Geology Reference
In-Depth Information
slowly receding ice margin. They are formed of similar
material to kames and slope down-valley in accordance
with the former ice level and often slope up the adjacent
hillside.
Kame deltas
or
delta moraines
are related to kames
but are usually much bigger. They are flat-topped, fan-
shaped mounds formed by meltwater coming from a
glacier snout or flank and running into a proglacial lake or
the sea. They lie at right-angles to the direction of ice flow
and contain debris from the ice itself, as well as glacioflu-
vial debris. The three Salpausselkä moraines, Finland,
are probably the biggest delta-moraine complexes in
the world. They are associated with a lake that was
impounded by the Fennoscandian ice sheet, which
covered the southern Baltic Sea region.
main road along the southern coast of Iceland, including
a bridge over the Skeidarásandur. Catastrophic though
the Skeidarásandur jökulhlaup was, it was tame in com-
parison with the 1918 Katla jökulhlaup, which involved
a flood of about 300,000 m
3
/s of water that carried
25,000 tons of ice and an equal amount of sediment
every second (Tómasson 1996).
Outwash plains, valley trains, and braided
outwash fans
Much of the vast quantity of sediment normally carried
by meltwaters is laid down in the proglacial environment.
Where glaciers end on land, systems of braided rivers,
called
outwash plains
or
sandar
(singular
sandur
)
develop (Plate 10.15; Colour Plate 12). In south-eastern
Iceland, outwash plains may be as wide as they are long
and full of active braids. When jökulhlaups occur, the
entire plain may be flooded. In mountainous terrain,
braided river systems may extend across the full width of
the valley floor, with mountains rising steeply from either
edge. Such elongated and flat systems are called
valley
trains
. Good examples come from the Southern Alps,
New Zealand.
Braided outwash fans
occur where river
systems hemmed in by valleys discharge on to lowlands
beyond a mountain range. Many examples are found
north of the European Alps.
Proglacial landforms
Scablands and spillways
Meltwater streams issuing from a glacier are usu-
ally charged with sediment and fast-flowing. They
deposit the sediment in front of a glacier, and streams
become clogged, leading to braiding. Lakes are com-
mon in this proglacial environment, and tend to fill
and overflow through
spillways
during the summer.
The impounding sediments are often soft and, once
breached, are cut through quickly, lowering the lake
level. Although uncommon today, large proglacial lakes
were plentiful near the southern limits of the Pleistocene
ice sheets and many abandoned spillways are known
(Figure 10.8). Where huge glacial lakes broke through
their containing dams, the rush of water produced
scablands (p. 246).
Jökulhlaups
are outbursts of meltwater stored
beneath a glacier or ice sheet as a subglacial lake. These
best-known jökulhlaups occurred in the last century, with
major ones in 1918 (Katla) and 1996 (Skeidarásandur).
Skeidarásandur jökulhlaup resulted from the rapid melt-
ing of some 3.8 km
3
of ice after a volcanic eruption on
30 September 1996 underneath the Vatnajökull ice cap
(Gudmundsson
et al
. 1997). The ensuing flood involved
a discharge of about 20,000 m
3
/s, running at its peak at
around 6 m/s and capable of transporting ice blocks at
least 25 m large (van Loon 2004). It destroyed part of the
Kettle holes and pitted plains
Many braided-river plains carry water-filled pits. These
pits are called
kettles
,
kettle holes
,or
ice pits
. They
form as a block of 'dead' ice decays and is buried. The
ice block may be an ice remnant left stranded when the
glacier retreated or a lump of ice washed down a stream
during a flood. The water-filled kettles are called
kettle
lakes
(Plate 10.16). An outwash plain pocked with many
kettle holes is called a
pitted plain
.
PARAGLACIAL LANDFORMS
Paraglacial processes occur after a glacier retreats, expos-
ing a landscape susceptible of rapid change. They do
not involve glacial ice; rather they modify landforms
