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In the relatively recent geological past, several great floods are widely suspected to have occurred, with varying amounts of supporting evidence, usually as a result of the last Ice Age ending.

Great flood

At the most recent glacial maximum, so much of the planet's water was locked up in the vast ice-sheets kilometres thick, that the sea level dropped by about 120 to 130 metres. As the sheets melted starting around 18,000 years ago sea levels rose. Most of the glacial melt had occurred by around 8,000 years ago, but the changes have not been as regular as a constant drip at the edges of the world's glaciers might suggest.

Sea levels have changed significantly since Late Paleolithic time, and shorelines have migrated. The sea has not always steadily encroached upon the land, for the immense weight of the ice-sheets depressed the continental plates under them and caused isostatic rebound around their edges, which are still adjusting today. Averaged rates of sea-level-rise are misleading. Also, parts of Scandinavia are rising isostatically for this reason, by up to 1 cm a year in some places; it rises as fast as mantle rock can flow in under it, and that mantle rock must come from somewhere around, including from under the Netherlandsmarker, which are slowly sinking as a result.

These floods happened in various ways, which can be categorised into 5 types:-
  1. Very flat land being steadily flooded over a long time as the sea rises, sometimes fast enough to be easily noticed in a human's lifetime.
  2. The same, plus the occasional stormflood submerging land and washing the loose soil and subsoil away, leaving the flooded land too deep to be reclaimed. This is more noticeable if the people try to defend their land with dike, for example in the Netherlandsmarker and in the Solentmarker.
  3. The rising sea overflowing a natural sill and entering an enclosed basin. The sill may then erode away catastrophically, like a dike in the Netherlandsmarker. The ocean could fill vast basins in matters of weeks or months, in catastrophes that are unimaginable in today's world. Some people argue that these events may have sparked the flood myths found in many cultures.
  4. Big glacier-dammed lakes bursting as their ice dams melt until they suddenly collapse.
  5. Other causes, for example megatsunamis.


Megafloods

Several examples where such rapid encroachment of the sea occurred are provoking geologists' and archaeologists' investigations.

The Black Sea (around 7,600 years ago)

A rising sea (type 3) flood, recently disclosed and much-discussed refilling of the freshwater glacial Black Seamarker with water from the Aegeanmarker, was described as "a violent rush of salt water into a depressed fresh-water lake in a single catastrophe that has been the inspiration for the flood mythology" (Ryan and Pitman, 1998) . The marine incursion, which was caused by the rising level of the Mediterranean, occurred around 7,600 years ago. It remains an active subject of debate among geologists, with subsequent evidence discovered to both support and discredit the existence of the flood, while the theory that it formed the basis for later flood myths is subjective and unprovable.

The Caspian and Black Seas (around 16,000 years ago)

A type 1 alternative theory proposed by Andrey Tchepalyga of the Russian Academy of Sciencesmarker dates the flooding of the Black Sea basin to an earlier time and from a different cause. According to Tchepalyga, global warming beginning from about 16,000 BP caused the melting of the Scandinavia Ice Sheet, resulting in massive river discharge that flowed into the Caspian Seamarker, raising it to as much as 50m above normal present-day levels. The rise was extremely rapid and the Caspian basin could not contain all the floodwater, which flowed through the Kuma-Manych Depression and Kerch Straitmarker into the ancient Black Sea basin. By the end of the Pleistocene this would have raised the level of the Black Sea by some 60–70m to about 20m below its present-day level, and flooding large areas that were formerly available for settlement or hunting. Tchepalyga suggests this may have formed the basis for legends of the great Deluge.

The lower Tigris-Euphrates Valley, reflooding the Persian Gulf (12,000 years ago)

Another type 1 theory. When sea levels were low, the combined Tigrismarker-Euphrates river flowed through a wide flat marshy landscape. The Persian Gulfmarker today has an average depth of only 35 m. During the most recent glaciation, which ended 12,000 years ago, worldwide sea levels dropped 120 to 130 m, leaving the bed of the Persian Gulf well above sea level during the glacial maximum. It had to have been a swampy freshwater floodplain, where water was retained in all the hollows. High in the Taurus Mountainsmarker glaciation would have been extensive.

