Fluvioglacial Processes and Landforms

Meltwater is one of the most powerful agents of erosion, transport, and deposition in glacial environments. The landforms created by glacial meltwater — collectively called fluvioglacial (or glaciofluvial) landforms — are fundamentally different from those produced by direct ice action. Understanding these differences is a key requirement of the AQA A-Level specification.

---

Characteristics of Glacial Meltwater

Meltwater originates from:

• Surface melting of the glacier (supraglacial meltwater)
• Basal melting caused by pressure and geothermal heat (subglacial meltwater)
• Englacial channels within the ice body
• Seasonal snowmelt from surrounding slopes

Glacial meltwater has distinctive properties:

Property	Description
Discharge	Highly variable — peak flow in summer, minimal in winter; diurnal variation linked to daytime temperatures
Sediment load	Very high — meltwater carries enormous volumes of fine sediment (rock flour), giving it a characteristic milky blue-grey colour
Energy	High — meltwater often flows under hydrostatic pressure beneath the glacier, giving it considerable erosive power
Temperature	Near 0°C year-round

---

Meltwater Erosion

Meltwater erodes through the same processes as normal rivers (hydraulic action, corrasion, corrosion, cavitation) but is often more effective because:

• It flows under high pressure in confined subglacial tunnels
• It carries a high sediment load (rock flour and coarser material)
• Discharge can be extremely high during jökulhlaups (catastrophic outburst floods from glacial lakes)

Meltwater Channels

• Subglacial channels — carved by meltwater flowing under the glacier. These can cut deeply into bedrock, forming steep-sided gorges and potholes. The Nye channel concept (John Nye, 1976) describes channels incised into bedrock beneath the ice.
• Marginal channels — formed along the edges of the glacier where meltwater flows between the ice and the valley side. After glaciation, these appear as channels cut into the hillside at an angle to the slope, often without an obvious modern water source.
• Proglacial channels — formed in front of the glacier snout by meltwater discharge. These often carry braided streams due to the high sediment load and variable discharge.
• Overflow channels — formed when meltwater from a glacially dammed lake overflows at the lowest point of the surrounding topography. The Newtondale gorge in the North York Moors was carved by overflow from glacial Lake Pickering during the Devensian glaciation (approximately 20,000 years ago).

---

Fluvioglacial Deposition

When meltwater loses energy (due to reduced gradient, widening channel, or distance from the glacier), it deposits its sediment load. Fluvioglacial deposits are fundamentally different from glacial till:

Feature	Glacial Till	Fluvioglacial Deposits
Sorting	Unsorted — random mixture of sizes	Sorted — graded by size (largest deposited first, finest last)
Stratification	Unstratified — no layers	Stratified — clearly bedded/layered
Shape of clasts	Sub-angular to sub-rounded	Rounded and smooth — worn by water transport
Surface texture	May be striated	Smooth, water-worn
Depositional agent	Ice	Water

Exam Tip: The sorted-vs-unsorted distinction is the most important difference between glacial and fluvioglacial deposits. If an exam question asks you to identify the origin of a deposit, particle size distribution and the presence or absence of stratification are the key diagnostic features.

---

Landforms of Fluvioglacial Deposition

Outwash Plains (Sandur)

An outwash plain (sandur, plural sandar) is a broad, gently sloping surface of fluvioglacial sediment deposited in front of the glacier snout by braided meltwater streams.

• Sediment is sorted by size — coarsest material (gravels, cobbles) deposited nearest the glacier, finest material (sands, silts) deposited furthest away
• Braided stream channels constantly shift across the surface due to variable discharge and high sediment load
• Skeiðarársandur, Iceland — one of the world's largest outwash plains (approximately 1,000 km²), formed in front of the Skeiðarárjökull glacier. During the 1996 jökulhlaup (caused by the Gjálp volcanic eruption beneath Vatnajökull), discharge reached an estimated 50,000 m³/s, depositing huge volumes of sediment and destroying sections of Iceland's ring road

Eskers

An esker is a long, sinuous (winding) ridge of sorted sand and gravel deposited by a meltwater stream flowing through a subglacial tunnel (Röthlisberger channel).

• When the glacier melts, the tunnel deposits are left as a raised ridge on the landscape
• Typically 5–30 m high, up to several kilometres long
• The sediment is well-sorted, rounded, and stratified — evidence of water deposition
• Cross-section shows lens-shaped beds of alternating coarse and fine material
• Trim esker, County Meath, Ireland — one of the finest examples in the British Isles, approximately 15 km long and up to 20 m high