How does the Narmada River region form a boundary between northern and southern India?

The Narmada River serves as a symbolic boundary between northern and southern India by flowing westward through a belt of hills and forests that separates the Indo-Gangetic Plain from the Deccan plateau region.1

This geographic division is reinforced by the Vindhya Range, which runs generally parallel to the river and has long been regarded as the traditional marker between the two regions. While the northern part of the subcontinent is characterized by the expansive alluvial basins of the Indus and Ganges rivers, the area to the south of the Narmada consists of the block of Archean rocks forming the Deccan plateau. Historically, the hilly and forested belt containing the Narmada remained relatively isolated and thinly populated, further distinguishing the cultural and topographical development of the northern plains from the peninsula proper.1