Peneplain

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Aerial view of the almost flat and drowned peneplain at Belcher Islands, Hudson Bay, Canada, cutting across numerous geologic folds. Sanikiluaq peneplain.JPG
Aerial view of the almost flat and drowned peneplain at Belcher Islands, Hudson Bay, Canada, cutting across numerous geologic folds.
Sketch of a hypothetical peneplain formation after an orogeny. Peneplain.jpg
Sketch of a hypothetical peneplain formation after an orogeny.

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] [upper-alpha 1] but Davis and other workers have also used the term in a purely descriptive manner without any theory or particular genesis attached. [3]

Contents

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] [upper-alpha 2]

At the grand-scale peneplains are characterized by appearing to be sculpted in rock with disregard of rock structure and lithology, but in detail, their shape is structurally controlled, for example, drainage divides in peneplain can follow more resistant rock. [9] In the view of Davis large streams do became insensitive to lithology and structure, which they were not during the valley phase of erosion cycle. This may explain the existence of superimposed streams. [9]

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. Canisteo valley 1453.JPG
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 classification 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 [upper-alpha 3] and glacial erosion among processes that can contribute in shaping peneplains. [3]

In addition, epigene peneplains can be distinguished from exhumed peneplains. [5] Epigene peneplains are those that have never been buried or covered by sedimentary rock. [5] [11] 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] [upper-alpha 4] An example of a primary peneplain is the Sub-Cambrian peneplain in southern Sweden. [12]

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: [13]

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 the difference between pediplains and Davis’ peneplains is in the history and processes behind their formation, 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. [13] 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 the fact that his idealized peneplains had very gentle convex slopes instead. However, Davis' views on the subject are not fully clear. [13] 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]

Preservation and destruction of peneplains

The Hardangervidda plateau in southern Norway is a peneplain formed in the Miocene epoch and then uplifted to its present altitude of 1200 m a.s.l. Hardangerviddaflora.jpg
The Hardangervidda plateau in southern Norway is a peneplain formed in the Miocene epoch and then uplifted to its present altitude of 1200 m a.s.l.

Peneplains that are detached from their base level are identified by either hosting an accumulation of sediments that buries it or by being in an uplifted position. Burial preserves the peneplain. Any exposed peneplain detached from its baselevel can be considered a paleosurface or paleoplain. [5] [15] Uplift of a peneplain commonly results in renewed erosion. As Davis put it in 1885: [16]

"the decrepit surface must wait either until extinguished by submergence below the sea, or regenerated by elevation into a new cycle of life"

Uplifted peneplains can be preserved as fossil landforms in conditions of extreme aridity or under non-eroding cold-based glacier ice. [5] Erosion of peneplains by glaciers in shield regions is limited. [17] [18] In the Fennoscandian Shield average glacier erosion during the Quaternary amounts to tens of meters, albeit this was not evenly distributed. [18] For glacier erosion to be effective in shields a long "preparation period" of weathering under non-glacial conditions may be a requirement. [17]

Silicification of peneplain surfaces exposed to sub-tropical and tropical climate for long enough time can protect them from erosion. [17]

See also

Notes

  1. ↑ The term was coined around 1900 by William Morris Davis who described it as follows: Given sufficient time for the action of denuding forces on a mass of land standing fixed with reference to a constant base-level, and it must be worn down so low and so smooth, that it would fully deserve the name of a plain. But it is very unusual for a mass of land to maintain a fixed position as long as is here assumed.... I have therefore elsewhere suggested that an old region, nearly base-levelled, should be called an almost-plain; that is a peneplain. [1] [2]
  2. ↑ Example of this are the Sub-Mesozoic hilly peneplains of southern Sweden. [6]
  3. ↑ Coastal geomorphologist Douglas Wilson Johnson has proposed to use the term "peneplane" instead of "peneplain" when a planation surface is thought to be of marine origin. [10]
  4. ↑ Akin to what in German scientific literature is known as a Primärrumpf. [3]

Related Research Articles

<span class="mw-page-title-main">Baltic Shield</span> Ancient segment of Earths crust

The Baltic Shield is a segment of the Earth's crust belonging to the East European Craton, representing a large part of Fennoscandia, northwestern Russia and the northern Baltic Sea. It is composed mostly of Archean and Proterozoic gneisses and greenstone which have undergone numerous deformations through tectonic activity. It contains the oldest rocks of the European continent with a thickness of 250–300 km.