The drainage of the combined glacial era Tigrismarker-Euphrates made its way down the marshes of this proto-Shatt-al-Arabmarker to the Strait of Hormuzmarker into the Arabian Seamarker. Reports of the exploration ship "Meteor" have confirmed that the Persian Gulf was an entirely dry basin about 15,000 BC. Close to the steeper Iranian side a deep channel apparently marks the course of the ancient extended Shatt al-Arab, being called the "Ur-Schatt". A continuous shallow shelf across the top (north) of the Persian Gulf and down the west side (at 20 m) suggests that this section was the last to be inundated. At the Straits of Hormuz the bathymetric profile indicates a division into two main channels which continue across the Bieban Shelf before dropping to a depth of c 400 m in the Gulf of Omanmarker; the deeper parts of these channels may be due to delta deposits at the edge of the deep ocean collapsing in a succession of big underwater landslides, causing underwater erosion by the resulting turbidity currents.

There is a theory that there was also a Black-Sea-type sill collapse at the Strait of Hormuzmarker at the outlet of the Persian Gulfmarker, so converting this case into type 3.

In a 1981 Journal of Cuneiform Studies article, "The Tangible Evidence for the Earliest Dilmun", Theresa Howard-Carter espoused her theory identifying Dilmun with Qurna, an island at the Strait of Hormuz. Her scenario put the original mouths of the Tigris-Euphrates rivers, which she thought should be the site of the primeval Dilmun, at or even beyond the Straits of Hormuz. Mainstream archaeologists have avoided mentioning her article for fear of its apparent catastrophism, an awkward subject in geology. Theresa Howard-Carter also wrote: "It is more likely that the original Persian Gulf inhabitants lived along the banks of the lower or extended Shatt al-Arab, ranging some 800 km across the dry Persian Gulf bed. We can thus postulate that the pre-Sumerian cultures had more than ample time to be born and flourish in a riverine setting, encouraged by the agricultural potential and the blessings of a temperate climate. The fact that the body of proof for the existence of these societies must now lie at the bottom of the Persian Gulf furnishes at least a temporary excuse for the archaeologist's failure to produce evidence for their material culture."

In our time, mangrove edge habitat and coral reefs encrustation of fossil dunes characterize the Persian Gulf. Mangroves recolonize easily from established mangrove fringe colonies elsewhere in the Arabian Sea. Artificial reefs are being established today along the coast of Iran. The present-day natural reef developments in the Persian Gulf, corals grow on hardground substrates but have not yet formed the massive calcium carbonate structures familiar from, say, Australia's Great Barrier Reefmarker.

The article Dive conditions described by Eric Bjornstrom found in 1999 in Dubaimarker coral-encrusted sand barrier islands situated 32 km off the coast of the Saudimarker city of Jubailmarker. There lies a chain of five coral cays, barely above the tide. They appear to be formations called diapirs in which a mobile core containing minerals of low density such as salt, deforms under pressure. The core pushes upwards, deforming overlying rock to form a dome. An ancient diapir at Enorama formed an island in shallow seas, buoyed up by salt. There are similar examples today in the Persian Gulf.

In addition to this large scale flooding of the Persian Gulf there is confirmed evidence of relatively recent extended local flooding in this part of the world. Excavations in Iraq, for example, have shown evidence of a flood at Shuruppakmarker around 2900-2750 BCE which extended nearly as far as the city of Kish (whose king, Etana, supposedly founded the first Sumerian dynasty after the Deluge). Sir C. Leonard Woolley’s excavations at Ur south of Uruk in the 1920s found a more than 2.5 m thick homogeneous silty loam stratum that was void of artifacts, which Woolley in 1931 ascribed to Noah’sFlood.

Great Sunda wetlands, Indonesia

This is type 1. During glacial times a huge peaty swampland joined the Malay peninsula, Sumatra, Java and southwestern Borneo to the Asian mainland. The present landmasses were highlands framing a vast wetlands ecosystem larger than any on earth today which is now covered by the southern part of the South China Seamarker. Though the area never lost its tropical to subtropical vegetation, the monsoon weather system, which is powered by the continental mass, is likely to have been more intense than it is today. At one of the "pulses" of sea level rise, the combination of violent monsoons over a single drainage basin, in a landscape that dwarfed modern Bangladesh, provide a scenario for some of the most devastating flooding humans have ever witnessed anywhere .

