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Tuesday 31 July 2018

Kalottberg features in northern Sweden

The key period in the formation of the highest raised beaches is that of the Ancylus lake, when the last big remnant of the Scandinavian ice sheet was breaking up into smaller short-lived ice caps on the mountains of Swedish Lapland.  Shorelines could only form in places from which the ice had already retreated, where wave action could affect the deglaciated landscape -- for example on hill summits that stood above the water level as islands. 

Having made a reference in my last post to washed landscapes in the Stockholm Archipelago, here is some more info about the isostatic recovery of the inner parts of the Baltic -- around 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 the world, 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.

(The quote above is a bit misleading, since it seems to assume that isostatic rebound rates are more or less constant; however, there is abundant evidence that immediately following deglaciation there is a rapid "elastic rebound" in which the crust can rise at a rate of up to 10m per century.  The rate then slows exponentially -- but the rate of land rise in the southern part of Hudson Bay in Canada is still more than 1m per century.)

The current uplift rates for the Gulf of Bothnia are shown on the maps below.  Note that there are complex interactions between ongoing isostatic uplift and the post-glacial eustatic sea-level rise, which has thus far been approx 120m.  Then we have the current additional fact of sea-level rise attributable to global warming. Having worked on raised beaches, hinge lines and rates of isostatic uplift in Greenland, Iceland and Antarctica, it's all rather like doing a three-dimensional jigsaw puzzle..........

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