How glaciers transport debris

Glaciers carry rock material, known as debris, as they move across landscapes. The position of the debris within or on the glacier affects how it is transported and eventually deposited.

Locations of debris transport within glaciers

• Supraglacial debris - Material carried on the surface of the glacier, often from rockfalls onto the ice.
• Englacial debris - Material embedded within the body of the glacier, which may have fallen into crevasses or been incorporated during ice movement.
• Subglacial debris - Material dragged along the base of the glacier, in direct contact with the bedrock below.

Processes of glacial deposition

Glacial deposition happens when a glacier deposits its load as it moves and melts. The deposited material is called till, which is an unsorted mixture of rocks ranging from fine particles to large boulders. Till is typically angular. Deposition can occur at the glacier's edges or base through several key processes.

Ablation

Ablation is the melting of ice, which releases debris embedded in the glacier. As the ice thaws, meltwater can transport and redistribute this material.

Flow

Flow occurs when high volumes of meltwater cause material deposits to slide to different locations.

Lodgement

Lodgement takes place when subglacial debris gets stuck against uneven bedrock due to friction. The glacier continues moving forward, leaving the material behind. This process is more common in slow-moving glaciers where the ice cannot dislodge the trapped debris.

Deformation

Deformation happens over soft bedrock, where the glacier's weight and movement fold or reshape entire layers of underlying material. This is less common and results in distorted deposits.

Factors influencing deposition rates

Several factors determine how much and where deposition occurs.

Key factors affecting deposition

• Glacier speed - Faster glaciers can carry more debris; slowing down (e.g., when entering a narrower valley) reduces energy, leading to increased deposition.
• Carrying capacity - If a glacier encounters rough terrain, friction increases, causing partial melting and a reduction in size. This exceeds the glacier's capacity, forcing it to deposit excess material.
• Glacial regime - A negative regime, where ablation exceeds accumulation (ice buildup), causes the glacier to shrink and lose mass. This reduces carrying capacity and promotes deposition, especially over rough bedrock where lodgement is more likely.

Types of moraines and their formation

Moraines are linear ridges or mounds of till formed by glacial deposition. They are ice-contact features, meaning they build up directly from sediment dropped by the glacier. Moraines are unsorted and elongated, often stretching from mountain slopes to valley floors, reflecting ongoing melting as the glacier moves.

Lateral moraine

Lateral moraines form along the sides of a glacier, marking where its edges once scraped against valley walls. Much of the till comes from freeze-thaw weathering on these slopes. In relict (post-glacial) landscapes, they appear as tall ridges running parallel to the valley length.

Medial moraine

Medial moraines develop in the centre of a valley where two glaciers merge. They consist of combined lateral moraines from each glacier, aligned parallel to the ice flow direction. The till is primarily from valley-side weathering.

Terminal moraine

Terminal moraines accumulate at the glacier's snout, forming semi-circular ridges of till. Meltwater streams often erode them into a line of hills, steeper on the upglacier side due to repeated deposition. These mark the glacier's furthest advance in relict landscapes.

Recessional moraine

Recessional moraines form behind terminal moraines during short pauses in glacial retreat, when melting slows. They appear as lines of smaller ridges, indicating temporary snout positions. They are usually shorter than terminal moraines due to briefer accumulation periods.

Drumlins and their characteristics

Drumlins are streamlined, egg-shaped hills formed from till. They align parallel to the direction of ice flow, with a height of up to 100 m and length of up to 1,500 m. They occur in large clusters known as swarms.

Features of drumlins

• Shape - The stoss (upglacier) end is broad and tall, while the lee (downglacier) end is narrow and tapered.
• Formation theories - Drumlins may form when till accumulates around a bedrock obstacle, then gets streamlined by advancing ice. Multiple processes likely contribute, but the exact mechanism remains debated.

Examples include underwater drumlins in Clew Bay, Ireland, and those in the Ribble Valley, Lancashire, UK.

Other glacial depositional features

Types of till and related features

• Lodgement till - Compacted material pressed into the valley floor by moving ice; it contains slightly rounded, varied-sized debris.
• Ablation till - Angular debris dropped near the snout during melting; it forms close to where ablation is highest.
• Till plain - A broad, gently rolling expanse of till left when a detached ice section melts in place, typical in lowland areas (e.g., northern Ohio, USA).

Erratics

Erratics are large boulders transported by glaciers and deposited in areas with mismatched geology. For example, in the Yorkshire Dales at Norber, dark Silurian rocks sit atop white Carboniferous limestone, showing long-distance glacial transport.