<span class="mw-page-title-main">Base level</span> Lowest limit for erosion processes

In geology and geomorphology a base level is the lower limit for an erosion process. The modern term was introduced by John Wesley Powell in 1875. The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory. The "ultimate base level" is the plane that results from projection of the sea level under landmasses. It is to this base level that topography tends to approach due to erosion, eventually forming a peneplain close to the end of a cycle of erosion.

<span class="mw-page-title-main">Shield (geology)</span> Large stable area of exposed Precambrian crystalline rock

A shield is a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. These rocks are older than 570 million years and sometimes date back to around 2 to 3.5 billion years. They have been little affected by tectonic events following the end of the Precambrian, and are relatively flat regions where mountain building, faulting, and other tectonic processes are minor, compared with the activity at their margins and between tectonic plates. Shields occur on all continents.

<span class="mw-page-title-main">Glacial landform</span> Landform created by the action of glaciers

Glacial landforms are landforms created by the action of glaciers. Most of today's glacial landforms were created by the movement of large ice sheets during the Quaternary glaciations. Some areas, like Fennoscandia and the southern Andes, have extensive occurrences of glacial landforms; other areas, such as the Sahara, display rare and very old fossil glacial landforms.

The geographic cycle, or cycle of erosion, is an idealized model that explains the development of relief in landscapes. The model starts with the erosion that follows uplift of land above a base level and ends, if conditions allow, in the formation of a peneplain. Landscapes that show evidence of more than one cycle of erosion are termed "polycyclical". The cycle of erosion and some of its associated concepts have, despite their popularity, been a subject of much criticism.

<span class="mw-page-title-main">Pediplain</span> Extensive plain formed by the coalescence of pediments

In geology and geomorphology a pediplain is an extensive plain formed by the coalescence of pediments. The processes through which pediplains forms is known as pediplanation. The concepts of pediplain and pediplanation were first developed by geologist Lester Charles King in his 1942 book South African Scenery. The concept gained notoriety as it was juxtaposed to peneplanation.

In geology, epeirogenic movement is upheavals or depressions of land exhibiting long wavelengths and little folding apart from broad undulations. The broad central parts of continents are called cratons, and are subject to epeirogeny. The movement may be one of subsidence toward, or of uplift from, the center of Earth. The movement is caused by a set of forces acting along an Earth radius, such as those contributing to isostasy and faulting in the lithosphere.

<span class="mw-page-title-main">Scandinavian Mountains</span> Mountain range in Finland, Norway and Sweden

The Scandinavian Mountains or the Scandes is a mountain range that runs through the Scandinavian Peninsula. The western sides of the mountains drop precipitously into the North Sea and Norwegian Sea, forming the fjords of Norway, whereas to the northeast they gradually curve towards Finland. To the north they form the border between Norway and Sweden, reaching 2,000 metres (6,600 ft) high at the Arctic Circle. The mountain range just touches northwesternmost Finland but are scarcely more than hills at their northernmost extension at the North Cape.

<span class="mw-page-title-main">Sub-Cambrian peneplain</span> Ancient, extremely flat, erosion surface

The sub-Cambrian peneplain is an ancient, extremely flat, erosion surface (peneplain) that has been exhumed and exposed by erosion from under Cambrian strata over large swathes of Fennoscandia. Eastward, where this peneplain dips below Cambrian and other Lower Paleozoic cover rocks. The exposed parts of this peneplain are extraordinarily flat with relief of less than 20 m. The overlying cover rocks demonstrate that the peneplain was flooded by shallow seas during the Early Paleozoic. Being the oldest identifiable peneplain in its area the Sub-Cambrian peneplain qualifies as a primary peneplain.

