable to return to its initial position because the
space originally occupied by the pebble has been compressed by lateral frost heaving (frost
thrusting) during the freezing process and, during thaw, material has slumped into the
hollow.
A second hypothesis, termed “frost-push,” relies upon the greater thermal conductivity
of the stone resulting in the formation of ice preferentially around and beneath it, thereby
forcing the pebble upwards. Upon thawing, as in the frost-pull mechanism, the infi ll of
fi nes beneath the pebble prevents its return to its original position. Both hypotheses
assume that ice lenses grow on the cold (freezing) side of the object with water migration
from the warm (unfrozen) side. However, two-sided freezing of the active layer means this
mechanism cannot be directly applied to upward freezing. Also, the possibility of down-
ward water movement in summer from the thawed active layer into the lower portion of
the frozen active layer, where refreezing and associated ice lensing and heaving may occur,
complicates the process. J. R. Mackay (1984a) concludes, somewhat unsatisfactorily, that
the unfreezing of any object must be considered within the context of several variables.
These include (a) the direction of freezing, either upwards or downwards, (b) the degree
of frost-susceptibility of the enclosing soil, and (c) the degree of frost-susceptibility of the
object concerned.
A related characteristic is that frost action commonly leads to the tilting of stones, the
more angular of which may become aligned on-edge (Figure 6.19A). This phenomenon is
thought to result from differential frost heave at the top and bottom of the stone that
results in the rotation and tilting of its axis. The probable mechanism, illustrated in Figure
6.19B, assumes that “frost-pull,” by the downward-advancing freezing plane, grips the top
of the stone while the lower part remains within the unfrozen zone. Depending upon the
strength parameters of the unfrozen material, the axis of the stone undergoes rotation.
When all of the stone is totally within the frozen layer, rotation and tilting ceases. Thus,
with repeated annual cycles of freezing and thawing, there is a progressive increase in
angle of the axis of the stone towards the vertical, as well as a progressive upward move-
ment of the stone in general.
6.6.4. Frost Sorting
Under laboratory conditions, at least three types of sorting mechanisms have been simu-
lated: (a) sorting by uplift (i.e. frost heave), when freezing and thawing occur from the
top; (b) sorting by preferential migration of fi ner particles ahead of a moving freezing
plane, when freezing and thawing occur from either the top or the sides; and (c) mechani-
cal sorting, when larger particles migrate under gravity when mounds and frost-heaved
structures are produced. Experiments have shown that fi ne particles migrate under a wider
range of freezing rates than coarser particles (Corte, 1966, 1971). This means that a het-
erogeneous material inevitably becomes sorted by freezing.
The effects of frost sorting can be demonstrated both in the fi eld and in the laboratory.
For example, on Banks Island, numerous complex deformations can be observed in the
near-surface sediments (French, 1986, p. 172; Pissart, 1975). Similar phenomena can be
produced by subjecting shallow trays fi lled with sediment layers of different particle size
and frost susceptibility to repeated freeze-thaw cycles (Corte, 1971). For example, in one
experiment when a relatively non-frost-susceptible material was the middle layer, this
layer became convoluted. In a second experiment, when frost-susceptible material was
