Strandflat

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View of the strandflat at Helgeland fra mountain Dønnesfjellet i Dønna. A number of rauks are seen, from left: Træna, Lovunda, Selvær, Nesøya, Hestmona, Rødøyløva og Lurøyfjellet, all of them landmarks on the Norwegian Coast. photo:Mahlum

Strandflat (Norwegian: strandflate[1]) is a landform typical of the Norwegian coast consisting of a flatish erosion surface on the coast and the near coast sea bottom. Within Norway strandflats provide room for settlements and agriculture and constitute thus important cultural landscapes.[1] Besides Norway proper strandflats can also be found in other high-latitude areas like Svalbard, Greenland, the South Shetland Islands, Alaska and western Scotland.[1][2]

The strandflats are usually bounded landward by a sharp break in slope leading to mountainous terrain or high plateux. Seaward strandflats end at submarine slopes.[3][4] The bedrock surface of strandflats is uneven and tilt gently towards the sea.[4]

Norwegian strandflat[edit]

Characteristics[edit]

Strandflats are not fully flat an may display some local relief, meaning that is usually not possible to assign them an unique altitude.[2] The Norwegian strandflats may go from 70-60 m a.s.l. to 40-30 m below sea level.[1] The undulations in the strandflat relief may result in an irregular coastline with skerries, small embayments and peninsulas.[3]

The mountain Trænstaven in Træna commune is a rauk amidst the strandflat of Norway's coast.

As with the height the width of the strandflat may also vary, going from a few kilometers to 50 km and occasionally reaching up to 80 km in width.[1][2] From land to sea the strandflat can be subdivided in the following zones: the supramarine zone, the skjærgård (skerry archipelago) and the submarine zone. Residual mountains sorrounded by the strandflat are called rauker.[5]

Landward the strandflat often terminates abruptly with the beginning of a steep slope that separates it from higher or more uneven terrain. Seaward the strandflat continues underwater down to depths of 30 to 60 meters where a steep submarine separates its from older low relief paleic surfaces. These paleic surfaces are known as bankflat and make up much of the continental shelf.[3] At some locations the landward end the strandflat or the region slightly above contains relict sea caves partly filled with sediments that predates the Last glacial period. These caves lie near the post-glacial marine limit or above it.[4]

Farms at the strandflat of Lofotfjella at Eggum in Vestvågøy commune.

Geological origin[edit]

The word and concept was introduced year 1894 by Norwegian geologist Hans Reusch.[6][7] Despite being the most studied coastal landform in Norway as of 2013 there was no consensus on the origin of the strandflat.[8][7] An analysis of the literature shows that during the course of the 20th century explanations for the strandflat shifted from involving one or two processes to include many more. Thus most modern explanation are of polygenetic type.[8]

In his original description Reusch regarded the strandflat as originating from marine abrasion that occurred prior to glaciation[7] but adding that some leveling could be indebted to non-marine erosion.[8] In his view the formation of the strandflat preceded the fjords of Norway.[9] Years later in 1919 Hans Ahlmann assumed the strandflat formed by erosion on land towards a base level.[7]

Arctic explorer Fritjof Nansen agreed with Reusch in that marine influences formed the strandflat but added in 1922 that frost weathering was also key in shaping the strandflat.[7][9] Ordinary marine abrasion was discarded by Nansen as forming the strandflat as he noted much of the strandflat lied in areas protected from major waves.[5] In his analysis Nansen argued that the strandflat formed after the fjords of Norway dissected the landscape. This he argued facilitated the marine erosion by creating more coast and by creating nearby sediment sinks for eroded material.[9]

Early on in 1929 Olaf Holtedahl favoured a glacial origin for the strandflat, an idea that was picked up by his son Hans Holtedahl. Hans Holtedahl and E. Larsen went on to argue in 1985 for an origin in connection to the Quaternary glaciations with frost weathering looseing material and sea-ice transporting loose material and making the relief flat.[6] Tormod Klemsdal added in 1982 that cirque glaciers could have made minor contributions in "widening, levelling and splitting the strandflat".[8][A]

