Here is your paragraph on the Great Plain of North India!
To the south of the Himalayas and to the north of the Peninsula lies the Great Plain of North India. It is an aggradational plain formed by the depositional work of three major river systems viz., the Indus, the Ganga and the Brahmaputra.
This arcuate plain is also known as Indo-Gangetic-Brahmaputra Plain.
This is the largest alluvial tract of the world extending for a length of 3,200 km from the mouth of the Indus to the mouth of the Ganga, of which the Indian sector alone accounts for 2,400 km in length. Its average width varies from 150 to 300 km. It is widest in the west where it stretches for about 500 km. Its width decreases in the east. It is about 280 km wide near Allahabad and 160 km near Rajmahal Hills.
It widens to about 460 km in West Bengal but narrows down in Assam where it is only 60-100 km wide. It covers a total area of 7.8 lakh sq km. The northern boundary of this plain is well defined by the foothills of the Shiwaliks but its southern boundary is a wavy irregular line along the northern edge of the Peninsular India.
Rivers flowing through this plain, especially those originating in the Himalayas have deposited a thick layer of alluvium throughout the length and breadth of this plain. Thus it is a classical example of an aggradational plain.
However, the thickness of the alluvium deposit varies from place to place and different estimates have been made about it. According to Oldham, the maximum depth of the soil is about 5,000 m near its southern edge. It has probably maximum depth between Delhi and Rajmahal Hills and is shallow in Rajasthan and Assam.
According to recent computations of seismic soundings, the maximum depth of the alluvium upto the basement rocks is about 6,100 m. The depth of alluvial deposits at some important locations are Meerut (Ц)66.8 m), Kalyan (2286.0 m), and Siliguri (5577.8 m).
The variation in thickness of the alluvium largely depends upon the morphological processes. The cones of Kosi in the north and those of Son in the south exhibit greater alluvial thickness while the intra-cone areas have relatively shallower deposits.
Extreme horizontality of this monotonous plain is its chief characteristic. Whereas its average elevation is about 200 m above mean sea level, its highest elevation is 291 m above mean sea level between Ambala and Saharanpur. The comparatively high area near Ambala forms the watershed which divides the drainage system of the Ganga from that of the Indus.
It is, in fact, a low land and is hardly perceptible as one enters Panjab-Haryana plain from the Uttar Pradesh plains. Its average gradient for about 1500 km from Saharanpur to Kolkata is 20 cm per km and it decreases to 15 cm per km from Varanasi to the Ganga delta.