Define Peneplains. Describe the landscape features associated with peneplains under different geomorphic cycles – UPSC Geography Optional Mains – 2023

A peneplain (literally meaning “almost a plain”) represents a low-relief, undulating, featureless plain produced at the end stage of a fluvial landscape evolution cycle. First conceptualized by the American geomorphologist William Morris Davis (1889), a peneplain is the penultimate stage (old age) of the normal cycle of erosion.

According to Davis, landforms undergo a systematic sequence of development through youth, maturity, and old age under humid temperate conditions. In this final stage, base-level erosion, downcutting, and lateral planar reduction bring the relief down close to sea level, beyond which landforms cannot degrade naturally under an uninterrupted cycle.

 

Conceptual Contrast: Peneplain (Davis) vs. Pediplain (L. C. King)

Understanding the distinction between Davis’s peneplain and L.C. King’s pediplain is critical for UPSC Geography Optional answers:

Landscape Features Associated with Peneplains Under Different Geomorphic Cycles

Geomorphic landforms vary significantly depending on whether the erosion cycle completes uninterrupted, experiences tectonic rejuvenation, or undergoes burial and resurrection.

A. Features under the Normal (Uninterrupted) Cycle of Erosion

1. Convexo-Concave Slope Profile:

   • The land surface exhibits gentle, low-gradient rolling topographies characterized by upper convex crests and lower concave slopes near wide floodplains.

2. Local / Incipient Peneplain:

   • Represents the first local manifestation of peneplanation along major river valleys while interfluves still retain moderate relief.

3. Regional Peneplain:

   • Formed when adjacent local peneplains coalesce over time, creating a vast regional expanse of subdued topography.

4. Monadnocks (Residual Hills):

   • Hard, resistant rock masses (e.g., granites or quartzites) that resist weathering stand isolated above the surrounding base plain.

   • Examples: Stone Mountain (Georgia, USA), residual remnants of the Aravalli Range, and isolated knolls on the Telangana Plateau.

B. Features under the Second Cycle of Erosion (Rejuvenation)

When a region undergoing peneplanation experiences crustal uplift or a base-level drop, a second cycle of erosion begins, rejuvenating the rivers:

1. Uplifted Peneplains:

   • Topographic expressions of rejuvenated landforms where former low-relief surfaces are elevated into plateau structures or high tablelands.

2. Accordant Summits & Planation Surfaces:

   • Flat-topped residual ridge summits aligning at uniform elevations, marking the height of the older peneplain before tectonic uplift.

3. Incised Meanders & Valley-in-Valley Structures:

   • Rejuvenated streams cut deeply into the older peneplain surface, creating step-like polycyclic profiles.

   • Examples: Schooley, Harrisburg, and Summerville peneplains in the Appalachian region, as well as the Patlands of the Ranchi Plateau (Chota Nagpur region, India).

C. Features under Shortened / Interrupted Cycles (Resurrected & Polycyclic Relief)

Erosion cycles are rarely uninterrupted due to eustatic changes, tectonic shifts, or climatic oscillations:

1. Resurrected (Buried) Peneplains:

   • Peneplain surfaces submerged under marine transgression or buried under heavy sedimentary/volcanic layers, and subsequently exposed again through denudation.

   • Examples: Granite-gneissic peneplain surface exposures, and the Chhechari Basin in the Palamau Upland (Jharkhand, India).

2. Polycyclic (Multi-cyclic) Reliefs:

   • Created when repeated interruptions produce multiple generations of peneplain remnants within a single regional landscape.

Frequently Asked Questions (FAQs)

Q1. What is the difference between a Peneplain and a Pediplain for UPSC?

A: A peneplain (Davis) forms mainly via downwasting and slope decay under humid environments, resulting in rolling convexo-concave terrain with Monadnocks. A pediplain (King) forms via parallel retreat of slopes and pedimentation under semi-arid conditions, resulting in flat surfaces punctuated by steep Inselbergs.

Q2. What is a Monadnock, and how is it formed?

A: A Monadnock is an isolated, erosion-resistant rock hill rising abruptly from a gently sloping or level surrounding peneplain. It represents resistant bedrock (like quartzite or granite) that survived downwasting during the old age stage of the Davisian cycle.

Q3. How do Patlands in India demonstrate an Uplifted Peneplain?

A: The Patlands of the Ranchi Plateau (e.g., Netarhat Pat) represent high-altitude basaltic tablelands that were originally part of a low-elevation peneplain. Tectonic rejuvenation during the Tertiary period uplifted these surfaces, turning them into elevated planation surfaces with steep scarps.