Geoscience Reference
In-Depth Information
stone stripes (Anderson,
1993
; Werner and Hallet,
1993
) and stone polygons, block
fields (
Felsenmeere
) of frost-shattered angular rubble, block streams or rock glaciers
and extensive mantles formed by
periglacial solifluction
. Solifluction simply means
soil flow, and can result from a variety of processes, such as soil creep, which do not
necessarily require frost action. The prefix periglacial denotes soil flow resulting from
freeze-thaw activity.
Needle-ice is a very visible aspect of frost action, with small pillars of ice a few
millimetres wide and a few centimetres high pushing up small pebbles and frozen
soil, which is later distributed downslope as the ice melts during the day. Needle-ice
is an effective agent at disrupting the surface plant cover.
Other common periglacial features are
terracettes
, or turf-banked terraces, which
are caused by frost disrupting the turf or grass cover, with downslope movement of the
turf layer so that it overrides the grass cover further down the slope. Vigilance is needed
not to confuse these landforms with terracettes caused by overgrazing or originating
as sheep or goat tracks in upland areas. In some instances, as in the highlands of
Botswana, overgrazing can exacerbate the effect of frost action (Hastenrath,
1972
;
Hastenrath and Wilkinson,
1973
).
Thufurs
(Troll,
1944
) are small, vegetation-covered
mounds about 30 cm high and are characteristic of sites with abundant soil moisture.
On gentle slopes in many high mountains, the plants often grow in rough circles up
to a metre in diameter.
Where periglacial solifluction is especially active, it tends to smooth out small
topographic irregularities in the landscape, filling hollows, and can sometimes dis-
guise the effect of previous glacial erosion. Periglacial processes can also modify
glacial moraines, and this is sometimes only evident by examining the micro-fabric
(Mills et al.,
2009
). By removing weathered mantles from irregular bedrock surfaces,
periglacial solifluction can in some instances help create
tors
, or isolated pillars of
rock. Once again, caution is needed, because tors can form in a variety of different
ways and do not require periglacial processes in order to form (Thomas,
1994
).
The distribution of these features reflects the temperature prevalent at that particular
elevation or latitude. On Mounts Kenya and Kilimanjaro, the lowest occurrence of turf
exfoliation from freeze-thaw processes coincides with the 3,500 m contour marking
the zone of 0
C temperatureminimum, while stone stripes are common onKilimanjaro
above 4,400 m (Hastenrath,
1973
).
Both Hastenrath (
1974
) and Williams et al. (
1978
) observed a variety of minor
periglacial freeze-thaw features above 4,300 m on Ras Dashan in the Semien High-
lands of Ethiopia, including stone-banked terraces, stone stripes and polygons, fine-
earth polygons, recently frost-shattered boulders and fields of unstable, angular basalt
blocks. Neither Budel (
1954
) nor Hastenrath (
1974
) nor Williams et al. (
1978
) found
any evidence of present-day frost shattering or of movement of the resulting angular
rubble below about 4,250-4,300 m, which is also the upper limit of tussock grass in
the Semien Mountains.
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