Landforms of Glaciation

The Himalayan System: Orogeny to Glaciers already covers India's own glaciers, Siachen, the Gangotri, and which Himalayan rivers they feed. This note does not repeat that: it builds the general vocabulary of glacial landforms GC Leong's own chapter is named for, the erosional and depositional features any moving ice sheet produces anywhere in the world, which is exactly the kind of named-term list UPSC's matching and statement questions draw on.

A glacier is simply a mass of ice moving under its own weight, driven by gravity rather than by the kind of flow that carries water or wind. Two broad families exist. Continental or piedmont glaciers are vast sheets that spread out over open plains or across the foot of mountains, Antarctica and Greenland's ice sheets being the extreme case. Mountain or valley glaciers, the type most relevant to landform vocabulary, are linear rivers of ice confined within existing valleys on a mountainside, moving slowly downslope, typically centimetres to a few metres a day. Because the ice is loaded with rock debris frozen into its base and sides, ranging from fine grit to large angular boulders, a glacier does not merely slide over the landscape, it grinds, scrapes and plucks at it, and it does so with far more force than running water because of the sheer weight of the ice column pressing down. This combination of abrasion (the debris-loaded ice scouring the bed like sandpaper) and plucking (the ice freezing onto joints and blocks of bedrock and tearing them away as it moves) is what lets glaciers reduce whole mountain ranges to lower hills over geological time, and it is worth holding onto as the single mechanism behind every erosional landform below.

Erosional landforms

Cirques are the most distinctive and most common landform in glaciated mountains, and the starting point for nearly every other erosional feature in this chapter. A cirque is a deep, steep-walled, bowl- or amphitheatre-shaped hollow cut into a mountainside at the head of a glacial valley, formed where accumulating snow and ice erode headward into the slope through repeated plucking and abrasion. The walls are steep to vertical on three sides, dropping into a floor that is often over-deepened below the level of the cirque's own lip. Once the glacier that carved it has melted away, a cirque very often holds a small lake, called a cirque lake or tarn, and it is common to find a stepped sequence of two or more cirques, one opening into another lower down the same slope.

Horns and arêtes are what forms when several cirques attack the same mass of rock from different sides at once. As three or more cirques cut headward into a peak simultaneously, their back walls retreat toward each other until almost nothing of the original mountain is left except a high, sharp, steep-sided peak at the point where they converge, a horn. The Matterhorn in the Alps is the type example the name itself comes from, and Everest, in the Himalaya, is a horn formed the same way, by the headward erosion of the cirques ringing it. Where two cirques erode toward each other from opposite sides of a ridge rather than converging on a single peak, the divide between them is narrowed from both sides until it is reduced to a thin, sharp-crested, saw-toothed ridge called an arête, with a jagged, zig-zag skyline. A pass or low saddle point along an arête, between two horns or high points, is called a col.

Glacial troughs, also called glacial valleys, are the single clearest sign that a valley was carved by ice rather than water: they are broadly U-shaped in cross-section, with a wide, relatively flat floor and steep, smooth sides, in direct contrast to the narrow, V-shaped valley a river cuts. This shape comes from the glacier's own bulk: unlike a river, which is confined to a channel far smaller than its valley, a valley glacier fills the whole width of the trough it sits in and grinds against both the floor and the sides at once as it moves, widening and deepening the valley uniformly rather than incising a narrow channel into it. Where a smaller tributary glacier joined a larger trunk glacier, its own, shallower valley is left stranded well above the floor of the main trough once the ice retreats, since the smaller glacier had far less erosive power than the trunk glacier and could not cut down as deep; this is a hanging valley, and it very often produces a waterfall or cascade where the tributary valley's stream now drops down to meet the main valley floor. The same difference in erosive power also truncates the spurs, the ridges that would otherwise project into the valley from its sides, shaving them back into steep, triangular-faced cliffs called truncated spurs, in place of the more gently interlocking spurs a winding river valley leaves behind. Where a glacial trough runs all the way down to the coast and sea level rises (or the land subsides) enough to drown it, the deep, steep-sided inlet that results is a fjord, the standard high-latitude coastal feature of Norway, Chile and New Zealand's south-west coast alike.

Depositional landforms

Everything a glacier has eroded, plucked and dragged along does not vanish when the ice finally melts; it is dropped more or less where the ice left it, as an unsorted mixture of angular to sub-angular rock fragments of every size called glacial till. Because till is dumped directly by melting ice rather than sorted by flowing water, it shows none of the size-grading a river deposit does, a useful diagnostic in its own right.

