Page 272 - Fundamentals of Geomorphology
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GLACIAL AND GLACIOFLUVIAL LANDSCAPES 255
EROSIONAL GLACIAL LANDFORMS The ‘knocks’ are rocky knolls and the ‘lochans’ are lakes
that lie in depressions.
Glaciers and ice sheets are very effective agents of erosion.
Large areas of lowland, including the Laurentian Shield Glacial troughs – glaciated valleys
of North America, bear the scars of past ice movements. and fjords
More spectacular are the effects of glacial erosion in
mountainous terrain, where ice carries material wrested Glacial troughs are dramatic landforms (Colour
from bedrock to lower-lying regions (Colour Plate 12, Plate 13, inserted between pages 208 and 209;
inserted between pages 208 and 209). Plate 10.4). They are either eroded by valley glaciers or
A panoply of landforms is moulded by glacial ero- develop beneath ice sheets and ice caps where ice stream-
sion. One way of grouping these landforms is by the ing occurs. Most glacial troughs have, to varying degrees,
dominant formative process: abrasion, abrasion and rock a U-shaped cross-section, and a very irregular long-
fracture combined, rock crushing, and erosion by glacier profile with short and steep sections alternating with long
ice and frost shattering (Table 10.2). Notice that abraded and flat sections. The long, flat sections often contain
landforms are ‘streamlined’, landforms resulting from rock basins filled by lakes. In glacial troughs where a line
the combined effects of abrasion and rock fracture are of basins holds lakes, the lakes are called paternoster
partly streamlined, while the landforms resulting from lakes after their likeness to beads on a string (a rosary).
rock fracture are not streamlined. The remaining group The irregular long-profile appears to result from uneven
of landforms is residual, representing the ruins of an ele- over-deepening by the ice, probably in response to vari-
vated mass of bedrock after abrasion, fracturing by ice, ations in the resistance of bedrock rather than to any
frost-shattering, and mass movements have operated. peculiarities of glacier flow.
There are two kinds of glacial trough: glaciated val-
leys and fjords. A glaciated-valley floor lies above sea
Abrasional landforms
level, while a fjord floor lies below sea level and is a
Glacial abrasion produces a range of streamlined land- glaciated valley that has been drowned by the sea. In
forms that range in size from millimetres to thousands mostrespects,glaciatedvalleysandfjordsaresimilarland-
of kilometres (Table 10.2). In sliding over obstacles, ice forms. Indeed, a glaciated valley may pass into a fjord.
tendstoabradetheup-icesideorstoss-sideandsmoothit. Many fjords, and especially those in Norway, are deeper
The down-ice side or leeside is subject to bedrock frac- in their inner reaches because ice action was greatest
ture, the loosening and displacement of rock fragments, there. In their outer reaches, where the fjord opens into
and the entrainment of these fragments into the sliding the sea, there is often a shallow sill or lip.The Sognefjord,
glacier base. In consequence, the downstream surfaces Norway, is 200 km long and has a maximum depth of
tend to be rough and are described as plucked and 1,308 m. At its entrance, it is just 3 km wide and is 160 m
quarried. deep, and its excavation required the removal of about
3
2,000 km of rock (Andersen and Borns 1994). Skelton
Scoured regions Inlet, Antarctica, is 1,933 m deep.
Breached watersheds and hanging valleys are of the
The largest abrasive feature is a low-amplitude but irregu- same order of size as glacial troughs, but perhaps generally
lar relief produced by the areal scouring of large regions a little smaller. Breached watersheds occur where ice
such as broad portions of the Laurentian Shield, North from one glacier spills over to an adjacent one, eroding
America.Scouredbedrockregionsusuallycompriseacol- the intervening col in the process. Indeed, the eroding
lection of streamlined bedrock features, rock basins, and may deepen the col to such an extent that the glacier itself
stoss and lee forms (see below and Colour Plate 12). In is diverted. Hanging valleys are the vestiges of tributary
Scotland, parts of the north-west Highlands were scoured glaciers that were less effective at eroding bedrock than
in this way to give ‘knock and lochan’ topography. the main trunk glacier, so that the tributary valley is cut