Short answers, pulled from the story.
The Indo-Gangetic Plain is the world's largest expanse of uninterrupted alluvium formed by the deposition of silt from the Indus, the Ganges, and the Brahmaputra rivers. This geological feature stretches from the Himalayas in the north to the northern edge of the Deccan Plateau in the south.
The Maurya Empire existed from the 4th to the 2nd century BCE and unified most of the Indian subcontinent within the Indo-Gangetic Plain. This empire became the demographic and political heart of the region before the Kushan Empire expanded from Afghanistan in the middle of the 1st century CE.
Recent research indicates that sediment deposited at the bed of the pre-historic Tethys Sea folded towards the northern end due to the northward drift of the Indian plate. A trough was formed later due to the emergence of the Himalayas in the north, creating a landscape that rests on hard crystalline rocks.
The four distinct geographical regions are the Bhabar, the Terai, the Bangar, and the Khadir. The Bhabar is a narrow region about 8 to 16 kilometers wide located below the Himalayan foothills, while the Terai is a strip of new silt that receives heavy rains and consists of thick wet evergreen vegetation.
The Indus River Delta receives only 100 to 200 millimeters of rainfall annually and covers an area of about 4,000 square kilometers. Since the 1940s, the delta has received less water due to large-scale irrigation works capturing large amounts of the Indus water before it reaches the delta.
The entire region came under the control of the British Raj after the Battle of Plassey in 1757 and the Battle of Buxar in 1764. This period of consolidation lasted until Indian Independence in 1947.