2A.6 - Glaciers as landform systems

From erosion to deposition

A glacier alters a landscape by removing material, incorporating it, moving it and eventually releasing it elsewhere.

Erosion is the removal or wearing away of rock and sediment. In abrasion, debris held against bedrock scrapes and grinds it as ice moves, producing grooves, polish and fine rock flour. In quarrying or plucking, jointed rock is detached and removed by moving ice, aided by stress variations and water/ice in fractures. The resulting debris can become another abrasive tool. Bare, debris-free ice is a much less effective abrasive agent than ice carrying rock fragments.

Entrainment is incorporation of sediment into the moving glacier. It can occur when water and sediment freeze onto the base, when basal ice picks up available debris, or when rockfall and avalanches add material at the surface. It differs from erosion: a glacier can entrain sediment already loosened by earlier weathering or deposited by a river.

Transport can be supraglacial, on the surface; englacial, within the ice; or subglacial, at the bed. Rockfall commonly supplies surface debris near valley sides. Crevasses, burial by snow and deformation can transfer it into the ice. Basal debris remains in contact with the bed and can be crushed, abraded or pushed along. Transported clasts can retain angular shapes because they do not tumble continuously as they would in a river, although basal grinding can alter them.

Deposition occurs when material is released or no longer moved. Melting ice leaves debris behind; particles may also be pressed into or lodge against the bed. Direct glacial deposits are called till and commonly contain a poorly sorted mixture from clay to boulders. Meltwater can then erode and redistribute that till, producing fluvioglacial deposits with sorting and layering that reflect water flow.

The sequence is connected but not obligatory for every particle. A boulder falling onto ice may be entrained, transported and deposited without having been eroded by the glacier. Material can be recycled several times. Sediment input, transfer and storage also vary through an advance–retreat cycle.

Landform scales and process environments

The size and position of a landform help identify the processes that created it, but scale classes have no universal fixed boundaries.

Macro-scale features include kilometre-sized cirques, entire glacial troughs, fjords and broad ice-scoured landscapes. Meso-scale features include individual smaller moraine ridges, drumlins and rounded bedrock hills. Micro-scale features include striations, polished bedrock patches and small depositional structures. These are relative categories: the measured dimensions and scale of the study matter more than assigning every feature name one fixed size.

The environments can migrate across the same location. A site beneath advancing ice may later become marginal, then proglacial, and finally a relict glaciated landscape. A sediment layer can therefore be overlain or modified by deposits from a different process environment. A feature's present position beside a river does not mean the river created its original form.

Morphology means shape and dimensions: profile, plan form, steepness, height and orientation. Sediment characteristics and internal structure add evidence that shape alone cannot supply. A ridge of unsorted debris near a former snout is consistent with a moraine, whereas layered sorted gravel suggests significant meltwater deposition. Ice and water can operate together, so overlapping evidence needs explanation rather than forcing every feature into one exclusive label.

Upland and lowland landscape assemblages

A landscape assemblage is a spatially related group of features whose combined evidence is stronger than an isolated landform.

In an upland valley system, snow accumulation in high basins supplies ice that flows down valleys. Concentrated erosion can widen and deepen former river valleys into glacial troughs. Smaller tributary glaciers may erode less deeply, leaving their valley floors above the main trough after deglaciation. Debris from steep sides and from the bed is transferred down-valley and released along margins or near the snout. Cirques, troughs, hanging tributary valleys and moraines therefore form a connected system rather than unrelated shapes.

The Lake District supplies a British relict example: high basins and steep-sided glacial valleys survive although no permanent glaciers remain. Alpine valleys offer a contrasting setting where some upper glaciers remain active, while lower troughs and older moraines record larger former ice systems. Active ice extent and the full area of the glaciated landscape need separate boundaries.

In lowland settings, broad ice-sheet movement can scour exposed bedrock, streamline material and leave widespread till. Drumlin swarms comprise elongated hills with broadly aligned axes, while moraines can mark former ice margins and outwash spreads beyond them. Topography, bedrock resistance, sediment supply and ice dynamics decide whether erosion or deposition dominates. “Uplands are erosional and lowlands depositional” is only a rough tendency: uplands also preserve moraines and lowlands can be intensely scoured.

Scandinavian relict ice-sheet terrain and Canada's formerly glaciated lowlands illustrate the broad scale of these assemblages beyond Britain. Different places preserve different proportions of bedrock and sediment evidence. Multiple glaciations can superimpose flow directions or remove older features, so the visible landscape is not necessarily the complete product of the final glaciation alone.

Inferring ice extent and flow direction

Reconstructing former ice cover requires different kinds of evidence for former presence, flow and margin position.

A striated bedrock surface establishes abrasion by moving debris-bearing ice and its line of movement, but a simple groove usually gives an axis, not an unambiguous arrow. A long drumlin axis similarly suggests flow alignment; stronger directional inference requires diagnostic shape and corroborating evidence. A moraine ridge can mark a former margin, but a recessional moraine inside an older limit need not be the maximum extent. An erratic's rock type, compared with mapped possible source rocks, can constrain transport from one area towards another.

Consider an original illustrative valley reconstruction. Several bedrock striations align northwest–southeast. Erratics have a distinctive rock type exposed only in the surveyed northwest source area. Till extends southeast to a curved moraine ridge, with sorted gravels beyond it. Together these support northwest-to-southeast ice movement and a margin at the ridge at some stage. Striations alone would leave the reverse direction possible; the erratic provenance provides the directional connection. Outwash beyond the ridge does not prove grounded ice extended to its outer edge.

Record locations, orientation, height and sediment properties consistently. Map an assemblage at a scale showing both local features and their regional relationships. Use BGS superficial-geology maps for sediment distributions, topographic maps for shape and elevation, and GIS for overlays; keep survey scale, mapping date and legend meanings explicit. These sources help formulate a reconstruction, but mapped “till” can include deposits from different events and boundaries contain uncertainty.

Cross-check evidence before drawing a maximum limit. Later rivers can rework gravels, later ice can overwrite grooves, weathering can obscure surfaces and erratics can be moved by people. Dating distinguishes a maximum from a later stillstand. An absence of features does not prove ice was absent, especially where deposits were eroded. A defensible map shows an approximate limit or confidence band where evidence is sparse and states which observations support each inference.