Os (landscape)

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An Os or Wallberg in an idealized representation
Os in the Swedish national park Fulufjället ( aerial view )

An Os ( Ås , Äsar , Esker ) or Wallberg is a wall-like, often curved terrain elevation made of sand and gravel , which was formed in the Quaternary Ice Age ("Ice Age"). The typical, narrow, elongated shape of the Oser was formed by glaciers and the meltwater that flows from them . This embankment-shaped land form made of meltwater sediments , the length of which usually extends to a few dozen, rarely several hundred kilometers, is an element of the ground moraine landscape .

Etymologies and type locality

Ireland

The word Esker is derived from Irish eiscir or from Old Irish escir which means ridge between two lower levels . The name Esker example, carries also in eastern County Galway location Town Country Ahascragh , formerly Áth Eascrach meaning ford at Esker . As eponymous type locality can Eiscir Riadha be considered, which across the whole of Ireland from Dublin to Galway extends thereby km traverses a distance of 200th

Scandinavia

The ideal typical Ås from St. Olavsormen near Egersund in Norway

In the Scandinavian languages Danish , Norwegian and Swedish , Ås (pronunciation [os] , plural form åser or åsar , derived from Old Norse áss ) denotes an elongated ridge or elevation whose core consists of deposits such as B. consists of sand. Ås is also part of many place names in the Nordic region. B. Jyske Ås in Jutland and Hallandsåsen between Skåne and Halland , even if the latter is geologically referred to as Horst and not Os.

The term Os (with o) in Scandinavian refers to otherwise warm haze or smoke. or in Norwegian (pronunciation [us] ) also an estuary. The term “-os” is also part of the name of numerous places at river mouths, for example in “ Namsos ”, although there are also names on -ås such as in Västerås .

Definition and shape description

Drewry (1986) defined Oser / Esker as follows:

Åser / Eskers are winding, narrow, relatively steep ridges. Their irregularly structured sediment layers were deposited in direct contact with glacier ice either in an exposed watercourse or in a closed tunnel.

Åser, railway embankments rise similarly in the landscape, as they were surrounded on both sides by ice masses. Their elevations are usually flattened, but they can also taper to a point. The embankments, which sometimes meander like a snake, can also have branches. Even complex, dendritic, vein-like networks are encountered, such as at Monroe in Maine .

The usual dimensions of Åsern are several kilometers in length with an average height of 40 m to 50 m (rarely up to 80 meters) and widths less than 150 m (small Oser are in the hundred meter range). Their sides usually dip at 10 ° to 20 °, but they can also be steeper. In northern Germany they rarely reach a height of more than 20 m, but can be several dozen kilometers long. Extremely long Åser are the 800 km long Thelon-Esker in Canada , the 250 km long Uppsalaåsen in Sweden and the Irish Eiscir Riadha with 200 km length.

Åser are generally the deposit product of highly organized meltwater systems in the inland ice / glacier and are often the result of abundant meltwater supply. They usually appear in parallel flocks in front of the defrosting front and their spatial arrangement underlines the direction of flow of the ice masses. They often establish themselves in the neighborhood or even within glacial channels ( tunnel valleys , also called Nye channels or just N channels ), which were created subglacial due to the abrasive effect of the meltwater in the loose and also in the solid subsoil.

Delimitation criterion

Åser differ from lateral moraines in their composition: glaciers transport unsorted sediments of various grain sizes. In flowing water, on the other hand, it depends on its flow speed whether and where sediments of which size are deposited. Therefore, only in Åsern is there a clear sorting and stratification of the deposits.

Influence of the topography

Wallberg at Sims Corner Eskers and Kames , Washington state

Due to the water flowing in from the surface of the glacier, the water under the ice is often under high hydrostatic pressure and can accordingly also flow upwards. Frequently changing pressure conditions explain the strongly changing height of Osern as well as the interruptions in the Oszügen, which are more the rule than the exception. For example, in sloping meltwater tunnels, the viscosity of the water generates heat that melts the tunnel walls. As a result of the increase in height of the tunnel that is achieved, more sediment can consequently be deposited, and the resulting esker deposits increase in height. Conversely, less heat is generated in ascending sections, so that the esker is lower here. But not only the overall height is influenced, but also the shape. In sloping or straight tunnels, tapering eskers are created, whereas in ascending sections water freezes at the upper edge of the tunnel and thus only lower, rounded or flattened esker can be formed.

Emergence

Profile representation of the emergence of sub- and proglacial land forms

Åser were formed in meltwater tunnels or crevasses by glacifluvial melt rivers, which either carry their debris

  • under (subglacial),
  • im (englacial) or
  • on (supraglacial)

tempered glaciers accumulated.

