Parallel Scarp Retreat and Drainage Evolution, Pachmarhi Area, Madhya Pradesh, Central India

This study is the outcome of field and topographic map interpretations to evaluate the relationship between scarp retreat and associated drainage pattern evolution. The area under study is composed of gently north-dipping series of Gondwana (Triassic) sediments overlam by Deccan Trap basalts (Cenozoic). These sediments have been exhumed within an elliptical iniler, the Central India Gondwana Basin, due to domal uplift parallel with the Narmada river rift valley which lies to the north of the study area.

The near east-west trending Pachmarhi Scarp developed within the Pachmarhi Sandstones is about 160 kilometers long and as much as 280 meters 10 height along the north-south minor axis of the dome. Outliers of the Pachmarhi Plateau (Upland Landscape) aligned parallel to the present scarp and some distance from it suggests parallel scarp retreat to the north.

Numerous drainage lines on the Lowland Landscape south of the Pachmarhi Plateau are aligned parallel to the Pachmarhi Scarp and are interspaced with sandstone outilers astride the interfluves. The resulting rectangular drainage pattern also suggests former scarp positions. Continuing scarp retreat exhumes the underlying Bijori Shales on which evolves the Lowland Landscape.

Scarp recession is believed to have been initiated by strike-streams developed along the feather edge of the northerly-tilted Pachmarhi Sandstones. Post-Deccan Trap uplift parallel to the Narmada rift valley lead to the entrenchment of major drainage lines that maintained their course across the axis of uplift and are hence considered antecedent. Minor streams that were defeated by the uplift occupied litho-tectonicc 'weak zones' both parallel to the scarp and at large angles with it, creating the rectangular and barbed drainage patterns. The Narmada river rift valley has acted as a recurrently active local base level at least since the post-Deccan Trap uplift and has had a major influence on 'stepped landform' evolution in the study area.