The Himalayan upliftment out of the Tethys Sea and the subsidence of the northern flank of the Peninsular Plateau resulted in the formation of a large basin. Gradually, the rivers flowing from the mountains in the north and the peninsular plateau in the south filled up this depression with deposits of sediments. This led to the formation of the Northern Plains of India.The formation of the Northern Plains owes largely to the interplay of three major river systems, namely − the Indus, the Ganga and the Brahmaputra along with their tributaries. This physiographic division spreads over an area of 7 lakh square kilometres, and is 2,400 kilometres long and 320 kilometres broad. It is a densely populated region. The rich soil cover, adequate water supply and favourable climate make the region agriculturally very productive. The Northern Plains are broadly divided into three sections.1) The Punjab Plains −Western part of the Northern Plains; formed by the Indus and its tributaries.2)The Ganga Plains − Largest part of the Northern Plains; extends between Ghaggar and Teesta rivers.3)The Brahmaputra Plains − Eastern part of the Northern Plains; formed by the Brahmaputra and its tributaries.According to the variations in relief features, the Northern plains can be divided into four regions. 1)The rivers, after descending from the mountains deposit pebbles in a narrow belt of about 8 to 16 km in width lying parallel to the slopes of the Shiwaliks. It is known as bhabar. All the streams disappear in this bhabar belt. 2)South of this belt, the streams and rivers re-emerge and create a wet, swampy and marshy region known as terai. This was a thickly forested region full of wildlife. 3)The largest part of the northern plain is formed of older alluvium. They lie above the flood plains of the rivers and present a terrace like feature. This part is known as bhangar.4) The soil in this region contains calcareous deposites locally known as kankar. The newer, younger deposits of the flood plains are called khadar. They are renewed almost every year and so are fertile, thus, ideal for intensive agriculture.