The subglacial origin is represented by authors such as Bärtling (1905) and Shreve (1985), who assume tunnel systems on the underside of the ice as the place of origin. Philipp (1912) defends the englacial point of view, who assumes that the tunnel sediments slowly sink to the ground when the ice melts. Holst (1876) and Liedtke (1975) consider a supraglacial origin to be more likely if the Oser run over ridges and valleys in the glacier bed without varying their thickness. (Note: the development milieus are not mutually exclusive, but rather should be combined with one another.)

Like every glacier, inland ice also contains moraine material ( grain size : fine clay to coarse blocks in the meter range). The melting streams on the ice, which after a more or less short run, find their way to the glacier base (water has a higher density than ice) absorb the moraine material and deposit it again along their course. Hence, oers are glacifluvial forms. They mainly consist of unlayered gravel as well as coarse sand and occasionally blocks. Since the meltwater flows parallel to the direction of the ice movement, Oszuges in northeast Germany mostly run in a northerly or northeastern direction.

The formation or maintenance of the Oser is associated with the standstill or retreat of the inland ice during late glacial phases (after the last glacial maximum ). After the final melting, they tower above the ground moraine landscape as positive relief forms. Oser are preferably found in valley areas of former glaciers or glacier tongues in the immediate vicinity of the former ice edge layers. Oser are only preserved under special conditions; they usually fall prey to later erosion. Therefore Oser are mainly from the Weichselian , from the previous Saalian almost no examples are unknown, but a likely saale temporal age should the os of Tellingstedt own. Examples of recent Esker can be found at Woodworth Glacier in Alaska , on the Greenland Ice Sheet in Frederikshaab and on Brúarjökull in Iceland .

Creation models

Banerjee and McDonald (1975) considered the following three origins for Oser:

  • Model of the free flowing watercourse
  • Tunnel model
  • Delta model

In the “free-flowing watercourse model”, sedimentation takes place in a pigtail stream that is surrounded on the sides by walls of ice. The facies distribution found indicates a lateral decrease in grain size. Anti-dunes with countercurrent stratification are very common. When viewed in cross section, the sediment units are lenticularly arranged, but flat in longitudinal section.

In the “tunnel model”, the masses of water flow constrained in a subglacial tunnel. The sediment units are generally flat and sustained in the longitudinal section. The most common are sloping and normal stratified sediment packages made of gravel and sand. Fine-grained sediments are missing. The paleo flow directions show only minor deviations. The degree of sorting of the sediments is poor (only the matrix is ​​better sorted) and indicates a sliding bed stage (sliding surface) - a characteristic feature of transport and deposition in an ice tunnel.

In the "Delta model", sedimentation takes place when entering the upstream meltwater lakes. Rapid, downstream facies changes are characteristic of these sediments. Proximal gravels merge into deeper seabed rhythms of silt and clay or interlock with them. Landslides , mud and suspension flows are also characteristic . Paleo currents also show a wide range.

However, if the edge of the ice is exposed, the sediments are deposited as alluvial fans .

Internal structure

Internal structure of the Os of Badelunda in Sweden

Glacifluvial sediments differ only slightly from normal river sediments. In Osern, depending on the physical properties of the meltwater flow, layers of rubble and sand are found in different thicknesses. With a very strong flow regime, very large pebbles can be transported, but with a low flow regime and stagnant conditions, only fine-grained sediments are formed. Inclined stratification and normal stratification can exist side by side.

Although the sediment bodies of Osern appear relatively uniform from the outside, their internal structure can reveal very complex relationships. In addition to the usual gravel and sand layers, there are often tills and basin sediments . Occasionally they are tectonically disturbed in their association , recognizable by faults and folds . This can be explained by compressions that were caused by differential movements of the surrounding ice masses (active, glaci-tectonic deformations). But also passively disturbances in the storage conditions (subsidence) occur through thawing of the surrounding, stabilizing ice or through the melting of dead ice masses.

Sediment composition and facies

Oyster sediments are characterized by sediment structures that were created in the flow environment. They can be subdivided based on their grain sizes as follows:

  • Gravel
  • Sands
  • Fine sands and silts
  • Silte and Tone

The most common are “gravel layers” in stable packs , and now and then also in open packs. Inclined layers are to be found in them quite often, which can be up to seven meters thick. They are created by the downstream migration of gravel banks . Occasionally, countercurrent stratification is observed. Some sections also show dominant, graded parallel stratification . However, the gravels can also be completely non-stratified and were evidently precipitated from a sediment cloud. Even mud flows occur, recognizable by clay pebbles.

