There are two main sources of meltwater from glaciers
- : surface melting and basal melting.
Surface melting
- contributes most of the supply and peaks in late summer; it is the only source of meltwater for cold basal glaciers.
- Supraglacial surface streams form running along the top of the ice, especially in the ablation zone.
- These supraglacial channels are often very fast flowing and may plunge down into the ice either through a crevasse or, more commonly, via a moulin (a cylindrical, vertical tunnel rather like a pothole in limestone landscapes), becoming englacial streams.
- As meltwater moves through a glacier it may refreeze or contribute to further melting and reach the subglacial supply, depending on the temperature of the ice inside the glacier.
Basal melting
- occurs if the temperature of the ice at the base of a glacier is at pressure melting point (in a warm-based glacier).
- The basal meltwater flows under hydrostatic pressure beneath the glacier and can excavate subglacial tunnels by cutting through the bedrock.
- The meltwater streams eventually emerge from subglacial tunnels at the glacier snout via portals (caves).
Processes of fluvio-glacial erosion and deposition
- It is important to remember that away from the glacier, outlet streams behave similarly to normal streams, although their discharge, sediment loads and the lack of vegetation lead to some variations in operation.
- Within the glacier, however, fluvio-glacial streams operate very differently because of the high pressure and velocity of flow.
- This causes the erosion of underlying bedrock by abrasion, cavitation and chemical means beneath the glacier ice, and can also lead to intense erosion by meltwater streams as they exit the glacier snout.
- The ablation rates are very high during deglaciation, and many of the meltwater streams have very high discharges leading to powerful erosion.
When meltwater deposits material subglacially, englacially and supraglacially, the material is referred to as an
ice-contact fluvio-glacial deposit.
Where the fluvio-glacial material is deposited at or beyond the ice margin, by streams coming out of the snout, it is known as
outwash or proglacial.
Characteristics of fluvio-glacial deposits
In comparison to glacial deposits (tills), fluvio-glacial deposits tend to be:
- generally smaller than glacial till as meltwater streams; although having seasonally high discharge, they still have less energy than large valley glaciers so they generally carry finer material
- generally smoother and rounder through water contact and attrition
- sorted horizontally, especially in the case of outwash deposition, with the largest material found up-valley or nearer the glacier snout, and progressively finer material down-valley, due to the sequential nature of deposition mechanisms
- stratified vertically with distinctive layers that reflect either seasonal or annual sediment accumulation.
In contrast, glacial till is classed as a
diamicton, being angular, poorly sorted and non-stratified.
A further distinction can be made between ice-contact fluvio-glacial deposits and outwash deposits.
Outwash deposits experience more attrition, causing clasts to become more rounded, and the material is better sorted horizontally.
Three main zones of outwash deposition extend from the front of the glacier, and the characteristics change through these zones, as shown in Table 6.4.
Note that varved deposits form in meltwater lakes along or beyond a glacier margin (see page 87).
Ice contact landforms of fluvioglacial deposition
Proglacial landforms of fluvioglacial deposition
Fluvio-glacial landforms
- Subglacial meltwater can excavate large meltwater channels.
- These can cut across contours as the direction of the meltwater flow is controlled by the hydrostatic pressure gradient.
- Subglacial meltwater can even flow uphill, so these channels can have a ‘humped’ long profile.
- Examples of these meltwater channels can be found in many parts of the UK including the Gwaun Valley in North Pembrokeshire.
Ice-contact features - eskers
- Eskers result from subglacial meltwater deposition.
- They are sinuous ridges of relatively coarse sand and gravel deposited by meltwater flowing through tunnels, sometimes englacially, but normally subglacially.
- Eskers came in a variety of sizes.
- A small esker can be found in the UK at Wark on the River Tweed; it is about 1 km long, 40 m wide and about 20 m high.
Eskers are thought to occur when
- a subglacial or englacial channel becomes obstructed, leading to deposition of material upstream from the blockage.