Contrary to the mentioned glacial and periglacial ideas, weathering of rock into saprolite is regarded important in shaping the strandflat by Julius Büdel and Jean-Pierre Peulvast. These authors differ in that Büdel held that weathering took place in a distant past with tropical and sub-tropical climate while Peulvast consider that present-day conditions and a lack of glaciation were enough to produce the weathering. As such Peulvast consider the saprolite found in the strandflat and the weathering that produced it to predate the Last glacial period and possibly the Quaternary glaciations.[6] For Büdel the strandflat was an etchplain with inselbergs.[7]

A 2013 publication Odleiv and co-workers put forward a mixed origin for the strandflat of Nordland. It is argued that the strandflat in northern Norway could represent the remnants of weathered peneplain of Triassic age that was exhumed and made flat again in Pliocene and Pleistocene times.[7]

Footnotes[edit]

  1. ^ Cirques in southern Norway can be found both near sea level and at 2,000 m.[10]

Literature[edit]

  • Hans Holtedahl 1959: Den norske strandflate. Med særlig henblikk på dens utvikling i kystområdene på Møre. Norwegian Journal of Geography. 16, 285-385.
  • Fridtjof Nansen: 1904: The bathymetrical features of the North polar seas. In Nansen F. (ed.): The Norwegian North Polar Expedition 1893-1896. Scientific results, Vol IV. J. Dybwad, Christiania, 1-232.
  • Hans Reusch: Strandflaten, et nyt træk i Norges geografi. Norges geologiske undersokelse, 14, 1-14. 1894.

References[edit]

  1. ^ a b c d e Bryhni, Inge. "strandflate". In Helle, Knut. Store norske leksikon (in Norwegian). Oslo: Kunnskapsforlaget. 
  2. ^ a b c Dawson, Alasdair D. (2004). "Strandflat". In Goudie, A.S. Encyclopedia of Geomorphology. pp. 345–347. 
  3. ^ a b c Klemsdal, Tormod (2005). "Strandflat". In Schwartz, Maurice L. Encyclopedia of Coastal Science. Encyclopedia of Earth Sciences Series. pp. 914–915. ISBN 978-1-4020-3880-8. 
  4. ^ a b c Corner, Geoffrey (2004). "Scandes Mountains". In Seppälä, Matti. The Physical Geography of Fennoscandia. Oxford University Press. pp. 240–254. ISBN 0-19-924590-8. 
  5. ^ a b Motrøen, Terje (2000). Strandflatens dannelse – kystlandskapet som spiser seg inn i landblokken (PDF) (Report) (in Norwegian). Høgskolen i Hedmark. ISBN 82-7671-104-9. Retrieved September 6, 2017. 
  6. ^ a b c Lidmar-Bergström, K.; Olsson, S.; Roaldset, E. (1999). "Relief features and palaeoweathering remnants in formerly glaciated Scandinavian basement areas". In Thiry, Médard; Simon-Coinçon, Régine. Palaeoweathering, Palaeosurfaces and Related Continental Deposits. Special publication of the International Association of Sedimentologists. 27. Blackwell Science Ltd. pp. 275–301. ISBN 0-632 -05311-9. 
  7. ^ a b c d e f g Olesen, Odleiv; Kierulf, Halfdan Pascal; Brönner, Marco; Dalsegg, Einar; Fredin, Ola; Solbakk, Terje (2013). "Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway". Norwegian Journal of Geology. 93: 189–213. 
  8. ^ a b c d Klemsdal, Tormod (1982). "Coastal classification and the coast of Norway". Norwegian Journal of Geography. 36: 129–152. 
  9. ^ a b c "Nansen og den norske strandflaten". ngu.no (in Norwegian). Norwegian Geological Survey. October 25, 2011. Retrieved September 6, 2017. 
  10. ^ Hall, Adrian M.; Ebert, Karin; Kleman, Johan; Nesje, Atle; Ottesen, Dag (2013). "Selective glacial erosion on the Norwegian passive margin". Geology. 41 (12): 1203–1206.