Geography Reference
In-Depth Information
FIGURE 5.9 Extensive blockslope located in the Indian Peaks Wilderness, Front Range, Colorado.
(Photo by J. R. Janke.)
Blockfields may be actively forming in some areas. Freshly shattered surfaces and
the absence of soil and vegetation indicate recent activity. Active blockfields are relat-
ively unstable, with unbalanced rocks. If you have ever crossed a blockfield, you know
that it is safest to walk over the most heavily lichen-covered areas, since they exhib-
it greater stability, except when the rock surfaces are wet, when the lichens can be
extremely slippery! Inactive or relict block-fields may provide evidence of a past cli-
mate (Potter and Moss 1968). During the Pleistocene, snowline and glaciers extended
to lower elevations, as did colder climatic conditions; thus, the presence of inactive
periglacial features like patterned ground and blockfields can help establish the extent
of past environmental conditions. In some mountain areas, processes similar to that
which formed these relict features at lower elevations may now operate at higher elev-
ations. If the high-elevation surfaces were once covered by ice and protected from deep
frost penetration, block-fields there have probably formed since deglaciation. Where
peaks and ridges were not covered with ice, but stood like islands ( nunataks ) amid the
ice, they would have been susceptible to frost shattering and other weathering pro-
cesses for much longer. Such areas offer critical information about past environments,
but their identification and interpretation are difficult and controversial (Ives 1966;
Dahl 1966). For instance, Andre et al. (2008) suggested that the internal structure of
stone runs (blockstreams, stone stripes, etc.) in the Falkland Islands implies that they
are not of periglacial origin, but formed under subtropical or temperate climates.
Mass Wasting
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