PHYSICAL GEOGRAPHYANDARCHAEOLOGYOF CENTRAL INDIA
Broadly speaking, the hilly regions can be divided into two major geological sectors: the northern sector consists of Vindhyan sandstone scarpland, which extends from Rohtasgarhon on the River Son to Ginnurgarh, just to the west of Bhopal. Situated in the north-east plateau of Malwa, Sanchi forms an outcrop of this formation. The southern sector comprises the Deccan basalt trap of the Vindhyan range (not to be confused with Vindhyan sandstone). Due to the rapid weathering of the horizontal lava flows, these basaltic hills are usually much lower and less steep and dissected than the sandstone hills to the north. A smaller third zone, largely restricted to the northern area around Gwalior, consists of a much older igneous formation called Bundelkhand gneiss. This is a granitic stone which can sometimes be found underlying sandstone cliffs.