explain the formation of himalayas and northern plains

The Indian Peninsula drifted towards the north and finally collided with the much larger Eurasian Plate. As a result of this collision, the sedimentary rocks which were accumulated in the geosynclines (known as Tethys) got folded and formed the mountain systems of the West Asia and Himalaya.

Due to the uplift of the Himalayas in the Tethys Sea, the northern flank of the Indian Peninsula got subsided and formed a large basin. That basin was filled with sediments from the rivers which came from the mountains in the north and from the peninsula in the south. Thus, an extensive flat land of alluvial soil was formed which is known as the northern plains of India.