Fluvial, Glacial and Aeolian Landforms

Why this topic matters for CDS

Geomorphology — the study of landforms — is a steady scoring area in the CDS General Studies paper. Questions on landforms made by rivers, glaciers and wind appear almost every year because they are factual, fixed and easy to set as one-liners or matching items. The syllabus does not change, so the effort you put in now rewards you in every future attempt.

Examiners love asking you to match a landform to its agent — for example, is an oxbow lake made by a river or a glacier? Is a barchan a wind feature or a coastal one? Once you understand which agent does erosion and which does deposition, dozens of such questions answer themselves.

Agents of gradation: how the land is reshaped

The surface of the Earth is constantly being lowered in some places and built up in others. The internal forces (volcanoes, earthquakes, plate movement) are endogenic; the external forces that wear down and rebuild the surface are exogenic or gradational agents.

The main exogenic agents are running water (rivers), moving ice (glaciers), wind, groundwater and sea waves. This chapter focuses on the three that CDS asks about most — rivers, glaciers and wind.

Two processes, two sets of landforms

• Erosion — the wearing away and removal of rock and soil, which carves erosional landforms.
• Deposition — the dropping of the transported material when the agent loses energy, which builds depositional landforms.

Fluvial landforms of erosion (made by rivers)

The work of running water is called fluvial activity, and rivers are the most important land-shaping agent on Earth. A river is most energetic in its upper or youthful course in the hills, where it cuts downward (vertical erosion).

Chief erosional features

• V-shaped valley — the typical narrow, steep-sided valley of the upper course, cut by strong downcutting.
• Gorge and canyon — a very deep, narrow valley with near-vertical walls in hard rock.
• Waterfall — where the river drops over a sudden step of harder rock.
• Pothole — a circular hole drilled into the rocky bed by pebbles swirled by the current.
• River meander cliffs — in the middle course the river erodes sideways (lateral erosion) on the outer bank of bends.

Fluvial landforms of deposition

As a river leaves the hills and flows over flatter land in its middle and lower course, it slows down and drops the load it was carrying. This builds a series of well-known depositional landforms.

• Alluvial fan — a fan-shaped spread of sediment formed where a river suddenly emerges from hills onto a plain.
• Meander — a large loop-like bend; the river erodes the outer bank and deposits on the inner bank.
• Ox-bow lake — a horseshoe-shaped lake left behind when a meander loop is cut off from the main channel.
• Flood plain — the flat, fertile land on either side, built up by silt left during floods.
• Levee — a natural raised bank of coarse sediment along the river's edge.
• Delta — a triangular deposit of fine sediment where the river meets the sea, e.g. the Sundarbans delta of the Ganga-Brahmaputra.

Glacial landforms of erosion (made by glaciers)

A glacier is a large mass of slowly moving ice. Glaciers occur in high mountains (valley glaciers) and over polar regions (continental ice sheets). Moving ice scrapes and plucks the rock beneath it, producing distinct erosional landforms.

• U-shaped valley — the classic glacial trough with a flat floor and steep sides (contrast with the river's V-shaped valley).
• Cirque (corrie) — an armchair-shaped hollow on a mountainside where a glacier begins; often holds a lake (a tarn) after the ice melts.
• Arête — a sharp, knife-edged ridge between two cirques.
• Horn — a sharp pyramidal peak where several cirques meet, e.g. the Matterhorn.
• Hanging valley — a smaller tributary valley left high above the main U-shaped valley, often with a waterfall.
• Fjord — a deep U-shaped glacial valley drowned by the sea, common along the Norwegian coast.

Glacial landforms of deposition

When a glacier melts, it dumps the unsorted rock debris it was carrying. This mixed material is called till or boulder clay, and the depositional features it forms are listed below.

• Moraine — a ridge of glacial debris. It is named by position: lateral (along the sides), medial (in the middle, where two glaciers join), terminal/end (at the snout) and ground (below the ice).
• Drumlin — a smooth, elongated, whale-back hill of till; many drumlins together form a “basket of eggs” landscape.
• Esker — a long, winding ridge of sand and gravel deposited by streams flowing under the melting ice.
• Outwash plain — a spread of sorted sand and gravel laid down by meltwater beyond the glacier's snout.
• Kettle lake — a hollow left when a buried block of ice melts, later filled with water.

Aeolian landforms of erosion (made by wind)

The work of the wind is called aeolian activity (after Aeolus, the Greek god of winds). Wind is the chief land-shaping agent in hot deserts such as the Sahara and the Thar, where there is little vegetation to hold the surface.

Wind erodes in two ways: deflation (blowing away loose sand and dust) and abrasion (sandblasting rock with carried particles).

• Mushroom rock (pedestal rock) — a rock worn thin at the base because most sand is carried near the ground, leaving a broad top — a classic desert sight.
• Yardang — a streamlined ridge carved parallel to the wind direction.
• Inselberg — an isolated residual hill of hard rock rising abruptly from a worn-down plain.
• Deflation hollow — a shallow basin scooped out where the wind has blown away loose material.
• Ventifact — a pebble polished and faceted by wind-blown sand.

Aeolian landforms of deposition

When wind speed falls, the sand it carries settles into hills and sheets. These wind-deposited features are the best-known landforms of deserts.

• Sand dune — a mound or ridge of wind-blown sand. The two main shapes are the crescent-shaped barchan and the long longitudinal (seif) dune.
• Barchan — a crescent dune whose horns point away from the wind (downwind); forms where sand is limited and the wind blows from one direction.
• Loess — a thick deposit of very fine, fertile, wind-blown dust. The famous Loess Plateau of northern China is the largest example.

Quick comparison: river vs glacier vs wind

Most CDS questions can be cracked by knowing which agent makes which feature. The table below in list form is the heart of this chapter — revise it until it is automatic.

Erosional features by agent

• River: V-shaped valley, gorge, waterfall, pothole.
• Glacier: U-shaped valley, cirque, arête, horn, hanging valley, fjord.
• Wind: mushroom rock, yardang, inselberg, deflation hollow.

Depositional features by agent

• River: alluvial fan, meander, ox-bow lake, flood plain, levee, delta.
• Glacier: moraine, drumlin, esker, outwash plain, kettle lake.
• Wind: sand dune, barchan, loess.

Worked example: reading the river's load

A river's ability to carry material depends sharply on its speed. A useful rule of thumb is that the transporting power of a stream rises very fast as its velocity increases — so even a small change in speed greatly changes what the river can move and where it will deposit. Here is a simplified illustration of that reasoning.

Common mistakes to avoid

• Mixing up the V-shaped valley (river) with the U-shaped valley (glacier) — the most common trap.
• Calling an ox-bow lake a glacial feature — it is made by a river abandoning a meander.
• Thinking a barchan's horns point into the wind — they point downwind (away from the wind).
• Treating loess as a kind of sand — it is fine, fertile, wind-blown dust.
• Confusing a medial moraine (where two glaciers meet) with a terminal moraine (at the snout).

Previous-year question and quick recap