The Carpentaria plain (12,000 to 10,000 years ago)

This is type 1. During glacial times, a stretch of level plain joined Australia with New Guineamarker and enabled humans to walk into Australia. That plain flooded to form the Gulf of Carpentariamarker around 12,000 to 10,000 years ago. Aboriginal Australian myth of the "dream time" includes a Great Flood which is not ordinarily a recognizable feature of the Australian climate and geography, except for infrequent filling of ordinarily dry lake basins (e.g. Lake Eyremarker).

The Aegean Basin

The comparatively shallow Aegean basin
Areas that have not been as widely discussed include the refilling of the Aegean basin. A look at a modern chart shows that it is 1 km or more deep in some places but that it has a sill along the line of the Peloponnesemarker - Cretemarker - Rhodesmarker - southwest Turkeymarker. However, that sill is very deep: the northern Aegean has an average depth that is variously estimated: 566 ± 234 m, or 350 m, depths well below the lowest estimate of sea level drop during the recent ice age. Thus, there is more likely to have been a sill-overflow effect at refloodings of the empty Mediterraneanmarker in the Miocene, if that sill existed then, but not later.

English Channel (Strait of Dover): Doggerland and the channel flood

The flooding of the English Channelmarker (Strait of Dovermarker) is considered a flood of type 1. In 1998, the archaeologist B.J. Coles identified as "Doggerland" the now-drowned habitable and huntable lands in the coastal plain that was formed in the North Seamarker when sea level dropped. Doggerland has not caught the popular imagination, but the terrain was available for settlement. Its gentle swells remain as the Dogger Banksmarker. Paleolithic reindeer hunters roamed the land; some traces of their encampments have been identified, but the timing of the submergence has not been fixed. The region was watered by the glacial River Rhine, into which flowed the River Thames as a tributary; the combined river flowed into the North Seamarker, permitting access to Britain by large mammals and humans.

During an earlier glacial maximum, the combined rivers had been blocked to the north by an ice dam; they filled a vast lake with freshwater glacial melt on the bed of what is now the North Seamarker. A gently upfolding chalk ridge linking the Weald of Kentmarker and Artoismarker, perhaps some thirty metres higher than the current sea level, contained the glacial lake at the Strait of Dovermarker. At a certain time, and apparently more than once, the barrier failed or was overtopped, loosing a catastrophic flood that permanently separated Britain from the continent of Europe; a sonar study of the sea bed of the English Channel published in Nature, July 2007, revealed the discovery of unmistakable marks of a megaflood on the English Channel seabed: deeply-eroded channels and braided features have left the remnants of streamlined islands among deeply gouged channels where the collapse occurred.

Glacial lake outburst floods in North America

In North America, during glacial maximum, there were no Great Lakesmarker as we know them, but "proglacial" (ice-frontage) lakes formed and shifted. They lay in the areas of the modern lakes, but their drainage sometimes passed south, into the Mississippi system, sometimes into the Arctic, or east into the Atlantic. The most famous of these proglacial lakes was Lake Agassizmarker. A series of floods, as ice-dam configurations failed (type 4) created a series of great floods from Lake Agassiz, resulting in massive pulses of freshwater added to the world's oceans.

The Missoula Floods of Washingtonmarker were also caused by breaking ice dams, resulting in the Channeled Scablandsmarker.

Lake Bonnevillemarker burst catastrophically due to its water overflowing and washing away a sill composed of two opposing alluvial fans which had blocked a gorge.

The last of the North American proglacial lakes, north of the present Great Lakes, has been designated Glacial Lake Ojibway by geologists. It reached its largest volume around 8,500 years ago, when joined with Lake Agassiz. But its outlet was blocked by the great wall of the glaciers and it drained by tributaries, into the Ottawa and St. Lawrence Riversmarker far to the south. About 8,300 to 7,700 years ago, the melting ice dam over Hudson Baymarker's southernmost extension narrowed to the point where pressure and its buoyancy lifted it free, and the ice-dam failed catastrophically. Lake Ojibway's beach terraces show that it was 250 m above sea level. The volume of Lake Ojibway is commonly estimated to have been about 163,000 cubic kilometres, more than enough water to cover a flattened-out Antarctica with a sheet of water 10 m deep. That volume was added to the world's oceans in a matter of months.

The detailed timing and rates of change after the onset of melting of the great ice-sheets are subjects of continuing study.

There is also a strong possibility that a global climatic change in recent geological time brought about some large deluge. Evidence is mounting from ice-cores in Greenlandmarker that the switch from a glacial to an inter-glacial period can occur over just a few months, rather than over the centuries that earlier research suggested.

