WHAT ARE FLUVIOGLACIAL

PROCESSES?

Destination: Understand fluvioglacial processes and landforms

Routes: Identify, describe and explain the formation of meltwater

channels, eskers, kames, kettle holes, outwash plains and varves

WHAT DOES THE TERM ‘FLUVIOGLACIAL’ MEAN? Fluvio – rivers Glacial – glaciers Glacial meltwater erosion and

deposition

What words could you use to describe the following?

• Angular?

• Sub-rounded?

• Sorted?

• Unsorted?

• Stratified (in layers)?

• Unstratified?

• Mainly gravel & sand?

• Clay & boulders?

Why might they be different?

AND THIS ONE?

AND THIS ONE?

Subglacial streamA stream that flows beneath a glacier, and which usually cuts into the ice above to form a tunnel.

Large subglacial stream channel that formed beneath the glacier Pastaruri, Peru when an ice-dammed lake drained.

Fluvioglacial landforms are created by the meltwater from glaciers, largely through deposition but also by erosion

MELTWATER STREAMS When glacial ice melts, water runs out

as streams of meltwater Warm-based glaciers produce lots of

meltwater Surface meltwater filters through the

glacier (e.g. through crevasses or moulins) and flows through tunnels underneath the glacier before running out of the snout

Put moulin dye video clip in here Wikipedia image of moulin

MELTWATER STREAMS - EROSION

Meltwater streams cause erosion in the same way as normal rivers – but they cause more erosion than rivers the same size

The confinement of the ice means they flow quickly under pressure, so they can carry lots of material which can erode the landscape

The streams form deep troughs in the landscape called meltwater channels (wide and deep)

After the glacier has retreated, the deep meltwater channels are left with shallow streams running through them

Glacier milk

Meltwater from a glacier, which commonly has a milky appearance from suspended fine sediment.

Meltwater from Vadret da Morteratsch, Grisons, Switzerland

Photo taken on a July morning

Photo taken in the afternoon after ablation and subsequent runoff had both increased considerably

MELTWATER STREAM EROSION The melting of ice produces a great deal

of water that has the ability to carry much debris.

The water often flows under pressure so is turbulent and of a high velocity.

It can pick up and transport more material than a river of a similar size.

This water, with its load, is responsible for the creation of deep subglacial valleys with potholes.

MELTWATER DEPOSITION When the meltwater discharge decreases,

the loss of energy causes debris to be deposited.

Heavier particles are dropped first, resulting in sorting of the material.

Deposits may be found in layers (stratified) as a result of seasonal variations in meltwater flow.

The main features produced by fluvioglacial deposition are eskers, kames and the outwash plain.

Lakes on the outwash plain may have deposits in them called varves.

FEATURES OF LOWLAND GLACIATION

Features of lowland glaciation

Area of fluvio-glacial landforms

Braided stream A relatively shallow stream with many branches that commonly recombine and migrate across a valley floor. Braided streams typically form downstream of a glacier.

Tasman River, South Island, New Zealand, fed by Tasman Glacier off the picture to the right, and the Hooker and Mueller glaciers in the valley in the centre background.

ESKERS Long ridges of material running in the

direction of ice advance Sinuous form, 5-20m high Sorted, coarse material, usually coarse

sand and gravel Often stratified (in layers)

Esker A long, commonly sinuous ridge of sand and gravel, deposited by a stream in a subglacial tunnel.

Esker (arrowed) in NW Spitsbergen, Svalbard, Norway The ridge is about 3 m high

ESKERS Deposits made by subglacial streams. Channel of stream is restricted by ice walls, so

there is considerable hydrostatic pressure which enables a large load to be carried and also allows the stream to flow uphill for short distances.

Some eskers therefore run up gentle gradients. Bed of channel builds up above the

surrounding land and a ridge is left when the glacier retreats during deglaciation.

Sometimes, the ridge of an esker is combined with mounds of material, possibly kames. This is called a beaded esker.

KAMES Mounds of fluvioglacial material (sorted,

often stratified, coarse sands and gravel). Deltaic deposits left when meltwater

flows into a lake dammed up in front of the glacial snout by recessional moraine deposits.

When the ice retreats further, the delta kame often collapses.

Kame terraces are frequently found along the side of a glacial valley and are the deposits of meltwater streams flowing between the ice and the valley side.

Kame terrace Valley-side terrace or bench formed by the deposition of fluvial sediment along the margin of a glacier. The terrace is left stranded on the hillside after the glacier has receded.

