Plains constitute extensive, low-relief tracts of land characterized by gentle gradients and low absolute elevation relative to sea level, forming the cradles of human civilization. Geomorphologically, they are classified based on their genesis into structural, erosional, depositional, and glacio-fluvial plains, with fluvial deposition acting as the dominant sculpturing agent across the globe. From a tectonic and structural viewpoint, they frequently occupy cratonic interiors, foredeeps of collisional mountain chains, or coastal margins, integrating complex processes of subsidence, sedimentation, and eustatic sea-level fluctuations. For competitive exams like UP TGT/PGT Geography and UGC-NET, mastery requires dissecting the micro-relief of alluvial plains (Bhabar, Terai, Bhangar, Khadar), deltaic morphology, structural classifications, and anthropogenic carrying capacities.
1. Geomorphic Genesis and Genetic Classification of Plains
Plains are formed through diverse endogenetic and exogenetic processes operating across varying temporal and spatial scales.
- Structural plains are formed by tectonic stability, epeirogenic uplift of continental shelves, or cratonic downwarping.
- Depositional plains include alluvial plains (floodplains, meander belts), deltaic plains, loess plains, glacial outwash plains, and lacustrine plains.
- Erosional plains (peneplains and pediplains) are produced by subaerial denudation, weather-mass wasting, and lateral planation over prolonged geological eras.
- Glacio-fluvial or outwash plains (sandurs) form via meltwater deposition of stratified drift beyond terminal moraines during glacial retreats.
- Coastal plains form through marine sedimentation, wave-built platform emergence, or relative drop in sea levels.
- Alluvial depositional plains represent the highest concentration of global agrarian and demographic settlement.
- Loess plains (e.g., Loess Plateau of China) represent aeolian accumulation of fine wind-blown silt particles.
2. Alluvial Micro-Morphology of the Indo-Gangetic Plain
North Indian Plains exhibit systematic longitudinal and transverse zonation based on sediment load, stream competence, and permeability.
- Bhabar zone lies parallel to the Shiwalik foothills, characterized by high porosity, coarse boulders, and disappearing streams.
- Terai zone features marshy, water-releasing subterranean streams, high biodiversity, and dense fine-grained sedimentary accumulation.
- Bhangar represents older alluvial terraces above flood limits, often containing calcareous concretion nodules known as kankar.
- Khadar represents newer, fertile alluvial floodplains replenished annually by monsoon river flooding.
- Reh or Kalar refers to saline-alkaline efflorescences developing in canal-irrigated semi-arid tracts of the western Gangetic plain.
- Bhabar is non-arable due to boulder fragmentation; Khadar is agriculturally prime but prone to fluvial shifting.
- Canal irrigation without adequate drainage converts productive Bhangar into sterile Reh wasteland.
3. Structural, Tectonic, and Climatic Controls on Plain Formation
Plains are profoundly influenced by foreland subsidence, crustal flexure, and climatic regimes governing weathering and sediment yield.
- Foredeep plains (e.g., Indo-Gangetic-Brahmaputra trough) develop in flexural downwarping zones ahead of collision-induced orogenic belts.
- Pediplains form in arid and semi-arid environments through parallel retreat of slopes and sheetwash action.
- Coastal plains are heavily controlled by eustatic sea-level changes and continental shelf gradients.
- Glacial depositional plains reflect Pleistocene glacial advances, till deposition, and drumlinized till plains.
- Climatic zones dictate the mechanical vs chemical weathering balance, directly impacting sediment calibre reaching depositional basins.
- Foreland basins undergo continuous subsidence, accommodating thick sequences of continental clastic sediments.
- Arid plains show extensive pedimentation, whereas humid tropical plains are dominated by chemical weathering and thick alluvium.
Spatial Patterns & Regional Distribution
🌐 Global Geographic Belts & Spatial Variation
Major global plain belts include the Great Plains of North America (interior continental deposition/glacial outwash), the East European and Russian Platform Plains (cratonic stability), the Amazon Basin (tropical equatorial structural/alluvial plain), the Indo-Gangetic-Brahmaputra Plain (foredeep tectonic plain), and the North China Plain (alluvial-loessic plain).
🇮🇳 Indian Subcontinent Context & Regional Corridors
The Great Plains of India span from the Indus river system in the west to the Brahmaputra valley in the east, wedged between the Peninsular Plateau and the Himalayan orogen. It is sub-divided into the Rajasthan Plain (Thar desert, Luni basin, inland drainage), Punjab-Haryana Plain (Sutlej-Beas-Ravi interfluves or doabs), Ganga Plain (Upper, Middle, Lower sub-regions), and the Brahmaputra (Assam) Valley plain.
Mnemonics, Memory Shortcuts & Exam Traps
B = Bhabar (foothills), T = Terai (marshy belt), B = Bhangar (old alluvium), K = Khadar (new alluvium), F = Deltaic Facies.
Bist Doab (Beas-Sutlej), Bari Doab (Beas-Ravi), Rachna Doab (Ravi-Chenab), Chaj Doab (Chenab-Jhelum), Sindh Sagar Doab (Jhelum-Indus).