The Ramsey Sound giant erratic (2)

 

The giant erratic on the edge of the coastal slope at Penmaen-melyn, on Ramsey Sound.  

This is the view of it, looking north.

Today I took a bracing walk from the St Justinians lifeboat station to Penmaen-melyn, to check out the famous giant erratic.  

First things first -- it's still there, where I last saw it.  Altitude c 65m asl, and visible on the skyline from all directions.

Second, it is an erratic and not an outcrop of local bedrock; it sits partly on an undulating and ice-smoothed outcrop of dark blue Ordovician micro-gabbro (much finer grained than the unspotted dolerite of Preseli) and partly on smaller erratic boulders and patches of till.  On the west-facing cliffs here there are the remnants of a small Ordovician intrusion in rocks that are otherwise tuffs of Pre-Cambrian age.

Third, it's made of grey lava or tuff  with a pinkish or purple tint in the colouring.  It's very friable and crumbly and some exposures have "stringy" structures that remind me of the pahoehoe lava which I have seen in the Canary Islands.  Other exposures look pyroclastic, which would suggest an origin in an explosive volcanic eruption. That would explain the high amount of variability in texture and colour. There are abundant fractures, vesicles  and also jagged cavities.

Above:  Close-ups of the face of the giant

 erratic.

 

Above:  Exposures of the dark blue Ordovician micro-gabbro (dolerite)

 bedrock near the erratic.

I measured the erratic again, and this time I estimate the weight to be nearer 70 tonnes.  But the shape is quite complex, and there is one huge crack near the northern edge of the boulder which has effectively split the boulder into two segments.  Because of the nature of the rock, the boulder does not appear to be heavily abraded or ice-smoothed, and there is not much of a weathering crust, but in places the surface is crumbling away, and there is a considerable surface cover of lichens, liverwort and grasses.

So what might be the boulder's provenance?  I cannot be certain that it has travelled a great distance, since there are Precambrian ash flow tuffs (some of them welded)  outcropping on some parts of the mainland cliffline, and a detailed analysis would be needed in order to decide whether the rock is Pre-Cambrian or Ordovician in age and to determine a likely provenance. Many of the older rocks have a lighter buff, light grey or light brown colouring.   However, there are several exposures of darker-coloured Ordovician pyroclastic rocks on Ramsey, to the NW and about a kilometre away, in the middle part of the island.  I might hazard a guess that that is where the erratic has most likely come from..........

Looking under the erratic.  We can see that it rests on several smaller erratics and on patches of local till including flint fragments and sub-rounded cobbles.