Geology Reference
In-Depth Information
Areas fringing the Northern Hemisphere ice sheets
and other areas that were appreciably colder during the
Quaternary are rich in relict features of periglaciation.
The blockfields (p. 155) of the Appalachian Moun-
tains, eastern USA, are considered fossil periglacial
landforms, and in Norway some Tertiary blockfields
have been identified that seem to have formed under
a mediterranean-type climate. Studies in Europe have
yielded a large number of relict periglacial features
(Box 15.2). Periglacial landforms also survive from pre-
vious cold periods. Siltstones with fossil root traces
and surface mats of fossil plants occur in the mid-
Carboniferous Seaham Formation near Lochinvar, New
South Wales, Australia (Retallack 1999). They represent
ancient soils of tundra and bear signs of freeze-thaw
banding and earth hummocks.
Relicts and climatic geomorphology
Climatic geomorphologists have made careers out of
deciphering the generations of landforms derived from
past climates. Their arguments hinge on the assump-
tion that present climatic zones tend to foster dis-
tinctive suites of landforms (e.g. Tricart and Cailleux
1972; Büdel 1982; Bremer 1988). Such an assump-
tion is certainly not without foundation, but has been
questioned by many geomorphologists, particularly in
English-speaking countries. A close connection between
process regimes and process rates has been noted at
several points in the topic (e.g. pp. 159-60). Whether
the set of geomorphic processes within each climatic
zone creates characteristic landforms - whether a set of
morphogenetic regions may be established - is debatable.
Box 15.2
RELICT PERIGLACIAL FEATURES IN ENGLAND
steep slopes (7-10
◦
) that fluvial activity has reworked
a little.
Non-sorted
frost-wedge polygons
and stripes are
found over large areas of the Chalk outcrop in eastern
England, including many areas covered by Coombe
Deposits. They are readily apparent in soil and crop
marks in aerial photographs. Near Evesham, in south-
ern England, polygonal patterns with meshes 8 m
across have been noted. Remnants of
pingos
occur
in the south of Ireland, beyond the limits of the last
glaciation (Coxon and O'Callaghan 1987). The pingo
remnants are large (10-100 m in diameter) and occur
as individuals, as small groups, and as large clusters.
The tors, rock platforms, and debris slopes on the
Stiperstones in Shropshire appear to have formed con-
currently under periglacial conditions (Clark 1994).
The landscape is thus inherited. The crest-line cry-
oplanation platforms are probably the clearest of the
remnant and they display manifest relationships with
the tors and debris slopes.
England possesses many landforms formed under
periglacial conditions and surviving as relicts. A few
examples will illustrate the point.
'
Head
' is used to describe deposits of variable
composition that were mainly produced by a gelifluc-
tion or solifluction moving material from higher to
lower ground. Head deposits are widespread in east-
ern England and are a relict periglacial feature (Catt
1987). They occur on lower scarp and valley slopes
and overlie a variety of bedrock types. Thick
Coombe
Deposits
lie on the floors of dry chalkland valleys. They
consist of frost-shattered bedrock that has been carried
down slopes greater than 2
◦
by rolling, frost creep, or
mass sliding over melting ice lenses or a permafrost
table. The more extensive thin spreads of stony fine
loams - clay vale head deposits - that cover the floors
of clay vales occur on very gentle slopes (often less
than 1
◦
) or almost level ground but contain stones
from hard rock escarpments several kilometres away.
They appear to be cold climate mudflows initiated on
