Page 122 - Sedimentology and Stratigraphy

Continental Glacial Deposition 109

melts. Kames and kame terraces are mounds or
ridges of sediment formed by the collapse of crevasse
fills, sediment formed in lakes lying on the top of the
glacier or the products of the collapse of the edge of a
glacier.

7.4.3 Outwash plains

As the front of a glacier or ice sheet melts it releases
large volumes of water along with any detritus being
carried by the ice (Fig. 7.10). Rivers flow away from
the ice front over the broad area of the outwash
plain, also known by their Icelandic name sandur
Fig. 7.10 A dark ridge of material within a valley glacier
that will form a medial moraine when the ice retreats (plural sandar). The rivers transport and deposit in
(viewed from the air). the same manner as a braided river (9.2.1) (Boot-
hroyd & Ashley 1975; Boothroyd & Nummedal
1978; Maizels 1993; Russell & Knudsen 2002). The
melting. A few metres thickness of rock debris forms large volumes of water and detritus associated with
effective insulation and prevents the ice below it from the melting of a glacier mean that the outwash plain
melting. These ice-cored moraines (ablation mor- is a very active region with river channels depositing
aines) give the impression of being much larger sediment rapidly to form a thick, extensive braid plain
volumes of detritus than they really are because deposit (9.2.1). Outwash plain deposits (Fig 7.8) can
most of their bulk is made up of ice. be distinguished from other braided river deposits by
their association with other glacial features such as
moraines.
7.4.2 Other glacial landforms The most spectacular events associated with glacial
sedimentation are sudden glacial outburst events
Lodgement tills deposited beneath a glacier may form known by their Icelandic name as jo¨kulhlaups. The
sheets that can be tens of metres thick, or show outburst can be either the result of the failure of a
irregular ridges known as ribbed moraines. These natural dam holding back a lake at the front of the
tills also form smoothed mounds known as drumlins, glacier or a consequence of melting associated with a
which are oval-shaped hills tens of metres high and subglacial volcanic eruption. Very large volumes of
hundreds of metres to kilometres long, with the elon- meltwater create a dramatic surge of water and sedi-
gation in the direction of ice flow. In temperate gla- ment, which may include some very large blocks onto
ciers partial melting of the ice results in rivers flowing the outwash plain. Deposits of glacial outbursts are
in tunnels within or beneath the ice, carrying with thick beds of sand and gravel that are massive and
them any detritus held by the ice that melted. The poorly sorted or cross-bedded and stratified (Maizels
deposits of these rivers form sinuous ridges of material 1989; Russell & Knudsen 2002). Reworking of this
known as eskers (Fig. 7.7) and they are typically a material by ‘normal’ fluvial processes on the outwash
few metres to tens of metres high, tens to hundreds of plain may occur.
metres wide and stretch for kilometres across the area The absence of widespread vegetation under the
formerly covered by an ice sheet (Warren & Ashley cold climatic conditions means that fine-grained sedi-
1994). The deposits are bars of gravel and sand that ment on the outwash plain remains exposed and is
form cross-bedded and horizontally stratified lenses subject to aeolian reworking. Sand may be blown into
within the esker body. They may be distinguished accumulations within and marginal to the outwash
from river deposits by the absence of associated over- plain, forming deposits with the characteristics of
bank sediments (9.3) and by internal deformation wind-blown sediment (8.3). Silt- and clay-sized grains
(slump folds and faults) that forms when the sand may be transported long distances and be widely
and gravel layers collapse as the ice around the tunnel distributed: accumulations of wind-blown silt of