Peneplain

Aerial view of the almost flat and drowned peneplain at Belcher Islands, Hudson Bay, Canada. Note that the peneplain cuts across numerous folds.

In geomorphology and geology a peneplain is a low-relief plain formed by protracted erosion. This is the definition in the broadest of terms, albeit with frequency the usage of peneplain is meant to imply the representation of a near-final (or penultimate) stage of fluvial erosion during times of extended tectonic stability.[1] Peneplains are sometimes associated with the cycle of erosion theory of William Morris Davis,[1][A] but Davis and other workers have also used the term in a purely descriptive manner without any theory or particular genesis attached.[3]

The existence of some peneplains, and peneplanation as a process in nature, is not without controversy, due to a lack of contemporary examples and uncertainty in identifying relic examples.[1][4] By some definitions, peneplains grade down to a base level represented by sea level, yet in other definitions such a condition is ignored.[4] Geomorphologist Karna Lidmar-Bergström and co-workers consider the base level criterion crucial and above the precise mechanism of formation of peneplains, including this way some pediplains among peneplains.[5][6]

While peneplains are usually assumed to form near sea level it has also been posited that peneplains can form at height if extensive sedimentation raises the local base level sufficiently[7] or if river networks are continuously obstructed by tectonic deformation.[8] The peneplains of the Pyrenees and Tibetan Plateau may exemplify these two cases respectively.[7][8]

A common misconception about peneplains is that they ought to be so plain they are featureless.[4] In fact, some peneplains may be hilly as they reflect irregular deep weathering, forming a plain grading to a base level only at a grand-scale.[5][B]

Canisteo River Valley from Pinnacle State Park, New York. The distant peaks at the same elevation represent the remnants of a peneplain that was uplifted to form the Allegheny Plateau, which is a dissected plateau in southwestern New York. In this area, the sharp relief that is seen on some of the Allegheny Plateau has been rounded by glaciation.

Types of peneplains

There are various terms for landforms that are either alternatives to classical peneplains, a sub-set of peneplains or partially overlap with the term. The last is the case of planation surfaces that may be peneplains or not, while some peneplains are not planation surfaces.[5]

In their 2013 work Green, Lidmar-Bergström and co-workers provide the following classication scheme for peneplains:[5]

  1. Planation surfaces
    1. Pediplain
    2. Inselberg plain
    3. Etchplain
  2. Hilly relief
    1. Etched hilly relief

Rhodes Fairbridge and Charles Finkl argue that peneplains are often of mixed origin (polygenetic) as they may have been shaped by etchplanation during periods of humid climate and pediplanation during periods of arid and semi-arid climate. The long time spans under which some peneplains evolve ensures varied climatic influences.[3] The same authors do also list marine abrasion[C] and glacial erosion among processes that can contribute in shaping peneplains.[3]

In addition epigene peneplains can be distinguished from exhumed peneplains.[5] Epigene are those that have never been buried or covered by sedimentary rock.[5][10] Exhumed peneplains are those that are re-exposed after having been buried in sediments.[5]

The oldest identifiable peneplain in a region is known as a primary peneplain[3][D] An example of a primary peneplain is the Sub-Cambrian peneplain in southern Sweden.[11]

Pediplains

The peneplain concept is often juxtaposed to that of pediplain. However authors like Karna Lidmar-Bergström classify pediplains as a type of peneplain.[5] On the contrary Lester Charles King held them as incompatible landforms arguing that peneplains do not exist. King wrote:[12]

A peneplain in the Davisian sense, resulting from slope reduction and downwearing, does not exist in nature. It should be redefined as "an imaginary landform."

According to King pediplains main difference to Davis’ peneplains is in the history and processes behind, and less so in the final shape. A difference in form that may be present is that of residual hills which in Davis’ peneplains are to have gentle slopes while in pediplains they ought to have the same steepness as the slopes in the early stages of erosion leading to pediplanation.[12] Given that the coalesced pediments of the pediplains form a series of very gentle concave slopes, a difference with Davis' understanding of peneplains may lie in that his idealized peneplains had very gentle convex slopes instead. However, Davis' views on subject are not fully clear.[12] Contrary to this view Rhodes Fairbridge and Charles Finkl argue that the precise mechanism of formation (pediplanation, etc.) is irrelevant and that the term peneplain has been used and can be used in a purely descriptive manner. Further, alternation of processes with varying climate, relative sea level and biota make old surfaces unlikely to be of a single origin.[3]

Other Languages
български: Пенеплен
brezhoneg: Argompezenn
català: Peneplà
čeština: Parovina
Deutsch: Rumpffläche
español: Penillanura
Esperanto: Peneplano
euskara: Penezabaldi
فارسی: دشت‌گون
français: Pénéplaine
한국어: 준평원
հայերեն: Պենեպլեն
ქართული: პენეპლენი
қазақша: Пенеплен
Nederlands: Schiervlakte
norsk: Peneplan
norsk nynorsk: Peneplan
polski: Peneplena
português: Peneplanície
русский: Пенеплен
suomi: Peneplaani
svenska: Peneplan
Türkçe: Peneplen
українська: Пенеплен
中文: 準平原