Transcript Slide 1

GLACIAL ENVIRONMENTS 3
Fluvioglacial processes and landforms:
• fluvioglacial erosion, transportation and
deposition processes
• fluvioglacial landforms
Fluvioglacial activity results from the enormous amounts of water released
by ablation, particularly during periods of deglaciation. In the past, research
was confined to looking at depositional landforms and the processes that
formed them but recent research has revealed the dramatic power of
fluvioglacial erosion and its associated landforms.
Volumes of meltwater are highest in temperate glaciers, particularly in
summer. Much of the water flows within and under glaciers under pressure
(hydrostatic pressure) and so behaves differently to surface streams.
Meltwater streams are capable of transporting huge volumes of material and
consequently, mainly through abrasion, carrying out large amounts of erosion.
Deposition of the load occurs whenever there is a decrease in pressure
and/or velocity.
On the edges of ice sheets, a large number of meltwater streams transport an
immense amount of sand, silt, clay and rock particles from the melting ice. These
streams merge, sometimes into a single tangle of waterways and the discharge of
the channels varies both diurnally and seasonally. As a result, debris is constantly
being picked up and deposited. Such streams are referred to as braided or
anastomosing streams.
Photo source: U.S. Geological Survey (public domain)
Fluvioglacial deposits and glacial deposits are often found in
close proximity and sometimes overlying each other. The
fluvioglacial deposits can be distinguished from the glacial
deposits by the rounded nature of the rocks they contain, and
the fact that the deposits are both stratified and sorted
(graded).
Photo source: U.S. Geological Survey (public domain)
The largest materials are transported only short distances but the finest silts
and clays may be carried in suspension for many miles, only being
deposited in very still water. Many of the meltwater streams are milky in
colour due to the ‘rock flour’ carried in suspension. The layers of deposits
eventually form sandur or outwash plains which may cover huge areas
and be over 50 metres deep.
Skeiðarársandur in south-east Iceland, with an area of over 1,000 square
kilometres, is the world's largest active pro-glacial outwash plain. It has a
classic braided drainage pattern.
Photo source:
U.S. Geological Survey (public domain)
Map source:
http://www.skimountaineer.com/ROF/Beyond/Grimsvotn/VatnajokullMap.jpg
When meltwater streams flow into lakes, velocity is
reduced and deposition occurs. The load volume
is directly related to the time of year and in spring,
when discharge is greatest, lighter coloured and
coarser deposits are laid down on the lake bed.
Towards the autumn, discharge decreases and a
darker, finer layer of deposits is placed on top.
Each band of light and dark deposits therefore
represents one years accumulation.
The layers or varves provide useful evidence of
the age of the lake and variations in climate from
year to year. Glaciologists now take regular core
samples from the lake sediments in glaciated
areas to build up information about the glacial
history of the area.
Images sourced from: http://users.utu.fi/tijusa/
Kames are steep-sided mounds of fluvioglacial material of various shapes and
sizes with a variety of possible origins. Material transported by meltwater
streams may be deposited in crevasses, in hollows on the ice surface, in
spaces in ice tunnels. Once the ice melts, the material is dropped in a heap on
the valley floor.
These landforms although showing rounding of the larger deposits, may not be
well stratified or sorted. Kames formed by meltwater streams along the margins
of ice sheets or glaciers may well show better stratification and sorting but
these kame-terrraces also tend to collapse when the ice melts. Delta-kames
are formed when sub-glacial streams emerge into ice-contact lakes.
During deglaciation, ice sheets and glaciers
break up leaving blocks of ‘dead-ice’ lying in
amongst the fluvioglacial deposits that are
being created. As these ice blocks melt,
they form depressions or kettle holes which
may have lakes in them.
Eskers are long, sinuous ridges of sand,
gravel and pebbles that snake across areas
of gentle relief. Some are only a few metres
long but there examples which are hundreds
of kilometres long and tens of metres high in
Canada. In cross-section, eskers show the
characteristic fluvioglacial stratification and
sorting of deposited material.
It is thought that eskers are formed in
subglacial tunnels and that meltwater
streams gradually built up layers of deposits
which are exposed once the ice melts. The
sinuous nature of the esker reflects the
meandering course of the meltwater stream.
Many eskers have been destroyed by
modern-day quarrying for the sand and
gravel that they contain.
It is now generally agreed that fluvioglacial erosion has played an important
part in shaping many glaciated landscapes. Meltwater streams exhibit extreme
discharges and load volumes which allow active abrasion of both bedrock and
any deposited material.
The largest landform of fluvioglacial erosion is the meltwater channel.
Meltwater channels have a number of characteristics which distinguish them
from conventional river valleys. Although sometimes exhibiting valley
dimensions, they are channels rather than valleys which must at some time
have been full of water (bankfull stage).
They are relatively short in length and may be intermittent
i.e. they display gaps along their length. They tend not to
widen downstream and in a modern non-glacial landscape
are often dry, although some have misfit streams along
their length.
Newtondale in Yorkshire is a classic example of a
meltwater channel formed by the overflowing of a series of
pro-glacial lakes. Newtondale is about 40 metres wide and
80 metres deep.
Photo source: http://www.multimap.com
There are many alternative theories (other than overflow channels) for the
existence of the meltwater channels. Some are seen as marginal meltwater
channels carrying water along the sides of glaciers at times of extreme
ablation, probably during deglaciation. Others are regarded as subglacial
meltwater channels eroded to great depths by the power of the hydrostatic
pressure in these subglacial channels.
Evidence of glacial lakes comes from the strandlines created by waves
breaking on the shores of these lakes. The Parallel Roads of Glen Roy in
Scotland provide some of the best examples in the UK.
Meltwater streams often deposited vast quantities of material in glacial lakes
as deltas. These inland glacial deltas provide another valuable source of
material to the quarrying industry. The town of Pickering in Yorkshire is
located on a large glacial delta formed at the end of the Newtondale
meltwater channel.
Many of the courses of the UK’s current rivers were modified in glacial times.
Interference with drainage patterns happened when traditional routes
became blocked by ice or deposits left by the ice sheets and glaciers. The
River Thames formerly flowed much further north but was forced to adopt a
more southerly route during the Anglian Glaciation.
Summary of key points:
• fluvioglacial activity results from the enormous amounts of water released by ablation
• fluvioglacial deposits can be distinguished from glacial deposits by the rounded
nature of the rocks they contain, and the fact that the deposits are both stratified
and sorted (graded)
• braided streams transport debris from the melting glacier and deposit it across
huge areas to form outwash plains or sandur
• when meltwater streams flow into lakes, layers of summer and winter sediment
are deposited to form varves
• kames are steep-sided mounds of fluvioglacial material of various shapes and
sizes with a variety of possible origins
• Eskers are long, sinuous ridges of sand, gravel and pebbles that snake across
areas of gentle relief – possibly formed from subglacial streams
• the largest landform of fluvioglacial erosion is the meltwater channel. These
carried excess water in great volumes from melting glaciers or overflowing lakes