Geology Reference
In-Depth Information
A
riegel
is a rock barrier that sits across a valley, often
where a band of hard rock outcrops. It may impound
a lake.
from glacial abrasion. An example is found on slate in
North Wales, where pyrite crystals have small tails of rock
that indicate the orientation and direction of ice flow
(Gray 1982), and on carbonate rocks in Arctic Canada,
where limestone ridges less than 5 cm high and 25 cm
long form in the lee of more resistant chert nodules
(England 1986).
Cirques
Cirques
are typically armchair-shaped hollows that form
in mountainous terrain, though their form and size are
varied (Figure 10.6). The classical shape is a deep rock
basin, with a steep headwall at its back and a residual
lip or low bedrock rim at its front, and often contain-
ing a lake. The lip is commonly buried under a terminal
moraine. They possess several local names, including
cor-
rie
in England and Scotland and
cwm
in Wales. They
form through the conjoint action of warm-based ice and
abundant meltwater. Corries are commonly deemed to
be indisputable indicators of past glacial activity, and geo-
morphologists use them to reconstruct former regional
snowlines (Box 10.2).
Rock-crushed landforms
Small-scale, crescent-shaped features, ranging in size
from a few centimetres up to a couple of metres, occur
on striated and polished rock surfaces. These features
are the outcome of rock crushing by debris lodged at
the bottom of a glacier. They come in a variety of forms
and include lunate fractures, crescentic gouges, crescentic
fractures, and chattermarks.
Lunate features
are frac-
tures shaped like crescents with the concavity facing the
direction of ice flow.
Crescentic gouges
are crescent-
shaped gouges, but unlike lunate features they face away
from the direction of ice flow. Crescentic fractures are
similar to crescentic gouges but are fractures rather than
gouges.
Chattermarks
are also crescent-shaped. They
are friction marks on bedrock formed as moving ice
judders and are comparable to the rib-like markings
sometimes left on wood and metal by cutting tools
(Plate 10.9).
Stoss and lee forms
Roches moutonnées, flyggbergs, and crag-and-tail fea-
tures are all asymmetrical, being streamlined on the
stoss-side and 'craggy' on the leeside. They are the
productions of glacial abrasion and quarrying.
Roches
moutonnées
are common in glacially eroded terrain.
They are named after the wavy wigs (moutonnées) that
were popular in Europe at the close of the eighteenth
century (Embleton and King 1975a, 152). Roches mou-
tonnées are probably small hills that existed before the
ice came and that were then modified by glacial action.
They vary from a few tens to a few hundreds of metres
long, are best-developed in jointed crystalline rocks, and
cover large areas (Plate 10.8; see also Plate 10.6). In
general, they provide a good pointer to the direction
of past ice flow if used in conjunction with striations,
grooves, and other features.
Flyggbergs
are large roches
moutonnées, more than 1,000 m long.
Crag-and-tail
features
are eroded on the rugged stoss-side (the crag)
but sediment (till) is deposited in the smooth leeside.
Slieve Gullion, County Down, Northern Ireland, is an
example: the core of a Tertiary volcano has a tail of
glacial debris in its lee. Small crag-and-tail features occur
where resistant grains or mineral crystals protect rock
Residual landforms
Arêtes, cols, and horns
In glaciated mountains, abrasion, fracturing by ice, frost-
shattering, and mass movements erode the mountain
mass and in doing so sculpt a set of related landforms:
arêtes, cols, and horns (Figure 10.6). These landforms
tend to survive as relict features long after the ice has
melted.
Arêtes
are formed by two adjacent cirques eating
away at the intervening ridge until it becomes a knife-
edge, serrated ridge. Frost shattering helps to give the
ridge its serrated appearance, upstanding pinnacles on
which are called
gendarmes
('policemen'). The ridges,
or arêtes, are sometimes breached in places by
cols
.
If three or more cirques eat into a mountain mass from
