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Sunday 25 June 2017

Striations in the Stockholm Archipelago

Not all rock surfaces are susceptible to the process of scratching or striation.  In this part of the archipelago this grey fine-grained gneiss (?) reveals crossing fractures and a smoothed striated surface.  The pink granite (?) in the background contains large masses of quartz crystals, and reveals no striations at all, breaking up into a broken and rough surface.  In the foreground is a scar left by the removal of a large bedrock block on the down-glacier or lee side of this slight ridge.

Another heavily striated surface on fine-grained gneiss.  The crossing fractures are lines of weakness which are exploited by the quarrying process by which one block after anther is dragged away by the overriding ice.

 Heavily smoothed and striated bedrock, with a marked fracture scar in the bottom left quadrant of the photo.  This scar shows no traces at all of ice action, suggesting that it is very fresh.

Striations in  the Stockholm Archipelago.  Here, in an area of Pre-Cambrian basement rock with relatively low relief, there are abundant traces of glacial erosion on rock surfaces just above present sea level. The ice of the Scandinavian ice sheet retreated from this area around 11,200 years ago, at a time when the land surface was greatly depressed.  The retreating ice edge was probably floating, and the whole landscape was then deeply inundated by the sea —  to emerge bit by bit as isostatic recovery ran its course.  Isostatic recovery is still going on today, at a rate of c 1 cm per year. 

The last erosive ice to affect this landscape was flowing almost exactly north— south, and this is the direction followed by most striations.  However, there are deviations of as much as 30 degrees, since the ice adapted its flow patterns to the details of bedrock morphology.  We can even say that it adapted to micromorphology, with measurable deviations across an area of a few square metres where there are ridges, gullies and knolls.

How old are the striations?  There are occasional crossing striations, but not many.  I suspect that they are quite fresh, dating from the Weichselian / Devensian glacial episode.  I also suspect that during that glaciation, over a timespan of maybe 20,000 years, there were not many deviations in ice movement direction.  The ice always flowed from the north towards the south.  Given the micro-adaptations we can see on the striated rock surfaces, the ice was remarkably fluid, suggesting considerable meltwater lubrication — but this was not a “wasting ice” situation, since the effects of gouging, rock fracturing and block removal are in many cases quite spectacular, suggestive of situations in which the ice was “frozen on” to the bedrock surface. Striations are in themselves indicators of hard basal ice capable of pressing rocks and rock fragments into the bedrock surface over which it was flowing. I suspect rapid alternations between freezing and basal thawing and sliding — a classic situation for optimal glacial erosion and areal scouring. 

Most of the striations in the photos are less than 5 mm deep, but some are up to 2 cm deep.

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