- The ice needs to be stagnant for englacial eskers to form, otherwise the material would be reworked by glacier ice movement.
- Another possible mode of formation is where a delta of fluvio-glacial material extends outward, perpendicular to the ice margin, taking on an elongated form under conditions of rapid ice retreat.
Kames
- are generally steep-sided conical hills although they come in a variety of shapes and sizes.
- They are formed by deposition of material in ice in either surface depressions or crevasses, or alternatively as deltas along the sides of a glacier between the ice margin and the hillside.
- They show some evidence of stratification, although the bedding can be disturbed by subsidence as the ice melts away.
- e.g Kirriemuir,Scotland
Kame terraces
- form relatively continuous bench-like features along the valley side, when a gap or a lake between the valley side and the ice margin is filled with fluvio-glacial deposits.
- Some regions of extensive kame deposits also contain kettle holes and can be described as areas of kame and kettle topography.
- This type of topography is usually developed when there is a large amount of fuvio-glacial material deposited over the surface of stagnant ice, which melts in situ.
- The Loch Etive kame terraces
Diagram of formation of fluvioglacial features
Proglacial features - outwash plain or sandur.
- When an englacial or subglacial stream exits the snout of a glacier there is a rapid decrease in water pressure and velocity, causing the deposition of coarse fluvio-glacial material as an outwash fan.
- Outwash fans merge to become part of a debris-rich, anastomosing braided drainage system.
- As the discharge of meltwater decreases with deglaciation, the broad expanse of fuvio-glacial material that was deposited and spread out by the braided river systems is left behind as an outwash plain or sandur.
- An outwash plain is a gently sloping surface made up of rounded, sorted and stratified sands and gravels, with the particle size becoming progressively smaller away from the ice front.
Outwash plains may contain kettle holes
- where any surviving blocks of ice during deglaciation were buried by outwash material.
- After the ice melted, the ground above it subsided leading to the formation of a depression, which subsequently filled with water.
- As these kettle holes are only fed by rainwater, many of the smaller ones are gradually colonised by vegetation (hydroseres) and subsequently dry up.
- In the Ellesmere area of north Shropshire there is an area with many kettle holes varying from 400 m to 1.5 km in size.
- They are also very common in the Alpine Foreland area of southwest Germany near Wolfegg.
Proglacial lakes (also known as ice-margin lakes)
- are formed along the front of glaciers and ice sheets where meltwater from the glacier becomes impounded within a depression blocked by glacier ice and bounded by high ground.
- These lakes are ephemeral (temporary) features; depending on the rapidity of deglaciation the proglacial lake can empty completely, usually via a pre-existing col, or could stabilise at a lower level if the ice margin had not completely disappeared.
- Lake Algonquin is an example of a proglacial lake that existed in east-central North America
The dimensions of former proglacial lakes can be inferred from
both erosional and depositional forms.
The parallel ‘roads’ of Glen Roy in the Scottish Highlands mark the former shoreline of a proglacial lake formed during the Loch Lomond Stadial.
Strandlines marking the shore of the proglacial lake may occur if the water level was stationary for a relatively long time.
It may also be possible to find former lake deltas, where
- meltwater streams deposited outwash as they entered the lake.
- If the water was relatively calm in the lake, varved deposits form.
- They are characterised by alternate bands of relatively coarse grained sand, reflecting the rapid ice melt in summer at the base of the layer, overlaid by fine, dark-coloured silt or clay that came out of suspension when the lake’s surface (and the streams that fed it) were frozen in winter.
- These annual bands of sediment reflect the seasonal variation in discharge from the glacier.
Proglacial lakes were a very common feature with
many forming in the English Midlands, such as Lake Harrison or Lake Lapworth. Many proglacial lakes, such as those formed along the margins of the Laurentide Ice Sheet in North America, were enormous; at its maximum Lake Agassiz covered an area of around 300,000 km2.
