Geology Reference
In-Depth Information
Plate 10.9 Chattermarks on Cambrian quartzite, An Teallach, north-west Highlands, Scotland.
( Photograph by Mike Hambrey )
different sides, a pyramidal peak or horn may eventu-
ally form. The classic example is the Matterhorn on the
Swiss-Italian border.
Supraglacial landforms
Debris on a glacier surface lasts only as long as the glacier,
but it produces eye-catching features in current glacial
environments. Lateral moraines and medial moraines
lie parallel to the glacier. Shear or thrust moraines ,
produced by longitudinal compression forcing debris to
the surface, and rockfalls, which spread debris across a
glacier, lie transversely on the glacier surface. Dirt cones ,
erratics (Plate 3.3), and crevasse fills have no particular
orientation with respect to the ice movement.
Many features of supraglacial origin survive in the
landscape once the ice has gone. The chief such forms
are lateral moraines and moraine dumps, both of which
lie parallel to the ice flow, and hummocky moraines
and erratics, which have no particular orientation. Lat-
eral moraines are impressive landforms. They form from
frost-shattered debris falling from cliffs above the glacier
and from debris trapped between the glacier and the
valley sides (Figure 10.6c). Once the ice has gone,
lateral moraines collapse. But even in Britain, where
glaciers disappeared 10,000 years ago, traces of lateral
moraines are still visible as small steps on mountain-
sides (Plate 10.10). Moraine dumps rarely survive glacial
recession.
Nunataks
Nunataks are rock outcrops, ranging from less than a
kilometre to hundreds of kilometres in size, surrounded
by ice. They include parts of mountains where
ice has not formed, or entire mountain ranges,
including the Transantarctic Mountains on Antarctica
(see Figure 10.2), that have escaped ice formation
everywhere but their flanks.
DEPOSITIONAL GLACIAL LANDFORMS
Debris carried by ice is eventually dumped to produce
an array of landforms (Table 10.3). It is expedient to
group these landforms primarily according to their posi-
tion in relation to the ice (supraglacial, subglacial, and
marginal) and secondarily according to their orienta-
tion with respect to the direction of ice flow (parallel,
transverse, and non-orientated).
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