The coarse to medium-grained “sand” is mostly stratified at an angle and was deposited in mega-ripples or in wandering, 15 to 30 centimeter thick sandbank packages. The sedimentary structures are combined with one another in a very complicated way. For example, sloping sandbanks, mega-ripples and small ripples on top of each other can be observed. There are also massive layers with indistinct stratification, which indicate a high suspension load. Occasional parallel stratification indicates the Engl. Flat surface form phase of the low flow regime known as the plane bed phase .

The “fine sands and silts” mostly consist of small ripples and various types of climbing ripple lamination . The latter can document high flow velocities due to its steep increase (supercritical behavior, Fr> 1). Occasionally these sediments also show signs of simultaneous deformation such as B. landslides and twisted stratification.

The “silts and clays” come from the distal facies of Åsern and were sedimented in the ice masses of the offshore defrosting lakes. These deposits consist of well-developed rhythms and varves .

Facially, Oser belong to the following deposit areas (Facies classification according to Brodzikowski and van Loon):

  • subglacial tunnel facies (IC-1-a).
  • Terminoglacial, fluvial tunnel mouth facies (II-A-2-a).
  • terminoglacial, lacustrine tunnel mouth facies (II-A-1-c)
  • Terminoglacial, marine tunnel mouth facies (II-D-2-b).

Åser associations

This Ås runs along the edge of a glacial channel from Gatschow in the north to Zettemin in the south and has a length of approx. 30 km

In the history of their origins, Åser are often associated with drumlins or drumlin-like shapes that look similar to them at first glance, but have a different history. They must therefore not be confused with one another. Drumlins were created during the active flow of the glacier under the ice and therefore show a streamlined shape. Other socializations of Åsern are adjacent press-on structures from Till, ice edge layers (terminal moraines), tunnel valleys, overflow channels as well as kame formations or dead ice areas. The transition from escarpments and crevice fillings to kames and sand surfaces can also be observed quite often .

Special shapes

In addition to the usual Eskern there are also various special forms:

  • Impact Esker .
  • String of pearls esker .
  • Till-covered-esker .
  • Overlay Esker .
  • Parallel Esker .
  • Guttering Esker .

In the case of “Aufpressungs-Eskern”, the pressure front acting from below creates disturbances which can squeeze up the till of the underlying ground moraine, but also cohesive pitches in the subglacial tunnel and consequently enrich the core of the Osers.

The relatively rare "Perlenschnur-Esker" or "Perl-Oser" can be traced back to blocks of ice that remained in the tunnels or crevices and thus caused interruptions in the sedimentation (called Os eyes due to their rounded shape ). However, it is also possible that a rearward extension of the subglacial tunnel system into sediment-rich layers of the glacier was periodically omitted. But maybe they just document changes in the acceleration of the meltwater flow.

"Till-covered Esker" have a coat made of Till, which directly proves their subglacial origin at the base of the ice masses.

"Overlay Esker" are complex structures that arise from intersecting crevice systems in superimposed levels of the glacier.

With the "parallel Eskern" there are intergrown and separately running structural forms.

"Rinnenbildung-Esker" are surrounded either on one or both sides by edge depressions, which are, however, mostly difficult to recognize morphologically and are often muddy. The channels can either be explained by the formation of dead ice or they were created by flow rollers.

Physical parameters

Using sedimentological studies , Jackson (1995) was able to determine the following physical parameters for the Oser from Bridgenorth in Ontario :

The flow regime was turbulent ( Reynolds number Re> 500) and subcritical ( Froude number Fr <1).

Economic use

The trail Hassocky Meadow Trail runs on a Os in the Ipswich River Wildlife Sanctuary (Massachusetts) attached to the wetland of Hassocky Meadow borders

Oars are excellent groundwater reservoirs due to their granulometry (ie grain size composition , with coarse fractions predominating) and are therefore important reservoirs for water management . For the construction industry and road construction , they are an important supplier of gravel and sand and are consequently mined intensively. Oser also often cross lakes and moorlands and are therefore used as natural path and road substructures.

Distribution and occurrence

Åser are only found in once glaciated regions, for which they are characteristic of retreat. In North America they no longer occur north of 72 ° North. This fact suggests that oser are only formed by tempered ice masses with a water film at the base. Due to the enormous dimensions of the Würm Ice Age ( Fennoskand Ice Sheet , Laurentide Ice Sheet ), Oser are very widespread. However, Oser did not arise under rugged mountain glaciers as in the Alps , as no closed tunnel system with high hydrostatic overpressure at the tunnel exit could be established in these.

