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Tuesday, 20 January 2015

The Sleek Stone Giant Erratics (1)



Two giant erratics (from Ramsey Island?) resting on a rough platform of  Lower Coal Measures sandstones and shales.  The degrading cliff face is in the background, and the boulders in the foreground are all locally derived.  The larger boulder is about 3m across, and the smaller one about 2m across.  They lie about 100m from the Sleek Stone, and are apparently unmoved since they were described in 1916.

I had a jolly trip to Broad Haven this morning since there was a very low tide at a convenient time -- and although the temperature was just above freezing, with rain and sleet and a high wind, mission accomplished.  The big boulder of "columnar blue quartz-porphyry" discovered by Cantrill et al a century ago is still there, and is still washed by the tides every day.  It's the size of a small caravan -- by far the biggest erratic I have ever seen in Pembrokeshire.  It's got a very irregular shape, and is not at all smoothed or rounded off by either glacial transport or wave action.  It's difficult to estimate its volume, particularly in the middle of a deluge, but I would say it's at least 27 cubic metres -- which would make its weight about 75 tonnes.  (For comparison, the Freshwater Gut erratic is said to weigh about 50 tonnes......).

Next to it is a smaller boulder with an estimated weight of about 22 tonnes.  It's made of the same material, as far as I can see -- so is probably from the same source.  Maybe it has broken off the larger chunk of rock during or after glacial transport.  The colouring is dark blue, but there is heavy weathering on the rock surface.

That's not all.  I found another three giant erratics within 50 m of the ones pictured above,  and the satellite image below has been annotated to show their positions. 

Sleek Stone and the locations of the five giant erratics shown in the photos.


Giant erratics 3 and 4 (the rucksack lies between them) in the bay north of Sleek Stone, Broad Haven.  the degree of rounding on these is much greater, but this may be because of their position lower on the beach, where they are submerged by water for most of the time.  In this part of the shore zone there is a great deal of abrasion going on.....


This is giant erratic No 5 -- the closest one to the Sleek Stone.  This one is easily spotted, because it is standing rather than lying.......  the glove gives the scale.

These three erratics have similar textures to the two biggest ones, but the colour is greenish rather than dark blue.

I am sure there must be other erratics from the same source on this beach -- and I shall return one fine day in the spring or summer, when the rocks are not quite so slippery.

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