A variety of
landforms are associated with meltwater from glaciers, including Outwash plains
or Sandur, Varves, Braided Streams, Eskers, Kames and Kame terraces, Kettle
holes and proglacial lakes.
Fluvioglacial landforms are those that are created by the work of
meltwater streams in front of an often retreating glacier.
Glacial deposits tend to be unstratified (have no
layers), unsorted and angular.
Meltwater changes these glacial sediments or tills by
sorting them by size, stratifying them into layers and rounding the sediments.
This gives fluvioglacial landforms a very distinct look
or set of characteristics.
Melt water can create landforms through both erosion and
deposition, and the environment is highly dynamic and in constant flux.
In summer discharges will be very high, as melt water arrives from many
melt water streams from all over the glacier.
In winter, melt water discharge may even stop, as temperature may never
rise above the pressure melting point.
Discharges also vary on a DAILY basis; with greatest discharges
coinciding with the greatest daily temperatures (albeit with a LAGTIME to allow
the water to reach melt water channels).
Fluvioglacial landforms include melt water channels, kames, eskers, braided streams and outwash plains.
MELTWATER CHANNELS
The major erosion landform created by melt water are
Melt water channels
– also known as glacial overflow channels. These form as the original
course followed by a river before glaciation may be blocked by ice, or as an
overflow from a proglacial lake (one that results from melt water from glaciers).
These huge releases of water had much energy to erode and carve out deep gorges
using processes such as abrasion and hydraulic action, which today are occupied
by streams too small to have created the valleys they flow in. Examples are well
documented – Newtondale and Lake Pickering, Lake Lapworth and Ironbridge Gorge.
A fluvioglacial landform created by both erosion and deposition by
melt water is a Sandur or outwash plain that often has braided streams.
Outwash plains
are areas that may have been glaciated by ice sheets and then affected
by melt water or they may be areas in front of the snout. When melt water emerges
from the snout, it loses its energy as it is no longer flowing under hydrostatic
pressure. Consequently, the material it is carrying is
deposited
– the largest first often forming an alluvial fan at the end of the
glacier. When a number of these merge – an outwash plain is formed. The finest
material is carried furthest, sorting the sediment by size.
In addition these outwash plains are often stratified,
because the sediment is laid down in layers during annual flood events and
during periods of higher discharge (in summer when there is more melting).
Braided streams are often found in these outwash plain,
because the highly variable discharge of the melt water streams and deposition
cause the river to split into smaller streams.
Lateral erosion from these streams also helps to create
this flat layered feature.
Outwash plains can be huge, many 10s of kilometres long and
wide.
The rest of the landforms created by melt water are depositional in nature.
Eskers
are winding/sinuous ridges of often coarse sands and gravels that are
deposited
by melt water as it flowed in a channel beneath is under pressure and
much material is carried, to be dropped when melt water reduces in winter and
exposed when the glacier melts. Material is rounded due to
water erosion.
They vary in height – 5-20m for small eskers and length – from a km to 400km
(e.g.
Munro esker in Canada
is 250km long!).
KAMES
The next depositional feature is a Kame, which are rounded mounds/hills of fluvioglacial deposits. These are found near the end of the former glacier as it began to retreat. Melt water held back by terminal moraine led to formation of lakes and material was deposited where the melt water left the glaciers to flow into lakes – creating delta-like landforms. When the ice supporting the up-valley side melted, the material collapsed back to form the kame. There is the suggestion that hollows on the surface of a melting glacier would fill up with sediment and then gradually go down to lower levels as the ice melted – ultimately forming a mound on the ground surface. Kame terraces are found where meltwater runs between the glacier and the sidewall and material is deposited (sorted) when flow is reduced.
Fluvioglacial environments during
glaciation
Fluvioglacial environments after glaciation
KETTLE HOLES
The final depositional landform is Kettle lakes.
These small round features are found in zones where dead
ice was left on an outwash
plain by a retreating glacier.
As melt water streams emanated from the ice they buried
these blocks of dead ice under deposits of sorted and stratified sediments.
The ice melted and this left a depression in the outwash
plain known as a Kettle hole.