Holocene Impact-Generated Megatsunamis

The Holocene Impact Working Group has postulated that during the Holocene, oceanic impacts have created megatusamis, which have impacted the coasts of Western Australia, India, Madagascar, and other areas. They argue that these megatsunami created coastal landforms, which they call “chevron dunes”, and identifiable megatsunami deposits. Other Earth scientists dispute their interpretations.

The refilling of the Mediterranean Sea

An earlier catastrophe, called the Messinian salinity crisis, occurred too far back to be within human memory during the most recent re-flooding of the Mediterranean Seamarker's dry basin. It is dated by general consensus about 6 million years ago, before the emergence of modern humans.

The basin had previously become a desert once again, the most recent desiccation in a series, as deep cores in the seabed have revealed a series of several layers of salt, separated by loess deposits, after continental movement had closed the Strait of Gibraltarmarker, an event variously placed at 8 million or 5.96 million years ago. The Mediterranean did not dry out during the most recent glacial maximum.

References

  1. "Protecting the Persian Gulf": gives average depth 35 m.
  2. "Marine Transgression in the Arabo-Persian Gulf Basin". With extensive bibliography.
  3. The classic example is the decades-long resistance among North American geologists to J Harlen Bretz's theory of the formation of the Channeled scablands of Washington State in a series of post-glacial-age catastrophic floods.
  4. Baker, V., 1995, Surprise Endings to Catastrophism and Controversy on the Columbia: Joseph Thomas Pardee and the Spokane Flood Controversy. GSA Today. vol. 5, no. 9, pp. 169-173.
  5. Kennett, D.J., and J.P. Kennett. 2006. "Early State Formation in Southern Mesopotamia: Sea Levels, Shorelines, and Climate Change," Journal of Island and Coastal Archaeology 1:67-99. With extensive bibliography.
  6. G.F. Camoin, ed, Reefs and Carbonate Platforms in the Pacific and Indian Oceans (IAS International Workshop on reefs) held at Sydney 1995
  7. Dive conditions described by Eric Bjornstrom, Diver Magazine June 1999
  8. Woolley, "Ur und die Sintflut"
  9. Some Myths and Legends of the Australian Aborigines: A Legend of the Great Flood
  10. Estimate, for exampole, in A. Biachi, O. Tholosan et al., "Microbial activities at the benthic boundary layer in the Aegean Sea" (2007) ( on-line abstract).
  11. "The average depth of the Aegean Sea is 350 m, however, a significant part of it has depths of from 100 to 500 m." (Winfrid Schramm, P. H. Nienhuis, "Marine Benthic Vegetation: Recent Changes and the Effects of and the effects of eutrophication (Springer) 1996:425.
  12. The Aegean Basin formed very slowly by the crust thinning because of stretching due to tectonic events.
  13. Doggerland website (Danish), but the map redrawn from official Geological Surveys shows the landscape around 14,000-15,000 years ago in the first warm (interstadial) period after the glacial maximum.
  14. The area is subject to earth movements: see the Dover Straits earthquake of 1580
  15. Sanjeev Gupta et al. in Nature 448 (2007), pp 342-345.
  16. BBC News, "Megaflood' made 'Island Britain'"; News at Nature, "Geological evidence supports theory of surge down the English Channel."
  17. Abbott, D.H, S. Martos, H. Elkinton, E.F. Bryant, V. Gusiakov, and D. Breger, 2006, Impact Craters as Sources of Megatsunami Generated Chevron Dunes. Geological Society of America Abstracts with Programs. vol. 38, no. 7, p. 299.
  18. Kelletat, D., and A. Scheffers, 2003, Chevron-shaped Accumulations Along the Coastlines of Australia as Potential Tsunami Evidences? Science of Tsunami Hazards. vol. 21, no. 3, pp. 174-188.
  19. Pinter, N., and S.E. Ishman, 2008, Impacts, mega-tsunami, and other extraordinary claims PDF version, 304 KB. GSA Today. vol. 18, no. 1, pp. 37-38
  20. Hsu, K.J., 1983, The Mediterranean Was a Desert, Princeton University Press, Princeton, New Jersey.
  21. Anonymous, 2006, Profiles in Catastrophism: The Desertification and Deluge of the Mediterranean Basin Suburban Emergency Management Project. Chicago, Illinois.


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