Geology Reference
In-Depth Information
The climatic signifi cance of cracking is further complicated by the nature of the sub-
strate in which cracking occurs. Ground temperatures are generally lower in clay than in
sand and gravel. As a result, N. N. Romanovskii (1985) suggests that cracking in clay
substrate may occur with mean annual air temperatures as high as
−
2 °C, but in gravel,
the minimum temperature required may be
6 °C. Further caution is provided by C. R.
Burn (1990a), who reported upon elevated tritium concentrations in ice within frost cracks
near Mayo, Yukon Territory. These values mean that, despite a mean annual air tempera-
ture of
−
4 °C, thermal-contraction cracking must have been active at some time during
the previous 30 years. One possibility is that contraction cracking at Mayo is related to
below-normal winter temperatures, perhaps associated with cold-air drainage.
Because cracking is sensitive to a number of climate-related factors, it follows that a
change in crack frequency may refl ect a change in climate. In the western North American
Arctic there is a body of anecdotal evidence that supports this. For example, the sound
of cracking was reported in early literature (e.g. Leffi ngwell, 1915, pp. 638-639) from
along the northern Alaska coast. However, on Garry Island in the outermost Mackenzie
Delta, where cracking is known to be occurring today (Mackay, 1992a), no audible crack-
ing or ground tremors resulting from cracking have been recorded during more than 30
years of monitoring. Today, audible cracking appears only to be reported from the High
Arctic islands (Mackay, 1993a). Likewise, at Inuvik, 150 km south of the Arctic coast,
winter temperatures are similar to those on Garry Island yet no cracking has been
observed during the 1980-1988 period. The ice wedges are inactive. The critical factor
that allows cracking to occur on Garry Island and not at Inuvik is that the former has less
than half the winter snowfall of the latter. Therefore, the presence of inactive ice wedges
at Inuvik and the absence of audible cracking today along the mainland Arctic coast
suggest an increase in winter snowfall and higher winter temperatures during the last
hundred years.
In summary, any climatic interpretation of thermal-contraction cracking must focus
upon winter air temperatures, local site conditions, and winter snowfall amounts. In the
Pleistocene context, the use of ice-wedge casts to reconstruct paleo-temperatures, as
often attempted in the European periglacial literature, is fraught with uncertainty. A
recent questioning of this approach (Murton and Kolstrup, 2003) is highly appropriate.
The paleo-environmental signifi cance of thermal-contraction cracking is discussed more
fully in Part III.
Soil wedges pose a number of specifi c problems. First, it is not known what type of
frost-action environment is necessary for them to form and whether perennially-frozen
ground is necessary. Instances of seasonal-frost cracking have been reported from middle
latitudes (Svensson, 1977; Washburn et al., 1963). Second, as stressed by J. Dylik (1966,
p. 260), the reason for downturning of sediments adjacent to soil wedges is not well under-
stood, and certain Siberian structures developed in silty alluvial terrace sequences may
be subaqueous water-escape structures. Elsewhere, apparent frost cracks may be seismic-
related (Leshikov, 1999). Third, the close spacing of many soil wedges is diffi cult to
explain. Clearly, our understanding of soil wedges is incomplete and their use as indicators
of perennially-frozen ground not justifi ed.
−
6.3. ORGANIC TERRAIN
Organic material is particularly effective in protecting the ground beneath from atmos-
pheric heat. This is most clearly demonstrated towards the southern limit of the discon-
tinuous permafrost zone where permafrost is commonly restricted to peaty organic