<span class="mw-page-title-main">South Swedish highlands</span> Hilly area covering large parts of Sweden

The South Swedish highlands or South Swedish Uplands are a hilly area covering large parts of Götaland in southern Sweden. Except for a lack of deep valleys, the landscape is similar to the Norrland terrain found further north in Sweden. The central-eastern parts of the highlands contain about thirty narrow canyons locally known as skurus.

<span class="mw-page-title-main">Planation surface</span> Large-scale surface that is almost flat

In geology and geomorphology a planation surface is a large-scale surface that is almost flat with the possible exception of some residual hills. The processes that form planation surfaces are labelled collectively planation and are exogenic. Planation surfaces are planated regardless of bedrock structures. On Earth, they constitute some of the most common landscapes. Geological maps indicate that planation surfaces may comprise 65% of the landscapes on Saturn's largest moon, Titan, which hosts a hydrological cycle of liquid methane. Peneplains and pediplains are types of planation surfaces planated respectively by "peneplanation" and "pediplanation". In addition to these there are planation surfaces proposed to be formed by cryoplanation, marine processes, areal glacial erosion and salt weathering. The term planation surface is often preferred over others because some more specific planation surface types and processes remain controversial. Etchplains are weathered planation surfaces.

<span class="mw-page-title-main">South Småland peneplain</span>

The South Småland peneplain is a large flattish erosion surface, a peneplain, formed during the Tertiary, covering large swathes of southern Småland and nearby areas in Southern Sweden. To the east the South Småland peneplain bounds with the Sub-Cambrian peneplain uphill across an escarpment. While is almost as flat as the Sub-Cambrian peneplain the South Småland peneplain differs in that it contains far more residual hills and that it has never been covered by sedimentary rocks. To the south and west the peneplain transitions into Mesozoic-aged hilly surfaces.

<span class="mw-page-title-main">Muddus plains</span> Landscape in northern Sweden

The Muddus plains is a landscape type in northern Sweden characterized by its flat topography dotted with inselbergs. The topographic character of the Muddus plains was first described in detail by Walter Wråk in 1908. The Muddus plains are part of the Norrland terrain. The northern lower levels of Southern Norway's Paleic surface are considered to be equivalent to the Muddus plains.

<span class="mw-page-title-main">Paleic surface</span> Erosion surface in Southern Norway

The paleic surface or palaeic surface is an erosion surface of gentle slopes that exist in South Norway. Parts of it are a continuation of the Sub-Cambrian peneplain and Muddus Plains found further east or equivalent to the strandflat coastal plains of Norway. Hardangervidda, a particularly flat and elevated part of the Paleic surface formed in the Miocene at sea level.

Hügelland is a type of landscape consisting of low rolling hills whose topography or surface structure lies between that of a lowland region and that of a more rugged hill range or low mountain range. The term is German and has no exact equivalent in English, but is often translated as "hill country", "hilly terrain", "upland(s)" or "gently undulating" or "rolling country", or "rolling countryside". It is derived from Hügel, a low hill or hillock and appears frequently as a proper name for this type of terrain.

In geology and geomorphology a paleosurface is a surface made by erosion of considerable antiquity. Paleosurfaces might be flat or uneven in some cases having considerable relief. Flat and large paleosurfaces —that is planation surfaces— have higher potential to be preserved than small and irregular surfaces and are thus the most studied kind of paleosurfaces. Irregular paleosurfaces, albeit usually smaller than flat ones, occur across the globe, one example being the Sudetes etchsurfaces. In the case of peneplains it is argued that they become paleosurfaces once they are detached from the base level they grade to.