Kame terrace

KAMES VS ESKERS Kames – supraglacial material Eskers – subglacial material

EXAM QUESTION JUNE 2013 Draw a labelled sketch to show the

characteristics of eskers (4 marks) Explain the formation of eskers (5

marks)

OUTWASH PLAINS (SANDUR) Found in front of the glaciers snout. Deposited by meltwater streams issuing

from the ice. Consist of material brought down by the

glacier and then picked up, sorted and dropped by running water beyond the position of the ice front.

Coarsest material travels shortest distance before being deposited.

Deposits are layered vertically, which reflects the seasonal flow of meltwater streams.

Outwash plain (aerial view) A relatively flat spread of debris deposited by meltwater streams emanating from a glacier

Outwash plain in front of Thompson Glacier, Axel Heiberg Island, Canadian Arctic

OUTWASH PLAINS (SANDUR) Meltwater streams that cross the outwash plain

are braided. This happens as the channels become choked with

coarse material because of marked seasonal variations in discharge.

The outwash plain often contains small depressions filled with lakes or marshes – kettle holes.

Kettle holes form when blocks of ice, washed onto the plain, melt and leave a gap in the sediments.

The gap then fills with water to form a small lake. Aquatic plants become established in the lakes

which leads to the development of a marshy area, then peat.

Outwash plain (ground view) A relatively flat spread of debris deposited by meltwater streams emanating from a glacier.

Glaciologist crossing an outwash plain in front of Midre Lovénbreen in NW Spitsbergen, Svalbard

Kettle holes

Kettleholes form in the following way: As the glacier retreats it may leave large

blocks of buried ice. This ice slowly thaws over time and the

covering gravel collapses leaving a depression.

These depressions are called kettle holes. If the depressions are deep enough to tap

the water table a kettle hole lake forms.

Kettle (red arrows) (or kettlehole) A self-contained bowl-shaped depression within an area covered by glacial stream deposits, often containing a pond. A kettle forms from the burial of a mass of glacier ice by glacial or stream sediment, followed by its subsequent melting.

Kettles in the forefield of Glacier du Mont Miné, Valais, Switzerland.

VarvesA varve is a distinct layer of silt lying on top of a layer of sand, deposited annually in lakes found near to glacial margins

The coarser, lighter coloured sand is deposited during late spring when meltwater streams have their peak discharge and are carrying their maximum load.

As discharge decreases towards autumn when temperatures begin to drop, the finer, darker silt will settle.

VARVES Lakes on the fringe of the ice are filled with deposits

that show a distinct layering. A layer of silt lying on top of a layer of sand

represents one years deposition in the lake – a varve. The coarser, lighter coloured layer is the spring and

summer deposition when meltwater is at its peak and the meltwater streams are carrying maximum load.

The thinner, darker coloured and finer silt settles during autumn and winter as stream discharge decreases and the very fine sediment in the lake settles to the bottom.

A good indicator of age of lake sediments and past climates.

EROSIONProglacial lakes and overflow channels: Glacial meltwater has great erosive power because

of its volume and the large amounts of debris it contains.

During deglaciation, lakes develop on the edges of the ice, some occupying large areas.

Overflows from these lakes which cross the lowest points of watersheds will create new valleys.

When the ice damming these meltwater lakes totally melts, many of the new valleys are left dry, as drainage patterns revert to the preglacial stage.

In certain cases, however, the postglacial drainage adopts them, giving rise to new drainage patterns.

PROGLACIAL LAKES AND OVERFLOW CHANNELS Large meltwater lakes of this kind

occurred in the Midlands (Lake Harrison), the Vale of Pickering in North Yorkshire (Lake Pickering) and the Welsh borders at the end of the last glaciation.

The River Thames is thought to have followed a much more northerly course before the Quaternary – its modern course formed when ice filled the northern part of its basin and forced it to a different route.

Proglacial lake A lake developed immediately in front of the glacier, commonly bordered by the mounds of unconsolidated deposits that characterise the terminal zone of a glacier.

Proglacial lake at Sheridan Glacier near Cordova, Alaska.

Summary diagram – features produced by glaciation

EXAM QUESTION JAN 2011 Assess the role of meltwater erosion and

deposition in the formation of fluvioglacial landforms (15 marks)

EXAM QUESTION JUNE 2013 Draw a labelled sketch to show the

characteristics of eskers (4 marks) Explain the formation of eskers (5

marks)