Glacial Transport and Deposition

Glaciers transport vast quantities of rock debris and deposit it in distinctive landforms that provide evidence of past glacial activity. This lesson examines the processes of glacial transport and deposition and the landforms they create, continuing the exploration of Edexcel A-Level Geography Enquiry Question 1 (EQ1).

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Glacial Transport

A glacier transports debris from three main sources: material eroded from the valley floor and walls, material falling onto the glacier surface from weathering of surrounding slopes, and material reworked from older glacial deposits. The position of debris within or on the glacier determines how it is transported and the characteristics of the deposits it eventually forms.

Transport Pathways

Pathway	Location	Source of Debris	Characteristics
Supraglacial	On the glacier surface	Rockfall from valley walls, freeze-thaw weathering of exposed rock	Angular, unmodified by abrasion; forms lateral and medial moraines
Englacial	Within the body of the glacier	Supraglacial debris buried by accumulating snow; debris falling into crevasses	Protected from abrasion; retains angular shape
Subglacial	At the base of the glacier	Plucked from bedrock; abrasion products (rock flour)	Rounded, striated, faceted by abrasion; forms ground moraine and lodgement till

The distinction between transport pathways is important because it determines the shape and surface texture of deposited sediments:

• Supraglacial and englacial debris: Angular, varied sizes, unworn surfaces — it has not been in contact with the bed
• Subglacial debris: Rounded, striated, faceted surfaces — it has been ground against bedrock during transport
• Rock flour: Extremely fine-grained silt produced by abrasion; gives meltwater its milky appearance

Exam Tip: In an exam, distinguishing between supraglacial/englacial (angular) and subglacial (rounded, striated) debris demonstrates your understanding of glacial processes. This distinction also helps identify whether a deposit is lodgement till (subglacial) or ablation till (from the glacier surface).

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Glacial Deposition

Glacial deposition occurs when the glacier loses energy and can no longer transport its load. This happens when:

• The glacier melts (ablation exceeds accumulation)
• The glacier slows down (reduced gradient, widening valley, reaching a plain)
• The glacier retreats (releasing debris as ice melts)

Till (Boulder Clay)

Till is the general term for all sediment deposited directly by glacial ice (as distinct from fluvioglacial sediment deposited by meltwater). Till has several characteristic properties:

Property	Description
Sorting	Unsorted — till contains a chaotic mixture of particle sizes from clay to boulders
Stratification	Unstratified — no visible layers or bedding (unlike fluvioglacial deposits)
Clast shape	Variable — subglacial clasts are rounded and striated; supraglacial clasts are angular
Compaction	Often highly compacted (especially lodgement till) due to the weight of overlying ice
Composition	Reflects the lithology of the rocks over which the glacier passed — can be used to trace ice flow directions

Types of Till

Type	Depositional Process	Characteristics
Lodgement till	Plastered onto the bed beneath a moving glacier as basal debris is smeared across the bedrock	Very compact, dense; clasts often aligned with ice flow direction; deposited subglacially
Ablation till	Released from the glacier surface as ice melts during retreat	Less compact; more angular clasts; deposited from supraglacial and englacial positions
Deformation till	Pre-existing sediment reworked and deformed by the moving glacier	Mixed characteristics; may contain folded or thrust structures
Flow till	Saturated till that flows off the glacier surface under gravity	Often reworked and resedimented; may show crude sorting

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Moraine Types and Formation

Moraines are accumulations of glacial debris. They are classified by their position relative to the glacier:

Lateral Moraine

• Forms along the sides of a glacier
• Composed mainly of angular supraglacial debris derived from freeze-thaw weathering and rockfall from valley walls
• Appears as a ridge of angular, unsorted debris running parallel to the glacier margins
• After glaciation, lateral moraines form prominent ridges along the upper valley sides
• Example: Well-developed lateral moraines visible in the Lauterbrunnen Valley, Swiss Alps

Medial Moraine

• Forms along the centre of a glacier where two glaciers merge
• Created when the adjacent lateral moraines of two tributary glaciers join at a confluence
• Appears as a dark line of debris running down the centre of the glacier surface
• A glacier with multiple tributaries may have several medial moraines
• Example: The Aletsch Glacier (Switzerland) displays prominent medial moraines from the merger of three tributary glaciers

Terminal (End) Moraine

• A ridge of debris deposited at the furthest point of glacial advance (the maximum extent of the glacier)
• Forms when the glacier is in equilibrium — the rate of ice supply to the snout equals the rate of ablation, so debris is continuously delivered to and deposited at the same location
• Terminal moraines often form arc-shaped ridges across valleys
• They may dam meltwater to form lakes
• Example: The terminal moraine at the southern end of Lake Garda (Italy) was deposited during the Last Glacial Maximum

Recessional Moraine

• Formed during temporary pauses (stillstands) in glacial retreat
• Similar in form to terminal moraines but smaller and located up-valley from the terminal moraine
• A series of recessional moraines records the stages of glacial retreat — each ridge represents a period when the glacier snout was stationary
• Example: Multiple recessional moraines are visible in the Scottish Highlands, marking stages of Loch Lomond Stadial retreat

Push Moraine

• Formed when a glacier re-advances over previously deposited material, bulldozing and deforming it
• Characterised by folded, thrust and overturned structures within the moraine sediments
• Indicates a period of glacial re-advance (possibly due to a short-term climate deterioration or a surge)

Ground Moraine (Till Plain)

• A relatively flat or gently undulating sheet of till deposited across the landscape beneath a retreating ice sheet or glacier
• Forms extensive low-relief plains in lowland areas
• Example: Much of East Anglia and the English Midlands is covered by ground moraine (till plains) deposited by the Devensian ice sheet