An etchplain is a plain where the bedrock has been subject to considerable "etching" or subsurface weathering. Etchplanation is the process forming etchplains. Contrary to what the name might suggest, etchplains are seldom completely flat and usually display some relief, as weathering of the bedrock does not advance uniformly. This means that weathering is unrelated to the flatness which might be derivative of various other processes of planation including peneplanation and pediplanation. Erosion of etchplains can result in the exposure of inselbergs such as bornhardt and tors. Generally the topography exposed at a stripped etchplain, that is an etch surface, after erosion of regolith is one with many irregularities as result of structurally defined areas of rock strength.

The glacial buzzsaw is a hypothesis claiming erosion by warm-based glaciers is key to limit the height of mountains above certain threshold altitude. To this the hypothesis adds that great mountain massifs are leveled towards the equilibrium line altitude (ELA), which would act as a “climatic base level”. Starting from the hypothesis it has been predicted that local climate restricts the maximum height that mountain massifs can attain by effect of uplifting tectonic forces. It follows that as local climate is cooler at higher latitudes the highest mountains are lower there compared to the tropics where glaciation is and has been more limited. The mechanism behind the glacial buzzsaw effect would be the erosion of small glaciers that are mostly unable to erode much below the equilibrium line altitude since they do not reach these altitudes because of increased ablation. Instead, large valley glaciers may easily surpass the equilibrium line altitude and do therefore not contribute to a glacial buzzsaw effect. This is said to be the case of the Patagonian ice fields where lack of buzzsaw effect results in rapid tectonic uplift rates.

<span class="mw-page-title-main">Sub-Mesozoic hilly peneplains</span> Landscape in Scandinavia of undulating hills and joint valleys

The Sub-Mesozoic hilly peneplains or Sub-Mesozoic hilly relief is a landscape in Scandinavia made up of undulating hills and joint valleys and occasional kaolinized bedrock in valley bottoms. The landscape formed in the Mesozoic Era and was eventually drowned by the sea during the Campanian transgression and covered by a thick cover of Cretaceous sedimentary rocks. Later erosion of the cover rocks partly re-exposed this landscape. During the Quaternary epoch the re-exposed Mesozoic hilly relief escaped major glacier erosion being only surficially scoured in parts.

A piedmonttreppen or piedmont benchland is a conceived landform consisting in a succession of benches at different heights and that forms in sequence during the uplift of a geological dome. The concept was first proposed in a posthumous publication by Walther Penck in 1924.

References

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  2. ↑ Chorley, R.J. (1973). The History and Study of Landforms or The Development of Geomorphology. Vol. Two: The Life and Work of William Morris Davis, Methuen.
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  10. ↑ Miller, A.A. (1955). "The origin of the South Ireland Peneplane". Irish Geography. 3 (2): 79–86. doi:10.1080/00750775509555491.
  11. ↑ Twidale, C.R. (1985). "Old landsurfaces and their implications for models of landscape evolution". Revue de Géomorphologie Dynamique. 34: 131–147.
  12. ↑ Lidmar-Bergström (1988). "Denudation surfaces of a shield area in southern Sweden". Geografiska Annaler . 70 A (4): 337–350. doi:10.1080/04353676.1988.11880265.
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  14. ↑ Japsen, Peter; Green, Paul F.; Chalmers, James A.; Bonow, Johan M. (17 May 2018). "Mountains of southernmost Norway: uplifted Miocene peneplains and re-exposed Mesozoic surfaces". Journal of the Geological Society. 175 (5): 721–741. Bibcode:2018JGSoc.175..721J. doi:10.1144/jgs2017-157. S2CID   134575021.
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  18. 1 2 Lidmar-Bergström, Karna (1997). "A long-term perspective on glacial erosion". Earth Surface Processes and Landforms . 22 (3): 297–306. Bibcode:1997ESPL...22..297L. doi:10.1002/(SICI)1096-9837(199703)22:3<297::AID-ESP758>3.0.CO;2-R.
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