Landforms and their Evolution

Introduction

Landforms are the natural features of the Earth's surface. They are produced by the combined action of erosion and deposition by different geomorphic agents: running water (rivers), wind, glaciers, waves, and groundwater. Each agent creates distinctive landforms during erosion and deposition.

Landforms by Running Water (Fluvial Landforms)

Rivers are the most powerful and widespread geomorphic agents. River action is divided into three stages based on gradient and speed:

• A. Upper Course (Youthful Stage) — mainly erosion:
• Fast-flowing water in steep mountains; vertical erosion dominates
• Landforms:
• V-shaped valley: river cuts steeply downward, creating a narrow V-shaped cross-section
• Gorge/Canyon: very deep, steep-sided valley (e.g., Grand Canyon)
• Waterfall: water flows over a resistant rock layer (cap rock) that overlies softer rock; softer rock erodes, undercutting the cap rock until it collapses (retreating waterfall)
• Pot holes: cylindrical holes drilled in river bed by swirling stones (abrasion)
• Interlocking spurs: valley is so narrow the river winds around ridges that project alternately from either side

• B. Middle Course (Mature Stage) — erosion and deposition:
• Gentler gradient; lateral erosion increases; river meanders
• Landforms:
• Meanders: river bends sinuously due to erosion on the outer bank (cut bank) and deposition on the inner bank (point bar / slip-off slope)
• Ox-bow lakes: when a meander loop is cut off by the river during floods, forming a crescent-shaped lake
• Flood plains: flat, fertile land on either side of a river, formed by deposition of alluvium during floods

• C. Lower Course (Old Age Stage) — mainly deposition:
• Very gentle gradient; river deposits its load
• Landforms:
• Delta: triangular or fan-shaped deposit of sediment at the river mouth (e.g., Sundarbans delta of Ganga-Brahmaputra)
• Levees: natural embankments of sediment on river banks built up by repeated flooding
• Braided channels: the river splits into multiple shallow channels separated by sand bars

Landforms by Groundwater (Karst Topography)

When rainwater absorbs CO2 and becomes slightly acidic (carbonic acid), it dissolves limestone (calcium carbonate). This creates a distinctive landscape called karst topography.

• Erosional karst landforms:
• Sinkholes (Dolines): circular depressions formed by dissolution or collapse of limestone
• Uvalas: larger depressions formed by joining of several sinkholes
• Poljes: large flat-floored basins in karst regions
• Karst windows: holes in cave roofs opening to the surface
• Stalactites: icicle-like calcium carbonate formations hanging from cave ceilings (grow downward)
• Stalagmites: calcium carbonate formations rising from cave floors (grow upward)
• Columns/Pillars: when stalactites and stalagmites meet

Landforms by Glaciers (Glacial Landforms)

Glaciers are large, slow-moving bodies of ice that erode by plucking (pulling out rock fragments) and abrasion (scratching bedrock). They are extremely powerful erosional agents.

• Erosional glacial landforms:
• Cirque (Corrie): armchair-shaped hollow at the head of a valley where glacier originates
• Arete: sharp, knife-like ridge between two cirques
• Horn (Pyramidal peak): pointed peak formed when three or more cirques erode back from all sides (e.g., Matterhorn)
• U-shaped valley: valley carved by a glacier, with flat bottom and steep sides (contrast with river's V-shaped valley)
• Hanging valley: tributary glacier valley left high above the main valley floor after the main glacier retreats
• Fjord: drowned glacial U-shaped valley flooded by the sea (e.g., Norway's fjords)
• Roche moutonnee: asymmetric rock mound, smooth on the upstream side (abrasion) and rough on the downstream side (plucking)

• Depositional glacial landforms:
• Moraine: ridges of unsorted rock debris (till) deposited by glaciers
• Terminal (end) moraine, lateral moraine, medial moraine, ground moraine
• Drumlins: streamlined, egg-shaped hills of glacial till, with blunt end facing upstream
• Erratics: large boulders carried far from their origin and deposited
• Eskers: long, winding ridges of sorted sand and gravel deposited by meltwater streams under the glacier

Landforms by Wind (Aeolian Landforms)

Wind is most effective in arid and semi-arid regions (deserts) where there is little vegetation to protect the surface.

• Erosional wind landforms:
• Deflation hollows: shallow depressions from which wind removes fine particles
• Mushroom rocks / Pedestal rocks: rocks with narrow base and wide top, undercut by wind-blown sand near the ground
• Yardangs: wind-carved ridges aligned parallel to the prevailing wind

• Depositional wind landforms:
• Sand dunes: mounds of wind-deposited sand (Barchan = crescent-shaped; Seif = longitudinal; Transverse dune)
• Loess: very fine, wind-deposited silt (very fertile; e.g., North China Plains, Pampas of Argentina)

Landforms by Waves (Coastal Landforms)

• Erosional coastal landforms:
• Sea cliff: steep rock face formed by wave erosion at the base
• Sea arch: rock arch formed when a wave-cut cave breaks through a headland
• Sea stack: isolated pillar of rock left when a sea arch collapses
• Wave-cut platform: gently sloping rock surface exposed at low tide in front of a cliff

• Depositional coastal landforms:
• Beach: accumulation of sand/shingle along the shore
• Bar and lagoon: offshore bar of sand separating a shallow lagoon from the sea
• Spit: ridge of sand extending from a headland into the sea
• Tombolo: bar of sand connecting an island to the mainland

• Students confuse stalactites (hang from ceiling, like a "t" for top) and stalagmites (stand on ground). Trick: stalaCTite — C for ceiling; stalaGMite — G for ground.
• U-shaped valley = glacier; V-shaped valley = river. Do not mix these up.
• Ox-bow lake forms from river meanders — NOT from glacial action.
• A delta forms at the river MOUTH; a flood plain is along the river's middle/lower course.

Different geomorphic agents create distinctive landforms. Rivers create V-valleys, waterfalls, meanders, ox-bow lakes, and deltas. Groundwater dissolves limestone to create karst topography with sinkholes, stalactites, and stalagmites. Glaciers carve cirques, aretes, U-valleys, and deposit moraines and drumlins. Wind creates mushroom rocks and sand dunes in deserts. Waves shape coastlines with cliffs, arches, stacks, and beaches. Understanding these landforms and their origins is central to physical geography.