2-3-3-glacial-meltwater-landforms-and-landscapes

Process of Water Movement

• 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

Processes

• Fluvioglacial processes are through erosion and deposition by flowing meltwater 

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

• 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

Glacial & Fluvioglacial Deposits

• Depositional features through meltwater erosional channels is beneath and in front of the glacier

• These form distinctive landforms with well-sorted, stratified, rounded and smoothed debris

• 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 when found in meltwater lakes or beyond a glacier’s margin

• Fluvioglacial deposits are generally:

  • Smaller than glacial till debris

  • Carry finer material

  • Smoother and rounder through fluvial processes of attrition, abrasion and corrosion

  • Sorted horizontally with coarse material up-valley with progressively finer material being deposited as meltwater moves down-valley 

  • Have stratified layers that reflect seasonal and annual deposition variations

• Glacial till deposits are typically:

  • Unsorted, angular and non-stratified (non-layered)

Outwash deposits

• These are zoned into 3:

  • Proximal zone in front of the glacier and emerges from the snout

    • Meltwater has high velocity and particles are large and angular

    • Can be intermixed with finer glacial till

    • Outwash may form alluvial fans

  • Medial zone is where meltwater streams begin to form braided channels

    • Daily and seasonal changes in meltwater discharge 

    • Velocity is decreasing and particle size is rounder and smaller

    • Deposition begins in meanders of streams and across the outwash plain

  • Distal zone is the furthest from the glacial snout

    • The drainage pattern is now similar to normal fluvial drainage systems

    • Outwash is well-sorted, smaller, and rounded

Imbrication

• Sediments deposited in fast-flowing meltwater channels will show imbrication

• This is where rock fragments are pushed in one direction by the flow, which forces overlapping of each other

Formation of Fluvio-Glacial Landforms

• A fluvioglacial landscape can be divided into 2 categories:

  • Ice contact 

  • Proglacial meltwater

Category of Fluvioglacial Landforms

Meltwater channels

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

• 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 

• 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 moraines, 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 deposi