Glacial landforms

1. Glaciers as geomorphic agents

Glaciers develop where accumulated snow survives successive melt seasons and is progressively compacted into firn and glacier ice. Valley glaciers occupy mountain valleys, while ice sheets spread across extensive land areas and may override much of the underlying relief. Antarctica and Greenland contain the present ice sheets. The Himalaya, Karakoram, Alps, Andes and Alaska support major mountain glacier systems.

Glacial mass balance is the difference between accumulation and ablation over a specified period. Accumulation includes snowfall and avalanche inputs; ablation includes melting, sublimation and, where relevant, calving. The equilibrium line separates zones of net annual accumulation and net annual ablation. Its position is influenced by temperature, snowfall, aspect and local topography rather than latitude alone.

Glaciers shape landscapes by eroding their beds, carrying sediment within, beneath and on top of the ice, and depositing it when transport capacity declines or ice melts. Moving ice is not equivalent to flowing water: it can transport particles ranging from clay to enormous boulders together. Meltwater then reworks part of this debris, producing distinctly different deposits.

• Temperate or warm-based ice commonly permits basal sliding and substantial erosion; cold-based ice frozen to its bed may preserve older surfaces.
• Periglacial processes occur in cold environments around or beyond glaciers and include frost action and solifluction; they do not require glacier movement.

2. Erosion and high-mountain landforms

Abrasion occurs when debris carried at the glacier base scrapes and grinds bedrock, producing polished surfaces, grooves, striations and fine rock flour. Quarrying, commonly called plucking, removes fractured bedrock blocks as moving ice interacts with bedrock and fluctuating basal water pressure. Freeze-thaw weathering helps supply debris from exposed valley walls but should not be treated as identical to glacial erosion.

A cirque, also called a corrie or cwm, is an amphitheatre-shaped hollow near the head of a glaciated valley. It typically has steep enclosing walls, an overdeepened floor and a threshold or rock lip. After ice melts, a small lake called a tarn may occupy the hollow. Not every tarn has the same origin, but cirque lakes are its classic geographical expression.

An arête is a sharp ridge separating adjacent cirques or glacial valleys. Where several cirques erode a mountain from different directions, a steep pyramidal peak or horn may remain. The Matterhorn in the Alps is the standard example. A col is a saddle or pass along a ridge; glacial headward erosion can contribute to its formation.

• Cirque: hollow; arête: ridge; horn: peak. These are related but not interchangeable.
• Striations indicate the alignment of former ice movement; determining its direction usually requires additional evidence.

3. Glacial valleys and bedrock features

Valley glaciers widen, deepen and straighten pre-existing valleys, often transforming a river-cut V-shaped cross-section into a U-shaped glacial trough. Such troughs have steep sides and relatively broad floors. Interlocking river spurs may be cut back into truncated spurs. Glacial overdeepening can create rock basins that later hold ribbon lakes; however, lake shape alone does not establish a glacial origin.

A hanging valley is a tributary valley whose floor remains above that of the main glacial trough. It commonly develops because the larger trunk glacier excavates more deeply than its tributary. After deglaciation, a stream entering the main valley may descend as a waterfall. Yosemite Valley in the United States provides well-known examples of glacial troughs and hanging tributaries.

A roche moutonnée is an asymmetric bedrock knob, typically with a smooth, gently sloping upstream or stoss face and a rough, steeper downstream or lee face. Abrasion dominates the stoss side, while quarrying is characteristic of the lee. A fjord forms where the sea inundates a glacial trough, often producing a deep, steep-sided inlet with a shallower threshold near its mouth. Norway provides classic examples.

• Distinguish a fjord, which occupies a glacial valley, from a ria, which occupies a drowned river valley.
• A roche moutonnée is an eroded bedrock feature, whereas a drumlin is generally a streamlined sediment landform.

4. Glacial and fluvioglacial deposition

Till is deposited directly by glacier ice and is typically unsorted and unstratified, with mixed particle sizes. Moraine refers to accumulations or landforms of glacial debris rather than a single sediment size. Lateral moraines occur along glacier margins; medial moraines commonly develop where tributary glaciers merge and their adjoining lateral debris bands unite.

Terminal or end moraines mark a former ice-margin position; the terminal moraine commonly denotes the farthest advance in a particular glacial episode. Recessional moraines mark pauses or minor readvances during overall retreat. Ground moraine is a spread of till left beneath or behind ice. An erratic is a glacier-transported rock whose lithology differs from the local bedrock, providing clues to transport pathways.

Drumlins are streamlined hills commonly composed largely of till, sometimes with bedrock cores. Their long axes generally parallel former ice flow; the classic form has a blunt upstream end and a tapered downstream tail. Their formation is complex and may involve sediment deformation and erosion as well as deposition, so they should not be reduced to a single universal mechanism.

Meltwater sorts and stratifies sediment. Eskers are long, winding ridges of sand and gravel deposited by streams flowing within or beneath ice. Kames are irregular mounds of ice-contact sand and gravel. Outwash plains, or sandurs, form beyond glacier margins through braided meltwater streams. Kettle holes develop when buried or partly buried stagnant ice blocks melt; water-filled depressions become kettle lakes.

• Varves are paired seasonal sediment layers, commonly found in glacial lakes; coarser summer and finer winter layers can preserve annual records.
• A sinuous ridge suggests an esker, not a former surface river valley: sediment accumulated in an ice-confined channel and remained after melting.

5. Indian distribution, hazards and interpretation

Indian glacial landscapes occur mainly in the Himalaya and Karakoram. Gangotri Glacier lies in Uttarakhand, Zemu Glacier in Sikkim and Siachen Glacier in the eastern Karakoram. The Bhagirathi emerges at the Gangotri glacier terminus near Gaumukh. Moraines, glacial troughs and lake basins help reconstruct earlier glacier extents even where present ice has retreated.

Moraines may impound proglacial lakes, but such dams can contain loose sediment and buried ice. Overtopping waves, slope failures, intense inflows or internal erosion can contribute to a glacial lake outburst flood. Not every glacial lake is moraine-dammed: lakes also occupy bedrock basins or are dammed by ice. Hazard assessment therefore requires dam composition, lake geometry, surrounding slopes and downstream exposure.

For Prelims, classify features first by dominant agent and process. Contrast direct ice deposition with meltwater deposition, and distinguish a surviving landform from evidence of active glaciation. Ancient glacial features can persist in presently temperate regions. Remote sensing, field mapping, sediment analysis and dating of exposed rocks are used together to reconstruct glacier history.

• Glacier retreat does not automatically imply immediate river drying: runoff responses vary with season, precipitation and the stage of glacier loss.
• In mountain hazards, avoid labelling every debris flow or flash flood a glacial lake outburst flood without evidence of lake drainage.

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