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Tuesday, 20 August 2019

Where is the highest post-glacial raised beach in the world?



A geomorphologist's paradise.  A whole  landscape of raised marine  features -- mostly old storm beaches -- on the west side of Richmond Gulf, Hudson Bay. Here the highest beaches are at an altitude of c 235m, but further to the south the highest beach traces are more than
 310m above sea-level.........
(adjusted image -- exposure increased....)


In the Rivière Nastapoka area, eastern Hudson Bay (Quebec) the marine limit is at 248m, and there are abundant signs in the landscape of a dramatic inundation by the sea particularly beneath an altitude of 205m. See this paper:

Géographie physique et QuaternaireVolume 57, numéro 1, 2003, p. 65-83
Late Quaternary Deglaciation, Glaciomarine Sedimentation and Glacioisostatic Recovery in the Rivière Nastapoka Area, Eastern Hudson Bay, Northern Québec
Patrick Lajeunesse et Michel Allard
http://www.erudit.org/revue/gpq/2003/v57/n1/010331ar.html?vue=figtab&origine=integral&imID=im17&formatimg=imPlGr

But this is the paper that seems to describe the highest raised beaches -- as distinct from the highest washed surface or marine limit:

La déglaciation et le relèvement isostatique sur la côte est de la baie d’Hudson
Claude Hillaire-Marcel
Cahiers de géographie du Québec, Volume 20, Number 50, 1976

URI: https://id.erudit.org/iderudit/021319ar 

Article abstract

On the eastern coast of Hudson Bay, in the area between Great-Whale-River and the Richmond Gulf, the last wisconsinian ices flowed westward. The deglaciation took place between 8.100 l*C yr BP (Great-Whale-River) and 7.600 1 4 C yr BP (Richmond Gulf). The maximum limit of the Tyrrell Sea rises toward the south-east, from 230 m (north of Richmond Gulf) to 315 m (north of Manitounuk sound). In the Richmond Gulf, 185 successive raised beaches were built during the emergence of the land which followed the deglaciation. A 45 yr periodicity appears in their construction. Correlation between ï 4 C and sideral ages are in agreement with Suess' curve (1970). The emergence curve, established from the raised beaches, indicate a multiple component isostatic uplift. The land, first tilted toward NNE, is uplifted in relation to the main wisconsinian ice load on south-eastern Hudson Bay at the beginning, and later in relation to the more recent ice center of New Québec. The isostatic rate of uplift decreases from the 6,5 cm/yr at 6.000 14C yr BP, to a present rate of 1,1 cm/yr. A 20 m eustatic rise is observed between the deglaciation and 6.000 14C yr BP, when the sea level seemed similar to the present one. Variations in the rate of sea level changes indicate secondary eustatic oscillations of metric amplitude, which correspond to the main climatic events of that period of the Holocene.

The article is impressive, recording many locations where old strandlines, beach ridges and washing surfaces may be found.  The highest features, on the Hudson Bay coast to the south of Richmomd Gulf, are at about 315m above sea-level.    I have not come across any records of post-glacial beaches higher than this.   nHere are three of the illustrations from the paper:



In the Antarctic,  David Sugden and I found a disturbed raised beach at an altitude of 275m (over 900 ft) not far from the summit of Noel Hill on King George Island.  We werenconcerned since this was far higher than any other raised beaches ever found in Antarctica. Our initial conclusion was that beach dated from the last interglacial, and that the extraordinary high altitude had something to do with the isostatic forebulge effect, with this and other sites around the South Shetlands lifted by a considerable amount as the ice mass over the Antarctic Peninsula and West Antarctica expanded and depressed the crust further south.  I now think that this is not a satisfactory explanation, and that Eemian / Ipswichian raised beaches would have been destroyed during the course of a whole glacial episode.  I now think that the 275m raised beach is Holocene, and that it was apparently disturbed but not destroyed by a small glacial advance, in which the ice ran over it.

https://www.researchgate.net/publication/329656084_Raised_marine_features_and_phases_of_glaciation_in_the_South_Shetland_Islands

https://www.researchgate.net/publication/262414644_The_ages_of_glacier_fluctuations_in_the_South_Shetland_Islands_Antarctica

https://www.researchgate.net/publication/262414489_Evidence_from_the_South_Shetland_Islands_towards_a_glacial_history_of_West_Antarctica

In Sweden the greatest thickness of ice in the Devensian Scandinavian Ice Sheet was over the inner reaches of the Gulf of Bothnia.  In the district known as Höga Kusten (with high cliffs and steep slopes adjacent to the sea -- very unusual for the Baltic.....) we find the highest recorded isostatically lifted shoreline in Scandinavia, at 285 m above sea-level. The beach features at this altitude, on Skuleberget, are not very spectacular, but the hill summits are capped with unwashed till (and woodland) whereas the lower slopes are washed. Hence the word "Kalottberg."

Quote from Wikipedia:

At the height of the last ice age, 20,000 years ago, the ice sheet, which covered all of Northern Europe, had its center in the sea near the Swedish High Coast (Höga Kusten). The ice's thickness attained 3 kilometres (1.9 mi), exerting significant pressure on the ground surface, which was thus situated 800 metres (2,600 ft) below the current level of the High Coast. When the ice melted, the land surface rose progressively, a phenomenon called the post-glacial rebound, at a speed of 8 mm (0.31 in) per year. The zone was only freed of ice 9,600 years ago. As the land emerged from Lake Ancylus (ancestor of the Baltic Sea), the waves affected the terrain of today's park. The coastline of that era can now be found at an altitude of 285 metres (935 ft), measured from Skuleberget, southwest of the national park, which constitutes an absolute record. The peaks of the park were islands at that time.

The ancient coastline is notably made visible by vegetation caps, which cover the areas not submerged after the retreat of the glaciers, explaining the name Kalottberg ("mountain cap") given to certain mountains of the region and the park. These vegetation caps had been able to install themselves since, at these places, the moraines were not eroded by waves, and they thus constituted a place where vegetation could attach.






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