Formation of the Northern Plains

The Northern Plains of India represent one of the most fertile, youngest, and most densely populated physiographic divisions of the world. Extending over an area of approximately 7.5 lakh square kilometers, this region is a massive aggradational plain formed through complex tectonic movements and subsequent fluvial depositional processes over millions of years.

Tectonic Genesis and the Role of the Tethys Geosyncline

The origin of the Northern Plains is intrinsically linked to the formation of the Himalayas. During the Late Cretaceous and Early Tertiary periods, the northward drift of the Indian Plate led to its collision with the Eurasian Plate.

• The Collision: The collision caused the sediments of the intervening Tethys Ocean to fold, uplifting into the Himalayan mountain ranges.
• Creation of the Foredeep: As the Himalayas rose, the intense downward pressure exerted by the massive weight of the shifting plates caused the northern segment of the Indian Peninsular shield to tilt and subside. This structural warping created a massive, elongated geo-synclinal depression or foredeep parallel to the mountains, known as the Tethyan Foredeep.
• Initial Dimensions: This vast sag initially had a depth of several kilometers, forming a colossal trough between the newly risen Himalayas to the north and the stable Peninsular Plateau to the south.

Fluvial Sedimentation and Aggradation

The transformation of this deep structural depression into a fertile plain was achieved entirely through continuous alluvial deposition during the Miocene, Pliocene, and Pleistocene epochs.

• Sources of Alluvium: The newly formed, fast-flowing Himalayan rivers in the north and the older Peninsular rivers in the south carried immense loads of eroded sediments, including pebbles, gravel, sand, silt, and clay.
• The Process of Aggradation: As these rivers exited the steep gradients of the mountains and entered the low-lying depression, their velocity dropped drastically. This reduction in kinetic energy forced the rivers to deposit their heavy sediment load.
• Chronological Infilling: Over millions of years, this continuous depositional mechanism filled the deep foredeep completely, layer by layer, replacing the water body with a flat, low-relief alluvial plain.

Chronology of Geological Events in Plain Formation

Morphological Zones of the Northern Plains

The ongoing processes of river deposition and rejuvenation have differentiated the Northern Plains into four distinct, parallel morphological zones running from north to south.

Bhabar

This is a narrow belt, about 8 to 10 kilometers wide, running parallel to the Shiwalik foothills.

• Composition: It consists of coarse sediments, boulders, pebbles, and unassorted rock debris deposited by rivers as they break through the mountain fronts.
• Hydrological Feature: The porosity of this gravel-ridden tract is extremely high. As a result, smaller streams and rivers completely sink underground, disappearing from the surface and flowing as sub-surface water. The terrain is highly unsuitable for agricultural cultivation.

Terai

Located south of the Bhabar belt, the Terai zone is a 10 to 20-kilometer-wide marshy, damp, and thickly forested tract.

• Hydrological Feature: The underground streams that disappeared in the Bhabar zone re-emerge at the surface in the Terai, creating wet, swampy conditions.
• Soil Characteristics: The soil is rich in humus and composed of fine silt and clay. Historically a zone of dense forests and rich biodiversity, large tracts of Terai have been cleared for intensive agriculture, particularly for crops like sugarcane, rice, and wheat.

Bhangar

The Bhangar represents the older alluvium of the plains, forming the higher alluvial terraces above the current floodplains of the rivers.

• Composition: This zone is less prone to annual flooding. The soil is mature, clayey, and dark-colored.
• Key Features: It contains prominent calcareous concentrations or nodules known locally as Kankar. In drier parts of the Bhangar tract, intensive irrigation has led to the accumulation of saline and alkaline efflorescences on the surface, known as Reh or Kallar.

Khadar

The Khadar constitutes the younger alluvium found in the low-lying floodplains and deltaic regions of the river valleys.

• Composition: This tract is flooded almost every year by the overflowing rivers. The annual inundation deposits a fresh layer of fine silt, sand, and clay, making the soil highly fertile and self-renewing.
• Agricultural Value: Due to the perpetual replenishment of nutrients, Khadar tracts do not require heavy chemical fertilization and are heavily utilized for intensive, multi-crop cultivation.

Core Geographical Facts and Trivia for Prelims

• Depth of Alluvium: The depth of the alluvial deposits in the Northern Plains is not uniform. It increases from south to north, reaching its maximum thickness of nearly 6,000 meters near the Shiwalik foothills, while shallowing out toward the Peninsular shield.
• The Ambala Water Divide: The city of Ambala in Haryana acts as a critical water divide. It stands on a minor upland that separates the Indus River basin draining into the Arabian Sea from the Ganga River basin draining into the Bay of Bengal.
• Barind Plains: In the Bengal delta region, older alluvial terraces corresponding to the Bhangar formations have undergone lateritization, forming distinct, elevated geographical structures known as the Barind Plains.
• Chos and Dhayas: In the Punjab plains, regional erosion by seasonal streams (locally called Chos) has severely dissected the foothills of the Shiwaliks, leading to badland topography. The elevated blocks left between the floodplains are locally termed Dhayas.