The Newall Boulder -- more and more interesting

The broken surface of the boulder, showing the lithology and veining in some detail.  (Courtesy Salisbury Museum)

I have been in touch with Prof Peter Kokelaar, whose knowledge of the igneous rocks of N and NW Pembrokeshire is second to none.  He has kindly provided some advice based on my photos of the boulder -- with the proviso, of course, that there is really no substitute for the close examination of the boulder itself and of thin sections taken from it.

Anyway, he advises as follows:

1.  On possible provenance.  

He suggests that the initial IGS suggestion of a North Wales provenence for the rock was possibly based on a degree of bias, since in the 1970's a lot or work was being done in N Wales, and the IGS geologists were more familiar with the igneous rocks of Snowdonia than they were with those of N and NW Pembrokeshire.  He describes the rock as follows -- from the photos:  

Problem is that lavas commonly have various vesiculation bands that collapse and look vitroclastic and welded. I’d buy into the petrographer’s interpretation of welded igneous rock. One pic appears to contain a xenolith so that the streaky fabric perhaps on balance is welded pyroclastic, but then lavas contain xenoliths too. I suppose my main point for your purpose is it could well be of one of the minor Ordovician rhyolite sheets of north Pembs.

The source could well be Ramsey, especially Trwyn Llundain, the westernmost headland. There is moderately abundant petrographically distinct Cader Rhwydog Ignimbrite in reworked Anglian till here on Gower (Ram Grove), so no doubt the ice was collecting from Ramsey Island. Rather tantalising in my write-up of the Trwyn Llundain rhyolite (1985 paper, p 610) I note incorporated xenoliths that are rhyolite pebbles. It’s hard to tell from the photo, but that’s a possibility for your sample (picture below).

So, no question in my mind with the data available to me that the Newall stone can have originated in North Pembs. I’ve seen so much Ramsey erratic rock on Gower that Ramsey is a prime candidate, but there are numerous other Ordovician rhyolite sheet possibilities. Given that we know for certain Pembs supplied volcanic (and dolerite, and Skomer, and St Davids Head) erratics at least as far as Gower, it becomes an exceptional plea to have it come from Snowdonia.

So let's see what transpires when the rock is properly analysed........

Slickensiding features on the "bottom" of the boulder.  This face almost certainly coincides with a fault plane, and the streamlining probably has nothing to do with glacial processes. (Courtesy Salisbury Museum)

2. On slickensiding.

I have been thinking, since I saw the boulder 3 weeks ago, that one face shows all the signs of slickensiding on a fault plane. So I asked Peter about this, and he confirms that this does indeed seem to be a part of the "boulder story".   He says -- again just on the evidence of the photos:

In my book the surface shows quartz-mineral ribbon/fibre growth slickensiding. Top right the transverse bands are growth increments, where movement is missing-part down to left. The ribbons have stepped ends down-left where missing counterpart broke away.

The other photo shows layers of fibres with some changes of growth direction. Beneath a capping vein (white patch), part the fibres on the left are NW-SE, then middle W-E, then NW-SE again. Evidently part of a fault zone....

So that's pretty conclusive, and we can safely say that one of the faces on the boulder is a fault face, with its morphology and mineral characteristics fashioned or influenced by the movement of one rock face across another. There are plenty of examples in the literature:

On this basis we can, I think, rule out these striated or streamlined features as glacial features, and must attribute them to faulting instead, with the fault line acting as an obvious line of weakness in the rock, in turn facilitating glacial entrainment when the source area was deeply inundated by ice.

The other slight striations / scratches on other faces of the boulder may still of course have a glacial origin.

Grateful thanks to Peter for his comments.

The story of the boulder gets more and more interesting........