Fluvioglacial Processes & Landforms (AQA A Level Geography)

The Fluvial Glacial Landscape

• Fluvioglacial landscapes are seen at the edges of warm-based and retreating glaciers, 'downstream' of the main glacier field

• The landscape is associated with flowing meltwater in temperate, peri/glacial regions

• Unlike polar regions, meltwater is seasonal, plentiful and freely flows from a tunnel at the base of the glacier's snout or across the surface of the glacier and into moulins or crevasses 

•  As such, a fluvioglacial landscape is considered very dynamic; as meltwater channels frequently change course

Note that surface meltwater descends through crack/crevasses and moulin shafts to the base of the glacier and exits  through meltwater tunnels at the snout.

Meltwater

• Glacial meltwater is cold and loaded with suspended sediment

• Depending on the type of sediment, it can be milky, grey or brown in colour

• Subglacial meltwater exits the snout of the glacier under hydrostatic pressure through meltwater tunnels

• These tunnels begin under the ice as meltwater caves and vary in size from 50-150m wide, ~20 m high and >14 km in length

Fluvioglacial Processes

• Fluvioglacial processes are through erosion and deposition by flowing meltwater 

• Vast quantities of meltwater is produced, which transport large amounts of debris 

• Meltwater channels are typically steep sided, deep and straight 

• They have a high discharge rate and a turbulent flow 

• The larger the meltwater channels, the more significant the levels of meltwater erosion and size of deposition 

  • In spring and summer, when glaciers are ablating, levels of meltwater is higher, therefore, larger debris can be carried and deposited

  • In autumn and winter ablation is reduced, as is the capacity of fluvioglacial streams to carry and deposit sediment 

  • This annual cycle produces variations in deposition, and is responsible for contrasting layers within one year - known as glacial varves

• Other processes include:

  • Basal sliding where meltwater lubricates the warm-based glacier allowing it to flow more easily

  • Nivation is essential in freeze-thaw and meltwater removes the debris, at the edges, during the summer melt 

  • Plucking - the meltwater refreezes and glues to rock fragments 

  • Abrasion - debris 'rubs' the bedrock and produces rock flour

  • Depositional features through meltwater eroding channels beneath and in front of the glacier, forming distinctive landforms with well-sorted, stratified, rounded and smoothed debris

Fluvioglacial Landforms

• A fluvioglacial landscape usually contains the following features:

  • Meltwater channels

  • Outwash plains and varves

  • Kettle holes

  • Eskers

  • Kames

Meltwater channels

• Meltwater channels are formed from erosion due to the flow of meltwater beneath or close to an ice-sheet margin

• There are different types of channels:

  • Subglacial - found beneath the glacier, with an undulating long profile, and complex, braided stream systems

  • Englacial - where meltwater streams form within the body of the glacial ice - they do not have to exit

  • Lateral - meltwater streams that follow the glacial edge, either within the glacier or on its surface

  • Surface - meltwater flows over the surface of the glacier; the meltwater may flow into crevasses, moulins or supraglacial lakes

  • Proglacial - where meltwater drains from the front of the glacier, downslope and away from the ice margin, eventually forming a network of shallow, sedimented braided channels that are separated by gravel bars (eyots)

• These processes are the same as rivers 

  • Hydraulic action

  • Abrasion

  • Corrosion

  • Attrition

• However, meltwater is more erosive, due to the downward pressure of the ice 'squeezing' the meltwater, causing it to flow faster; plus the meltwater carries more debris, which aids in the abrasion and attrition processes

• Meltwater channels are deep, wide troughs that carry vast amounts of fast-flowing water and are, therefore, highly erosive 

• As the glacier retreats, the deep channels are left with shallow, slow-flowing streams of water

Outwash plains and varves

• As meltwater begins to descend, the velocity of the water begins to slow

• This allows for the formation of a network of shallow, sedimented split channels, that are separated by gravel bars that eventually make up the outwash plain or sandur

• Traction, saltation, suspension and solution processes transport the eroded material within the channels

• The decreasing velocity reduces the ability of the meltwater to 'hold' the debris, sorted sediment is deposited on the valley floor in layers also called varves

• Varves are frequently defined as a type of glacial lake sediment because they are common in glacial lakes 

• However, they occur in different environments where sediment layers are laid down annually and not just in glacial lakes

Kettle holes

• Kettle holes are hollows formed when blocks of ice calved from the main glacier and left on the outwash plain as the glacier retreated

• The ice block subsequently melts, leaving a depression in the sediment deposits (varves) of the outwash plain

• Water-filled kettle holes are known as kettle lakes

Eskers

• These are long, winding ridges of sand and gravel, running parallel to the glacier

• They are deposited by subglacial meltwater streams and can stretch for several kilometres and reach heights of 30m

• As the glacier retreats, the stream dries up, and the load remains as an esker

• Eskers show the position of past glacial tunnels

Kames

• These are mounds of sand and gravel found on the glacial valley floor

• Supraglacial meltwater streams collect in surface depressions and deposit layers of debris

• Glacial retreat dumps the sorted debris onto the glacial valley floor

• Kame terraces are piles of deposited debris, left by meltwater channels, running between the glacier and the valley sides

• Similar in appearance to the lateral moraine, however, kames are sorted layers (stratified) with the heaviest gravel at the base and finer sediments on top

• Proglacial lakes can form in front of glaciers, particularly when the terminal moraine acts as a dam for the meltwater

• As the proglacial lake develops, velocity is lost and sediment is deposited - these deposits are known as deltas

• Glacial retreat dumps these deltas on the glacial valley floor, forming delta kames 

• Crevasse kames are small hummocks of left behind, glacial surface deposited sediments