Fluvioglacial Processes and Landforms

Fluvioglacial Processes and Landforms

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• The word fluvioglacial relates to the meltwater of a glacier.
• This meltwater often flows under great pressure and has a large role in shaping glacial landscapes.
• The main sources of meltwater are surface melting and basal melting:
• Surface melting mainly occurs during the summer and is the only source of meltwater for cold-based glaciers.
• Basal melting occurs due to high pressure at the base of the glacier. This is more common with warm-based glaciers.

Fluvioglacial Erosion

• Meltwater streams from glaciers tend to act similarly to normal streams/rivers. However, the amount of sediment being carried and the lack of vegetation changes them slightly.
• Meltwater streams within a glacier are usually under a lot of pressure, causing erosion.
• Fluvioglacial erosion processes follow the same four types of erosion present in rivers:
• Abrasion - the force of sand and pebbles in the meltwater scraping away at the channel.
• Attrition - the wearing away of rocks in the river by hitting off one another.
• Hydraulic action - the wearing away of rocks by the force of the water. This can also occur through the compression of air in small gaps by water.
• Solution - the chemical breakdown of rock by the meltwater.

Landforms of Erosion: Meltwater Channels

• Meltwater channels form where the meltwater from a glacier follows a pre-existing river channel.
• The water released from the glacier has high energy and a large load in the form, which results in rapid erosion taking place.
• The river channels are deepened to form meltwater channels. These may also take the form of glacial overflow channels.
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Landforms of Erosion: Moulins

• A moulin is a roughly circular, vertical well-like shaft within a glacier which water enters from the surface.
• This feature is formed by the force of the meltwater and through abrasion.
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• Moulins are responsible for considerable quantities of meltwater flowing within the body of the glacier and contribute to many depositional fluvioglacial landforms.
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Fluvioglacial Deposition

• Fluvioglacial deposition occurs when glacial ablation periodically reduces so meltwater streams have a smaller discharge.
• When meltwater deposits material subglacially, englacially and supraglacially, the deposited material is called an ice-contact fluvioglacial deposit.
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• When material is deposited beyond the ice margin, it is referred to as an outwash or proglacial deposit.

Characteristics of Outwash Deposits

• Smaller than glacial till as the meltwater streams have less energy than large valley glaciers
• Smoother and rounder due to attrition
• Sorted horizontally
• Stratified vertically
• Are sorted into 3 zones (see next slide)
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Sorting of Outwash Deposits

• Proximal Zone:
• In front of the glacier snout
• Meltwater has the most energy here so deposits are large in particle size
• May be mixed with some glacial till
• Outwash occurs in alluvial fan forms
• Medial Zone:
• Further from the glacier
• Anastomosing/braided channels because of variability in discharge
• Particle size is smaller and clasts are more rounded
• Distal Zone:
• Furthest from the glacier
• Meanders and flood plain in river system
• Deposits are well sorted
• Particle size is very fine

Ice-Contact Deposition Landforms: Eskers

• Formed when there is a sub-glacial meltwater channel flowing within the body of the glacier and sediment is deposited within this channel.
• Eskers appear as long, sinuous ridges ranging from 5-20m high on the valley floor.
• After the ice age when the glacier melts, a ridge of sediment is left behind representing where the previous meltwater tunnel deposits descended to the valley floor as the surrounding ice melted.
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Ice-Contact Deposition Landforms: Kames

• Kames appear as small mounds on the valley floor. They are accumulations of partially-sorted material found at the front of a glacier.
• Kames are deltaic deposits formed when meltwater streams lose energy flowing into a lake.
• These lakes are formed from dammed meltwater building up between the retreating glacier and terminal moraines.
• The process continues and material is repeatedly deposited on top of the growing kame and may form a kame delta.
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• Bildunterschrift: Dude Hill, Yellowstone, USA

Ice-Contact Deposition: Kame Terraces

• Kame terraces are formed similarly to kames.
• These appear as ridges of material running along the edge of the valley floor.
• Meltwater streams flow along the convergence of the glacier’s lateral edge and the valley side. When they lose energy, they deposit their load into temporary channels.
• When the glacier retreats, these channels collapse to leave a ridge of partially sorted and rounded material to slump along the valley side.
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Proglacial Deposition Landforms: Kettle Holes

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• Kettle holes are small circular lakes that often accompany kames. When these features are found together, it is referred to as kettle and kame topography.
• Kettle holes are formed when large blocks of ice calve from the main glacier onto an outwash plain.
• Sediment builds up around these blocks of ice and when they melt, a small hollow is left behind.
• Water often accumulates in these hollows, forming lakes known as kettle lakes.

Proglacial Deposition Landforms: Lakes

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• Proglacial lakes are formed along the front of glaciers and ice sheets where meltwater becomes blocked.
• These features are often referred to as ice-margin lakes.
• Proglacial lakes are common in the English Midlands.

Proglacial Deposition Landforms: Varves

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• Varves are layers of sediment found at the bottom of glacial lakes.
• They are formed when sediment carried by meltwater loses energy entering a lake.
• In summer, the discharge of meltwater streams is higher, so the stream can carry coarser sediment and deposits accumulate more rapidly.
• In winter, there is less meltwater available, meaning that sediment is limited and smaller in size. This makes the bands of sediment thin and fine.

Proglacial Deposition Landforms: Outwash

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• Outwash plains (sandur) are flat expanses of deposited sediment.
• These are formed in front of a glacier and are where material is deposited over a wide area, carried out from the glacier by meltwater.
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• The largest material is deposited closest to the glacier and gradually gets smaller as the meltwater loses energy.