Moraines are long ridges built up of this glacial till, and they are named for where they form relative to the glacier rather than for any difference in the material itself. A terminal moraine is a ridge of debris dumped at the glacier's snout, marking the furthest point the ice ever reached. Lateral moraines build up as ridges running along both sides of a valley glacier, parallel to the valley walls, from debris that has fallen or been plucked from the slopes above; where two valley glaciers merge, their adjoining lateral moraines combine to form a medial moraine, a ridge of debris running down the centre of the combined glacier. Where the lateral moraines on either side of a valley curve around to join the terminal moraine at the glacier's snout, the whole assembly forms a continuous horse-shoe shaped ridge. A ground moraine is different again: it is not a single ridge but an irregular, often thin sheet of till left spread over the whole floor of a valley wherever a retreating glacier simply abandoned its load in place rather than pushing it forward, and it typically shows an uneven, hummocky surface rather than a distinct ridge line.

Meltwater does not stop working once the ice has gone, and the landforms it builds are distinct from till precisely because flowing water sorts material by size the way glacial ice never does. Outwash plains form at the foot of glaciated mountains, or beyond the limit of a continental ice sheet, where meltwater streams spread out and deposit broad, flat aprons of gravel, sand, silt and clay, roughly stratified and far better sorted than the till upstream of them, effectively a glacial equivalent of an alluvial fan. An esker is a long, narrow, sinuous ridge of sand and gravel, built by a stream of meltwater that once flowed in a tunnel beneath the glacier itself, with ice forming the tunnel's banks; when the surrounding ice eventually melts away, the coarse material that had settled inside the tunnel is left standing as a winding ridge across the landscape, tracing the former course of the sub-glacial stream. A drumlin is a smooth, oval, ridge-like hill made mainly of glacial till (sometimes with pockets of gravel and sand), typically up to about a kilometre long and up to roughly 30 metres high, and it forms not by simple dumping but by ice actively moulding and streamlining an existing patch of debris as it continues to move over it. A drumlin's long axis lies parallel to the direction the ice was moving, and the two ends are asymmetric: the end facing the oncoming glacier, the stoss end, is blunter and steeper, having been bluntly pushed and blunted by the moving ice, while the far, downstream end, the tail, tapers more gently. Because that asymmetry is fixed by the direction of ice flow, a field of drumlins is one of the more reliable ways to read the direction a vanished glacier was moving in, long after the ice itself is gone.

Quick revision points

• Glaciers are continental/piedmont (vast ice sheets) or mountain/valley (linear, confined to existing valleys); movement is gravity-driven and much slower than water. Erosion works through abrasion (debris-loaded ice scouring the bed) and plucking (ice freezing onto and tearing away jointed rock).
• Cirque: steep-walled, bowl-shaped hollow at a glacial valley's head, formed by headward erosion; often holds a tarn (cirque lake) once the ice melts; can occur in a stepped sequence.
• Horn: sharp peak left where three or more cirques erode toward each other (Matterhorn, Everest). Arête: sharp, saw-toothed ridge left where two cirques erode from opposite sides. Col: a low pass along an arête.
• Glacial trough (glacial valley): broad, U-shaped valley, in contrast to a river's narrow V-shaped valley, because the glacier fills and erodes the whole valley width at once.
• Hanging valley: a tributary glacier's shallower valley left stranded above the main trough floor, often producing a waterfall where it rejoins. Truncated spurs: valley-side ridges sheared back into steep triangular cliffs by the glacier's greater erosive power.
• Fjord: a glacial trough drowned by the sea at the coast (Norway, Chile, New Zealand).
• Glacial till: unsorted, angular-to-sub-angular debris dumped directly by melting ice, unlike water-sorted deposits.
• Moraines (all made of till): terminal (at the snout, marks the ice's furthest advance), lateral (along the valley sides), medial (where two glaciers' lateral moraines merge), ground (an irregular, hummocky sheet over the valley floor).
• Outwash plain: broad, stratified, well-sorted glacio-fluvial deposit at a glacier's foot, built by meltwater. Esker: sinuous ridge of sand and gravel marking a former sub-glacial meltwater tunnel.
• Drumlin: smooth, oval till hill, long axis parallel to ice flow; blunt, steep stoss end faces the oncoming ice, gentler tail end points downstream, so a drumlin field indicates the direction the glacier moved.