Page 276 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 259

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