Glacial Transport and Deposition

Glaciers transport vast quantities of rock debris, ranging from fine clay particles to house-sized boulders. When this material is deposited, it creates a distinctive suite of landforms that dominate lowland glaciated landscapes. Understanding how glaciers carry and deposit sediment is essential for interpreting both upland and lowland glacial environments.

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Glacial Transport

Rock debris enters a glacier through several pathways and is carried in three main positions:

Supraglacial Transport

Material carried on the surface of the glacier:

• Derived mainly from freeze-thaw weathering of exposed valley sides above the glacier
• Rock fragments fall as scree onto the glacier margins, forming lateral moraines
• When two glaciers merge, adjacent lateral moraines combine to form a medial moraine — a dark streak running along the centre of the combined glacier
• Supraglacial debris is typically angular and unsorted because it has not been subject to abrasion

Englacial Transport

Material carried within the body of the glacier:

• Supraglacial debris may be buried by snowfall or fall into crevasses, becoming incorporated into the ice
• This material is transported passively within the glacier
• Englacial debris is also typically angular

Subglacial Transport

Material carried at the base of the glacier, between the ice and the bedrock:

• Derived from plucking and abrasion of the bedrock floor
• Also from supraglacial and englacial debris that works its way down through the ice
• Subglacial debris is distinctive: fragments are typically rounded and striated (scratched) due to abrasion against the bedrock
• The basal debris layer is the glacier's primary tool for erosion — without it, abrasion would be minimal

graph TD
    A["Supraglacial<br/>Angular, unsorted<br/>On the surface"] --> B["Englacial<br/>Angular<br/>Within the ice"]
    B --> C["Subglacial<br/>Rounded, striated<br/>At the base"]
    D["Freeze-thaw from valley walls"] --> A
    E["Plucking and abrasion of bed"] --> C
    A -->|"Falls into crevasses"| B
    B -->|"Works down through ice"| C

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Glacial Deposition

When a glacier melts or slows down, it deposits the material it has been carrying. Glacial deposits are collectively known as drift. There are two main types:

Till (Boulder Clay)

Till is material deposited directly by the ice without reworking by meltwater. It was first described in detail by James Geikie (1877) in The Great Ice Age.

Characteristic	Description
Sorting	Unsorted — a chaotic mixture of clay, sand, gravel, and boulders
Shape of clasts	Variable — subglacial clasts are sub-rounded and striated; supraglacial clasts are angular
Stratification	Unstratified — no layering or bedding
Composition	Reflects the geology of the source area; may contain erratics

Two main types of till are recognised:

• Lodgement till — deposited beneath the glacier as basal debris is plastered onto the bed. It is compact, consolidated, and often has a preferred orientation of clasts (aligned with ice flow direction). This is the most common type of till.
• Ablation till — deposited on the surface as the ice melts. It is less compact, more variable in texture, and may include supraglacial and englacial debris. Often sandier and less clay-rich than lodgement till.

Exam Tip: A key exam question asks you to distinguish between glacial till and fluvioglacial deposits. Remember: till is unsorted, unstratified, and angular-to-sub-rounded; fluvioglacial deposits are sorted, stratified, and rounded. This distinction is fundamental.

Erratics

An erratic is a boulder or large rock fragment that has been transported by a glacier and deposited in an area of different geology from its source.

• Erratics can be used to trace the direction and extent of past ice movement
• Norber Erratics, Yorkshire Dales — dark Silurian greywacke boulders sitting on top of white Carboniferous limestone, transported several kilometres from their source by glacial action. Some rest on limestone pedestals where the erratic has protected the limestone beneath from chemical weathering while the surrounding surface has been lowered.
• Big Rock (Okotoks Erratic), Alberta, Canada — a quartzite erratic weighing approximately 16,500 tonnes, transported over 300 km from its source in the Rocky Mountains

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Moraine Landforms

Moraines are landforms composed of glacial till deposited at the margins or terminus of a glacier. They are classified by their position relative to the glacier: