Page 288 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 271

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