The great plains of northern India, also known as Ganga-Satlej Plains, are, in fact, transitional belt between the Himalayas and Peninsular India. The great plains cover an area 7,74,000 km2(3,00,000 square miles) having west-east length of 2400 km and north- south width of 144 km. Except Aravallis in the Rajas than plains no part of these extensive plains is higher than 150m AMSL. The plains have been formed by the deposition of alluvia brought by the rivers.
The exact depth of alluvia could not be ascertained as yet though the evidences of borings of tubewells have revealed the thickness of 400 to 500m but the basal rocks have not been encountered even during deepest borings.
As per scheme of R.L. Singh (1971) the great plains of India are divided into the following regions of three orders e.g., meso-level regions, first-order regions and second-order regions:
Origin of the Great Plains:
Though apparently the origin of the ‘great plains’ of India appears to be simple and less complicated as people think of their origin and development due to sedimentation of the foredeep created in front of the Himalayas during Alpine (Tertiary) mountain building but the process of the origin of the plains is still debatable as several scientists have postulated contrasting viewpoints.
(1) Concept of E. Suess:
According to E. Suess an extensive foredeep was formed between the Himalayas and Peninsular India due to Himalayan orogeny. The foredeep was in the form of an extensive syncline the bed of which being irregular and undulating was a broad synclinorium (consisting of numerous small anticlines and synclines).
The rivers coming from the Himalayas started to deposit sediments into the fore deep. The sediments were derived through the erosion of the Himalayas by southward flowing rivers. In due course of time the foredeep was filled with the sediments and thus the ‘great plains’ were formed.
According to Suess the bed of the foredeep was irregular, as its slope was gentle towards north but very steep towards south. It appears that Suess assumed greater depth of the foredeep in the south probably to explain the greater thickness of alluvia in the southern part of the ‘great plains’. He did not offer any explanation for the irregular bed of the foredeep. He further maintained that the Himalayas and Peninsular India were connected by consolidated parent rocks below the ‘great plain’.
(2) Concept of S. Burrard:
According to S. Burrard an extensive rift valley was formed at the time of the origin of the Himalayas. The northern and the southern blocks of this rift valley were formed by the southern ranges of the Himalayas and the foreland of Indian peninsula respectively. According to him this rift was formed due to formation of two normal faults.
He has presented the examples of Himalayan rift valley and Normada rift valley as evidences in support of his concept. Thus, according to Burrard an extensive rift valley, measuring 2400 km in length and 500 m in depth, formed due to the origin and upliftment of the Himalayas, was filled with the sediments brought by the rivers coming from the Himalayas and thus the great plains were formed due to gradual sedimentation in due course of time.
Burrard’s views of rift valley formation in front of the Himalayas are not tenable as no evidence could be found which can validate the idea of the formation of rift valley along the northern foreland of Indian Peninsula.
(3) General Concept:
The origin of the great plains of India should be considered as the outcome of several interconnected processes and mechanisms e.g. sedimentation, subsidence, upliftment, recession of sea etc. Evidences show that in geological past the Arabian Sea extended upto the present position of Garhwal and Kumaun regions as indicated by the presence of massive limestones and dolomites (these carborate rocks are always formed under marine conditions).
Similarly, the Bay of Bengal extended upto present Shillong plateau in the form of Eastern Gulf. It is believed that an extensive trench was formed in front of the Himalayas due to Himalayan orogeny. The rivers started to erode the Himalayan rocks and thus the sediments, so obtained, were deposited in the trench.
The ever-increasing weight of continuous sedimentation of enormous volume of eroded materials (alluvia) caused gradual subsidence of the trench. Thus, the repetition of twin processes of sedimentation and subsidence over long period of time resulted into the deposition of sediments upto great thickness (hundreds of metres).
The trench was ultimately filled with the sediments and the ‘great plains’ were formed. Arabian Sea continued to withdraw, with the result new land areas were freed from the sea and were added to plains. There are still evidences to demonstrate the recession of the Arabian Sea and progradation of coastal lands. The saline lakes, e.g. Panchbhadra and Sambhar Lakes, in Rajasthan validate the existence of sea in the geological past.