Page 307 - Fundamentals of Geomorphology

290 PROCESS AND FORM

Plate 11.6 Solifluction lobes, Drakensberg, South Africa.
(Photograph by Stefan Grab)

Antarctica and in the unglaciated northern Yukon, regions (Figure 11.6e). Unless they cut across geological
Canada. Type 3 comprises frost-shattered and gelifluc- structures, they are difficult to distinguish from struc-
tion debris with moderately smooth, concavo-convex tural benches. Lithological and structural controls are
profiles (Figure 11.6c). Residual hillside tors may project important in their development, which occurs in much
through the debris on the upper valley sides. Such the same way as cryoplanation terraces except that slope
profiles are often identified as relict periglacial forms dat- wash, rather than gelifluction, is more active in aiding
ing from the Pleistocene, but they are not widely reported scarp recession. The processes involved in their forma-
from present-day periglacial regions. Type 4 profiles are tion appear to be bedrock weathering by frost action
formed of gently sloping cryoplanation terraces (also combined with gravity-controlled cliff retreat and slope
called ‘goletz’ terraces, altiplanation terraces, nivation replacement from below. In profile types 3 and 4, resid-
terraces, and equiplanation terraces) in the middle and ual hilltop or summit tors surrounded by gentler slopes
upper portions of some slopes that are cut into bedrock are common on the interfluves. Many authorities argue
on hill summits or upper hillslopes (Figure 11.6d). Cry- that periglacial slopes evolve to become smoother and
oplanation terraces range from 10 m to 2 km across and flatter, as erosion is concentrated on the higher section
up to 10 km in length. The risers between the terraces and deposition on the lower section.
may be 70 m high and slope at angles of 30 or more
◦
where covered with debris or perpendicularly where cut
into bedrock. Cryoplanation terraces occur chiefly in HUMANS AND PERIGLACIAL
unglaciated northern Yukon and Alaska, and in Siberia. ENVIRONMENTS
They are attributed to nivation and scarp recession
through gelifluction (e.g. Nelson 1998), but substantive Attempts to develop periglacial regions face unique and
field research into their formation is very limited (see difficult problems associated with building on an icy sub-
Thorn and Hall 2002). Type 5 profiles are rectilinear strate (Box 11.3). Undeterred, humans have exploited
cryopediments, which are very gently concave erosional tundra landscapes for 150 years or more, with severe
surfaces that are usually cut into the base of valley-side or disturbances occurring after the Second World War
mountain slopes, and are common in very dry periglacial with the exploration for petroleum and other resource