So-called "Paläoeskers" are Eskers from pre-Pleistocene glaciations. Eskers from Mauritania , who come from the Upper Ordovician and belong to the Tichit group , are an example .

Even extraterrestrial Eskers have become known, for example the space probe Mars Odyssey has recorded several Esker systems on the planet Mars using THEMIS .

Examples

Typical view in the Finnish Lake District: An Os between two lakes

A few examples are given for clarification:

Ås in the Einunndalsranden nature reserve in Hedmark
Moraine ridge of Žagarė in Lithuania
Southern edge of the Argyre Planitia with the Charitum Montes . Several eskers cross the crater plane to the left of the crater rim.

literature

  • Frank Ahnert: Introduction to Geomorphology. (= Uni-Taschenbücher. 8103). 4th, updated and supplemented edition. Eugen Ulmer, Stuttgart 2009, ISBN 978-3-8252-8103-8 :
    • Cape. 24.5: Material, processes and forms of glacial deposition
    • Cape. 24.6: Glaciofluvial processes, deposits and forms
  • I. Banerjee, BC McDonald: Nature of esker sedimentation . In: V. Jopling, BC McDonald (ed.): Glaciofluvial and Glaciolacustrine Sedimentation (=  Soc. Econ. Paleont. Mineral. Spec. Publ. ). tape 23 , 1975, p. 132-154 .
  • H.-E. Reineck, IB Singh: Depositional Sedimentary Environments . Springer-Verlag, Berlin / Heidelberg / New York 1980, ISBN 0-387-10189-6 .

