Page 274 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 257

off sharply where it meets the steep wall of the main valley of meltwater under pressure. Bedrock bearing a multi-
(Colour Plate 13), often with a waterfall coursing over tude of tiny scratches has a polished look. The finer is
the edge. the abrading material, the higher is the polish. Stria-
tions are equivocal evidence of ice action, especially in the
geological record, as such other processes as avalanches
Domes and whalebacks and debris flows are capable of scratching bedrock. Rock
A variety of glacially abraded forms are less than about basins are depressions with diameters in the range sev-
100 m in size. Domes and whalebacks (rock drumlins, eral metres to hundreds of metres, carved into bedrock,
tadpole rocks, streamlined hills) form where flowing commonlyfoundinassociationwithrochesmoutonnées.
ice encounters an obstruction and, unable to obliterate They form where rocks contain structural weaknesses
it, leaves an upstanding, rounded hillock. exploitable by glacial erosion.

Striated, polished, and grooved bedrock Plastically moulded forms

Striated, polished, and grooved surfaces are all fashioned Some glaciated rock surfaces carry complex, smooth
byrockmaterialcarriedbyflowingice.Largeclasts(about forms known as plastically moulded forms,or p-forms
1 cm or bigger) erode by scratching and create striations (Plates 10.6 and 10.7). The origin of these puzzling fea-
and grooves. Finer material (less than a centimetre or tures is debatable. Possibilities are glacial abrasion, the
so), and especially the silt fractions, erodes by polish- motion of saturated till (till slurry) at the bottom or sides
ing bedrock surfaces. Striations are finely cut, U-shaped of a glacier, and meltwater erosion, especially meltwater
grooves or scratches, up to a metre long or more, scored under high pressure beneath a glacier. If a meltwater
into bedrock by the base of a sliding glacier. They come origin is certain, then the features are s-forms.
in a multiplicity of forms, some of which, such as rat-
tails, indicate the direction of ice flow. Large striations are Abrasion-cum-rock-fracture landforms
called grooves, which attain depths and widths of a few
metres and lengths of a few hundred metres (Plate 10.5). In combination, glacial abrasion and rock fracture pro-
Glacial valleys may be thought of as enormous grooves. duce partly streamlined landforms that range in size from
Grooves form through glacial abrasion or the generation about1mto10km(Table 10.2).

Trough heads, valley steps, and riegels
Trough heads (or trough ends) and valley steps are sim-
ilar to roche moutonnées (see below) but larger. Trough
headsaresteepandrockyfacesthatmarkthelimitofover-
deepening of glacial troughs. Their ‘plucked’ appearance
suggests that they may follow original breaks of slope
related to hard rock outcrops. In sliding over the break
of slope, the ice loses contact with the ground, creating a
cavity in which freeze–thaw processes aid the loosening of
blocks.The ice reconnects with the ground further down
the valley. Where another hard rock outcrop associated
with an original break of slope is met, a rock or valley step
Plate 10.5 Two-metre deep striated groove carved by the develops by a similar process. However, the formation of
Laurentide ice sheet, Whitefish Falls, Ontario, Canada. trough heads and rock steps is little researched and far
(Photograph by Mike Hambrey) from clear.