Page 118 - Sedimentology and Stratigraphy

Glaciers 105

the lower parts. Because little or no movement takes
place at the interface of the ice and the substrate, the
glacier does not remove material from the valley floor
or sides by glacial erosion. Cold glaciers are therefore
less important than polythermal and temperate
glaciers in terms of erosion and transport of sediment.
Material carried by cold glaciers is largely detritus
that has fallen under gravity down the upper part of
the valley sides and comes to rest on the top of the
glacier.
Polythermal glaciers are cold-based most of the
time, but as snow and ice accumulate in the upper
part of the glacier, the pressure near the base of it
increases to the point where it melts (the pressure Fig. 7.5 A valley glacier in a temperate mountain region
melting point, which decreases with increasing pres- partially covered by a carapace of detritus.
sure). When this happens there is a glacial surge as
the body of ice moves by basal sliding rapidly down-
slope and during this phase the glacier is capable of
eroding bedrock (Hambrey & Glasser 2003). The gla- Equator. Major mountain ranges in moderate and
cier returns to equilibrium as it reaches a position high latitudes also contain valley glaciers, bodies of
downslope where the pressure is no longer sufficient ice that are confined within the valley sides (Fig. 7.5).
to cause basal melting and the glacial snout breaks up In high latitudes valley glaciers may be fed by larger
and melts. The surge may take place over a matter of bodies of ice at higher altitudes, which are ice caps
months and the retreat of the snout to its former that wholly or partially blanket the higher parts of the
position takes a few years. Detritus eroded during mountains. The lower slopes of a mountain range
the surge is released during the subsequent retreat, may be the site of formation of a piedmont glacier,
so this process is capable of delivering sediment even where valley glaciers may merge and spread out as a
though the glacier is frozen to the bedrock most of body of ice hundreds of metres thick.
the time.
Temperate glaciers are typical of mountainous
regions in lower latitudes. The ice is above the pres- 7.3.1 Erosional glacial features
sure melting point throughout the glacier and it is
able to slide easily over the underlying bedrock The geomorphological features associated with the
(Fig. 7.4). Glacial action is an important erosional glaciations of the past few hundred thousand years
mechanism in mountainous areas with temperate are largely found in upland areas and therefore will
glaciers, with glacial abrasion and glacial plucking not be preserved in the geological record: cirques,
generating detritus ranging from fine-grained rock U-shaped valleys and hanging valleys are evidence
flour to large blocks of bedrock. The action of tempe- of past glaciation, which, in the framework of geolo-
rate glaciers provides an important source of detritus gical time, are ephemeral, lasting only until they are
that is carried downstream by rivers to supply other themselves eroded away. Smaller scale evidence such
depositional environments. as glacial striae produced by ice movement over bed-
rock may be seen on exposed surfaces, including
roche moutone ´e(6.5.4). Pieces of bedrock incorpo-
7.3 GLACIERS rated into a glacier by plucking may retain striae, and
contact between clasts within the ice also results in
In high mountain areas small cirque glaciers scratch marks on the surfaces of sand and gravel
(Fig. 7.1) form in protected hollows on mountain transported and deposited by ice. These clast surface
sides and are found at high altitudes all over the features are important criteria for the recognition of
world, even within a few degrees of latitude from the pre-Quaternary glacial deposits.