Individual evidence

  1. ↑ Ås. Det danske ordbog, accessed June 17, 2020 .
  2. ↑ Ås. Det Norske Akademis ordbok, accessed June 17, 2020 .
  3. ↑ Ås. Svensk ordbok (SO), accessed June 17, 2020 .
  4. ↑ Os. Det danske ordbog, accessed June 17, 2020 .
  5. ↑ Os. Det Norske Akademis ordbok, accessed June 17, 2020 .
  6. ↑ Os. Svensk ordbok (SO), accessed June 17, 2020 .
  7. ↑ Os. Det Norske Akademis ordbok, accessed June 17, 2020 .
  8. ↑ Langenscheidt's Universal Dictionary Norwegian
  9. ^ Ljunggren, Karl Gustav Till utvecklingen av os, öse i ortnamn , i Namn och Bygd årgång 24 (1936) s. 129 f
  10. ^ Drewry, D .: Glacial Geologic Processes . Edward Arnold, London 1986, p. 276 .
  11. ^ Easterbrook, DJ: Surface Processes and Landforms . Prentice Hall, New Jersey 1999, ISBN 0-13-860958-6 , pp. 352 .
  12. ^ A b Benn DI & Evans DJA: Glaciers and Glaciation . Arnold, London 1998, p. 734 .
  13. ↑ Reineck, H.-E. and Singh, IB: Depositional Sedimentary Environments . Springer-Verlag, Berlin, Heidelberg, New York 1980, ISBN 0-387-10189-6 .
  14. ↑ a b Shreve, RL: Esker characteristics in terms of glacier physics, Katahdin esker system, Maine . In: Geol. Soc. Amer. Bull. Band 96 , 1985, pp. 639-646 .
  15. ^ Bärtling, R .: The Ås at Neuenkirchener See on the Mecklenburg-Lauenburg border . In: Jahrb. Preuss. Geol. Landesanst. tape 26 , 1905, pp. 15-25 .
  16. ↑ Philipp, H .: About a recent alpine Os and its importance for the formation of diluvial Osar . In: Z. Deut. Geol. Ges. Band 64 , 1912, pp. 68-102 .
  17. ↑ Holst, NO: Om de glaciala rullstensa sarne . In: geol. Forums. Stockholm Forh. tape 3 , 1876, p. 97-112 .
  18. ^ Liedtke, H .: The Nordic glaciations in Central Europe . In: Research on German regional studies . tape 204 . Federal Research Institute for Regional Studies and Regional Planning, Bonn-Bad Godesberg 1975, p. 160 .
  19. ^ Scheidegger, AE: Theoretical Geomorphology, 2nd edition . Springer, Berlin, Heidelberg, New York 1970.
  20. ↑ a b Grube, A .: Geotopes in Schleswig-Holstein . In: State Office for Agriculture, Environment and Rural Areas Schleswig-Holstein (Ed.): Documentation of the geotopes of the state cadastre Schleswig-Holstein . 2011.
  21. ↑ a b Banerjee, I. and McDonald, BC: Nature of esker sedimentation . In: Jopling, V. and McDonald, BC: Glaciofluvial and Glaciolacustrine Sedimentation (eds.): Soc. Econ. Paleont. Mineral. Spec. Publ. Volume 23 , 1975, p. 132-154 .
  22. ↑ Saunderson, HC: The sliding bed facies in esker sands and gravels: a condition for fullpipe (tunnel) flow? In: Sedimentology . tape 24 , 1977, pp. 623-638 .
  23. ↑ Brodzikowski, K. and van Loon, AJ: A Systematic Classification of Glacial and Periglacial Environments, Facies and Deposits . In: Earth Science Reviews . tape 24 , 1987, pp. 297-381 .
  24. ↑ Alf Grube: On the structure of Eskern in Schleswig-Holstein, with special consideration of the "Esker-Kames System Forst Steinburg" in a morphological high position . In: E&G Quaternary Science Journal . tape 60 , no. 4 , 2011, p. 425-433 , doi : 10.3285 / eg.60.4.03 .
  25. ^ Schulz, W .: about Oser and similar formations in western Prignitz . In: Jb. Geol. Band 3 , 1970, p. 411-420 .
  26. ↑ Karl Gripp: The emergence of rubble Osern (Esker) . In: Ice Age and the Present . tape 28 , 1978, p. 92-108 .
  27. ^ R. Aario: Glacial and glaciofluvial sedimentation in Finnish valley environments . The river valley as a focus of interdisciplinary research. Finland 1977 (Conference June 21-23, 1977).
  28. ↑ Jackson, GR: Flow Velocity Estimation of Meltwater Streams in Subglacial Conduits: A Palæohydraulic Analysis of the Bridgenorth Esker, Peterborough County, Ontario . Department of Geography, Trent University, Peterborough, Ontario 1995.
  29. ↑ Mangold, N .: Giant paleo-eskers of Mauritania: analogs for martian esker-like landforms . Orsay-Terre 2000 (Equipe Planétologie, UMR 8616, CNRS et Université Paris-Sud).
  30. ↑ Humlum, O .: Sorø- og Stenlilleegnens geomorfologi . Unpublished report, Geographical Institute, University of Copenhagen, 1976, p. 383 .
  31. ↑ Geology on the information pages on the nature reserve "Lange-Damm-Wiesen und Unteres Annatal", accessed on September 30, 2018.
  32. ↑ Kölling, M. and Schlüter, M .: The Ahrensburg-Stellmoorer Tunneltal (northeast part) . In: Meyniana . tape 81 , 1988, pp. 85-95 .
  33. ↑ Wünnemann, B .: The emergence of the Langseerinne (fishing) in Schleswig-Holstein during the Vistula period . In: Dissertation, Geosciences Department, Free University of Berlin . Berlin 1990, p. 171 + appendix .
  34. ^ Strehl, E .: The Oser (Wallberge) in the old district of Eckernförde . In: Yearbook of the home community Eckernförde e. V. Band 64 . Schwansen, Amt Hütten and Dänischwohld 2006, p. 249-262 .
  35. ↑ Seifert, G .: The microscopic grain structure of the till as an image of the ice movement, at the same time the history of ice mining in Fehmarn, East Wagria and the Danish Wohld . In: Meyniana . tape 2 , 1953, p. 124-184 .
  36. ↑ Ohnesorge, W .: The Lübeck Os and his prehistoric antiquities . In: Mitt. Geogr. Ges. Ud Naturhist. Mus. Lübeck . 2 number = 32, 1928, p. 5-123 .
  37. ^ Gray, Charlotte: The Museum Called Canada: 25 Rooms of Wonder . Random House, 2004, ISBN 0-679-31220-X .
  38. ↑ caruba, R. and Dars, R .: Géologie de la Mauritanie . Université de Nice-Sophia Antipolis 1991, ISBN 2-86629-214-6 .
  39. ↑ Banks, ME and a .: An analysis of sinuous ridges in the southern Argyre Planitia, Mars using HiRISE and CTX images and MOLA data . In: Journal of Geophysical Research . 112 E09003, 2005, doi : 10.1029 / 2008JE003244 .
  40. ^ Head, JW and Pratt: Extensive Hesperian-aged south polar ice sheet on Mars: evidence for massive melting and retreat, and lateral flow and ponding of meltwater . In: Journal of Geophysical Research . tape 106 , 2001, pp. 12.275-12.299 .

Web links

Commons : Os  - collection of images, videos and audio files
Wiktionary: Os  - explanations of meanings, word origins, synonyms, translations