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Strandflat

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View of the strandflat at Helgeland from the mountain Dønnesfjellet in Dønna. A number of rauks can be seen, from left: Træna, Lovunda, Selvær, Nesøya, Hestmona, Rødøyløva and Lurøyfjellet, all landmarks on the Norwegian Coast.

Strandflat (Norwegian: strandflate[1]) is a landform typical of the Norwegian coast consisting of a flattish erosion surface on the coast and near-coast seabed. In Norway, strandflats provide room for settlements and agriculture and thus constitute important cultural landscapes.[1] Outside Norway, proper strandflats can 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 on the landward side by a sharp break in slope leading to mountainous terrain or high plateaux. On the seaward side, strandflats end at submarine slopes.[3][4] The bedrock surface of strandflats is uneven and tilts gently towards the sea.[4]

Norwegian strandflat

Schematic profile of a strandflat in Norway. In the right lies a higher terrain, to the left of this lies a steep slope leading to the strandflat. The strandflat is both flat and undulating and contains to the left a skerry zone. The skerries are separated from the submarine flat surfaces of the bankflat by an underwater slope. The large island in the middle hosts a rauk.

Characteristics

Strandflats are not fully flat and may display some local relief, meaning that it 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.

The width of the strandflat varies 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 into the following zones: the supramarine zone, the skjærgård (skerry archipelago), and the submarine zone. Residual mountains sorrounded by the strandflat are called rauks.[5]

On the landward side, the strandflat often terminates abruptly with the beginning of a steep slope that separates it from higher or more uneven terrain. On the seaward side, the strandflat continues underwater down to depths of 30 to 60 meters, where a steep submarine slope separates it 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 predate 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

The concept of a strandflat was introduced in 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 strandflats.[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 including many more. Thus most modern explanations are of polygenetic type.[8]

In his original description, Reusch regarded the strandflat as originating from marine abrasion prior to glaciation,[7] but adding that some leveling could have been caused by 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] In the mid-20th century, W. Evers argued for in a series of publications that the strandflat was a low erosion surface formed on land as part of a stepped sequence (piedmonttreppen) that included the Paleic surfaces. This idea was refuted by Olaf Holtedahl who wrote that the position of the surfaces were not that of a piedmonttreppen.[10]

Frost weathering, glaciers and sea ice

The Arctic explorer Fritjof Nansen agreed with Reusch that marine influences formed the strandflat, but added in 1922 that frost weathering was also of key importance.[7][9] Nansen discarded ordinary marine abrasion as an explanation for the formation of the strandflat, as he noted that much of the strandflat lay in areas protected from major waves.[5] In his analysis, Nansen argued that the strandflat formed after the fjords of Norway had 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 material loosened by frost weathering, 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]

Deep weathering

Contrary to the glacial and periglacial hypotheses, Julius Büdel and Jean-Pierre Peulvast regard weathering of rock into saprolite as important in shaping the strandflat. Büdel held that weathering took place in a distant past with tropical and sub-tropical climates, while Peulvast considered that present-day conditions and a lack of glaciation were enough to produce the weathering. As such, Peulvast considered 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 a surface shaped by weathering dotted with inselbergs.[7]

In 2013, Odleiv and co-workers put forward a mixed origin for the strandflat of Nordland. They argue that this strandflat in northern Norway could represent the remnants of a weathered peneplain of Triassic age[B] that was buried in sediment for long time before made flat again by erosion in Pliocene and Pleistocene times.[7]

Strandflats outside Norway

Strandflats have been identified in high-latitude areassuch as the coast of Svalbard, Greenland, the South Shetland Islands, Alaska and western Scotland.[1][2] In Scotland's Hebrides raised shore platforms are identified as strandflats formed possibly in Pliocene times and later modified by the Quaternary glaciations.[13] Raised strandflats have also been identified in Robert Island in the South Shetland Islands indicating a relative change in sea level.[14]

Footnotes

  1. ^ Cirques in southern Norway can be found both near sea level and at 2,000 m.[11]
  2. ^ Remnants of a peneplain formed in Triassic times do also exist in southwestern Sweden.[12]

References

  1. ^ a b c d e f Bryhni, Inge. "strandflate". In Helle, Knut (ed.). Store norske leksikon (in Norwegian). Oslo: Kunnskapsforlaget.
  2. ^ a b c d Dawson, Alasdair D. (2004). "Strandflat". In Goudie, A.S. (ed.). Encyclopedia of Geomorphology. pp. 345–347.
  3. ^ a b c Klemsdal, Tormod (2005). "Strandflat". In Schwartz, Maurice L. (ed.). 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 (ed.). 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 (eds.). Palaeoweathering, Palaeosurfaces and Related Continental Deposits. Special publication of the International Association of Sedimentologists. Vol. 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. doi:10.1080/00291958208552078.
  9. ^ a b c "Nansen og den norske strandflaten". ngu.no (in Norwegian). Norwegian Geological Survey. October 25, 2011. Retrieved September 6, 2017.
  10. ^ Holtedahl, Olaf (1965). "The South-Norwegian Piedmonttreppe of W. Evers". Norsk Geografisk Tidsskrift. 20 (3–4): 74–84. doi:10.1080/00291956508551831.
  11. ^ 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. doi:10.1130/g34806.1.
  12. ^ Lidmar-Bergström, Karna (1993). "Denudation surfaces and tectonics in the southernmost part of the Baltic Shield". Precambrian Research. 64: 337–345. doi:10.1016/0301-9268(93)90086-h.
  13. ^ Dawson, Alastrair G.; Dawson, Sue; Cooper, J. Andew G.; Gemmell, Alastair; Bates, Richard (2013). "A Pliocene age and origin for the strandflat of the Western Isles of Scotland: a speculative hypothesis". Geological Magazine. 150 (2): 360–366. {{cite journal}}: |access-date= requires |url= (help)
  14. ^ Serrano, Enrique; López-Martínez, Jerónimo (1997). "Geomorfología de la península Coppermine, isla Robert, islas Shetland del Sur, Antártica" (PDF). Serie Científica (in Spanish). 47. INACh: 19–29.

Literature

  • Holtedahl, Hans (1959). "Den norske strandflate. Med særlig henblikk på dens utvikling i kystområdene på Møre". Norwegian Journal of Geography. 16: 285–385.
  • Nansen, Fridtjof: 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.
  • Reusch, Hans: Strandflaten, et nyt træk i Norges geografi. Norges geologiske undersokelse, 14, 1–14. 1894.

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