Landforms are the physical features of the Earth's surface, such as mountains, valleys, plateaus, plains, dunes and deltas. No landform is permanent; every one of them is created and modified by the processes of weathering, erosion, transportation and deposition. The study of how landforms are created and how they change over time is called geomorphology, and the story of how a landform develops from its initial stage to its final stage is called the evolution of landforms. This chapter examines how running water, groundwater, glaciers, wind and sea waves create distinctive landforms.
Every geomorphic agent produces two sets of landforms: erosional landforms, which are carved out by the agent's erosive action, and depositional landforms, which are built up by the material the agent deposits. The nature of the landforms depends on the agent, the underlying rock structure, and the stage of development of the landform. Over long geological time, landforms pass through a sequence of youthful, mature and old stages, a concept called the cycle of erosion. Understanding landforms is important because they influence human settlement, agriculture, transport and economic activity.
Running water (rivers) erodes the land through hydraulic action, abrasion, attrition and solution. The erosional landforms include: - Valleys: In the youthful stage, rivers cut deep, narrow, V-shaped valleys with steep sides, often forming gorges and canyons. The Grand Canyon of the USA is a classic canyon. - Potholes: Circular depressions carved into the river bed by pebbles and stones swirling in the water. - Waterfalls: Formed where a river falls over a hard rock ledge onto softer rock that is eroded faster. - Gorges and canyons: Deep, narrow valleys; a canyon has steps on its sides while a gorge has nearly vertical walls.
When a river loses energy, it deposits its load. The depositional landforms include: - Alluvial fans: Cone-shaped deposits formed where a river or stream emerges from a mountain onto a plain. - Floodplains: Wide flat areas on either side of a river built by repeated deposition of silt during floods. - Natural levees: Low ridges of sediment built along the banks of a river during floods. - Deltas: Triangular deposits formed at the mouth of a river where it enters a sea or lake. Examples include the Ganga-Brahmaputra delta. - Meanders and oxbow lakes: Meanders are the winding curves of a river; when a meander loop is cut off from the main river, it forms an oxbow lake.
Groundwater acts on soluble rocks, mainly limestone, dissolving them to create a distinctive landscape called karst topography, named after the Karst region of Slovenia. Carbon dioxide dissolved in water forms carbonic acid, which slowly dissolves limestone.
Glaciers erode through abrasion (scratching by rock fragments embedded in the ice) and plucking (lifting out of rock blocks). The erosional landforms include: - Cirques (corries): Bowl-shaped hollows at the head of a glacial valley, formed by ice erosion and frost action. - Horns and arêtes: A horn is a sharp pyramidal peak formed when three or more cirques erode a mountain; an arête is a sharp ridge between two cirques. - U-shaped glacial troughs: Broad, deep valleys carved by glaciers, with steep sides and flat floors. - Fjords: Deep glaciated valleys that have been flooded by the sea.
Wind erodes through deflation (removal of loose particles) and abrasion (sandblasting). The erosional landforms include: - Mushroom rocks (pedestal rocks): Rocks shaped like mushrooms because the base is eroded more than the top. - Yardangs: Long, narrow ridges of rock aligned in the direction of the prevailing wind. - Zeugens: Table-like rock formations. - Inselbergs: Isolated residual hills rising abruptly from a desert plain.
Waves erode the coast through hydraulic pressure, abrasion and corrosion. The erosional landforms include: - Cliffs: Steep rock faces formed by wave erosion at the coast. - Sea caves: Hollows carved into the base of cliffs by wave action. - Sea arches: Arch-shaped openings formed when a sea cave is eroded through a headland. - Stacks: Isolated columns of rock left standing when a sea arch collapses. - Wave-cut platforms: Flat, rock platforms exposed at the base of cliffs.
| Agent | Erosional Landforms |
|---|---|
| Running water | V-shaped valleys, gorges, canyons, potholes, waterfalls |
| Groundwater | Sinkholes, caves, caverns, uvalas, poljes |
| Glacier | Cirques, horns, arêtes, U-shaped troughs, fjords |
| Wind | Mushroom rocks, yardangs, zeugens, inselbergs |
| Sea waves | Cliffs, sea caves, sea arches, stacks, wave-cut platforms |
| Agent | Depositional Landforms |
|---|---|
| Running water | Alluvial fans, floodplains, levees, deltas, oxbow lakes |
| Groundwater | Stalactites, stalagmites, pillars |
| Glacier | Moraines, drumlins, outwash plains, erratics |
| Wind | Sand dunes, barchans, loess |
| Sea waves | Beaches, bars, spits, lagoons, tombolos |
| Stage | Characteristics |
|---|---|
| Youthful | V-shaped valley, waterfalls, fast flow, vertical erosion |
| Mature | Meanders, wider valley, lateral erosion |
| Old | Floodplain, oxbow lakes, deltas, very gentle slope |
Landforms are dynamic features shaped by the ceaseless action of geomorphic agents. Running water carves V-shaped valleys, gorges and waterfalls and builds deltas and floodplains; groundwater dissolves limestone into caves, sinkholes and stalactite-studded caverns; glaciers gouge out cirques, horns and U-shaped valleys and deposit moraines and drumlins; wind sculpts mushroom rocks and yardangs and piles up barchans and loess; and sea waves cut cliffs, arches and stacks while building beaches, bars and spits. Each landform records a history of erosion and deposition that helps geographers read the landscape. Together with the processes studied in the previous chapter, these landforms reveal how the surface of the Earth is continuously evolving.
Keywords: Landforms, gorges, canyons, deltas, meanders, karst, stalactites, cirques, moraines, barchans, fjords, stacks.