The Ogof Golchfa sill -- a possible source for the Limeslade erratic?

 

At the eastern end of the Ogof Golchfa site (SM 742 236) there is a perfect example of a micro-tonalite sill, sitting conformably in a sequence of thin-bedded sandstones and shales belonging to the Cambrian Solva Beds.  The sill itself is assumed to be of Ordovician age, intruded along a bedding plane, joint or fracture.   It's a bit more complicated than that, because it varies in thickness between 5m and c 20m, and there are one or two offshoots which don't appear on the geological map at all.  These are difficult to spot because the sill and the argillaceous rocks have virtually the same colour -- a distinctive greenish-grey.

At the western end of the site the sill is c 20m thick.

Reading the features is made more difficult because the sedimentary beds are metamorphosed or baked close to the sill edges, so that they look more like slate. And in the sill itself, the geologists noted that the feldspar crystals are small close to the margins and larger in the middle as a consequence of differential cooling or solidification rates.

On those parts of the sill close to sea level, where marine processes operate, there are good fresh exposures of the rock surface.  The greenish colour is everywhere apparent, and there are many fractures with bands of quartz and other secondary minerals.

Fine grained micro-tonalite close to the edge of the sill

Larger crystals and a "coarser" or rougher texture some way in from the edge

A very rough texture close to the centre of the sill

Turning to the Limeslade boulder, the colour similarity is striking.  It has a greenish hue, unlike the Preseli dolerites which are predominantly blue or grey in colour.  Is this down to a different copper (Cu) content?  We know from recent work that the Limeslade boulder has a considerably higher copper content than most of the Preseli dolerites; the results will be published in due course. On the other hand a green colouration can also come from minerals containing iron, chromium or manganese, and other minerals can also be involved.

Visually, this photo taken on my visit to Ogof Golchfa yesterday reminds me of the Limeslade boulder.

Close up of a rock face at Ogof Golchfa where the feldspar crystals are of moderate size.

The Limeslade boulder, with a surface heavily abraded by shoreline processes.

Although the sill at Ogof Golchfa has been referred to as a dolerite / microgabbro / diabase sill by some geologists, the designation as a micro-tonalite is now preferred, with the BGS also using the labels "metagabbro" and "microgabbro" for this group of Ordovician intrusive rocks. 

According to Wikipedia, tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total feldspar content. Quartz (SiO2) is present as more than 20% of the total quartz-alkali feldspar-plagioclase-feldspathoid (QAPF) content of the rock. Amphiboles and biotite are common accessory minerals.  In older references tonalite is sometimes used as a synonym for quartz diorite.

I don't want to be carried away here, and I am not quite convinced that the rock at Ogof Golchfa is the same as that of the Limeslade boulder.  But there are other similar sills with a greenish colour in the St Davids area and along the North Pembrokeshire coast, and one obvious provenancing candidate is Carn Ysgubor on Ramsey Island, just a few km away, referred to by BGS as having a large micro-tonalite intrusion of approximately the same age as that of Ogof Golchfa.  As they say, watch this space........

The Ogof Golchfa giant erratic

 

While we are on about erratics, giant and otherwise, let's celebrate the Ogof Golchfa "giant erratic" which sits prominently on the raised beach rock platform, embedded in Devensian till.  This one is actually quite celebrated already, since it is used by the geocaching community as one of the key North Pembrokeshire locations.  Note the little box in the photo, which contains trinkets and other bits and pieces of memorabilia.........

The boulder, which is well rounded, has dimensions 2m x 1.40m x 1m, and it's made of a very coarse dark coloured gabbro -- almost certainly from St Davids Head,  around 5 km to the NNW.

Some of the old geologists who visited this site thought that the big well-rounded erratics on the rock platform are embedded in the raised beach, and indeed Leach thought that they are "cemented" into position.  Although I could see no unambiguously cemented material at the base of the raised beach, I have some sympathy with this view,  and on balance I agree with them that these boulders -- like those at Whitesands -- were already in position when they were incorporated into the raised beach shingle and pebbles.  So we have evidence of TWO glaciations at Ogof Golchfa.........

Ogof Golchfa striated bedrock

 

Surprisingly, given the history of glaciation in Pembrokeshire, there are very few examples of striated bedrock slabs.  But right now, at Ogof Golchfa, just to the west of Porthclais, sediment removal during exceptional storms has exposed some bedrock exposures that were previously buried.  The rocks are Middle Solva shales and mudstones (Cambrian) which are here very steeply dipping.  The striae are pretty chaotic, as you would expect on sloping surfaces -- but where striae are visible on surfaces that are more or less horizontal, the prevailing direction of ice movement is confirmed as NW towards SE.  

The picrite erratic -- lost, but now found.......

 

Grid ref: SM 73108 23383

Great is the rejoicing in Heaven when something lost without trace is found again and reinstated in the record books.  Anyway, that is what had happened today.  I went out and had a hunt all over a rather extensive patch of gorse and heather dry heath, on a very hot day, treading rather gingerly because this was perfect adder habitat and having discovered that the marked Google location was a long way out......

The boulder is quite small and insignificant, projecting just 30 cm above the surrounding vegetation and difficult to see because it is dark grey or black in colour and is well covered with assorted picturesque lichens and heather.  The dimensions of the well-rounded boulder are c 75 cm x 45 cm x 45 cm.  It looks very dense and heavy --rather like basalt.  Parts of the surface are quite smooth, and parts are rough and "knobbly".  I don't know of any other outcropping rocks in Pembrokeshire that look like this -- and I understand why the old geologists speculated that it might have come from Scotland or North Wales.

It is located on a gentle south-facing slope, and about 35m downslope from a series of prominent whitish outcrops of Precambrian volcanic rocks (the St Davids granophyre).  If you are walking south from the head of Porthlysgi Bay, on its eastern flank, you come to a point where the path swings sharply to your left (eastwards).  There are rock outcrops on the path.  There is a minor path leading out towards the tip of the peninsula, but ignore that one, and after turning the corner, leave the path and strike out across the dry heath.  You will see the boulder about 35m away -- if the gorse and the heather have not grown so high as to make in invisible!

Once upon a time the boulder was in a cage, to stop people from bashing bits off it -- now it has been set free in a different position, and I can see no damage on the surface.  Let's hope it stays that way.

This is an outcrop of the St Davids PreCambrian granophyre -- white, flinty and very different from the blackish picrite erratic........

Stonehenge Visitor Centre -- more nuanced messaging?

 

Thanks to Caroline Beaton for sending a photo taken recently in the Stonehenge Visitor Centre. It's good to see that the only decent book on the bluestones is displayed quite prominently, as one would expect. May it continue to sell well, as it has done over the past few years.

Caroline tells me that "there is now a panel in the interpretation centre which mentions that it is not known for sure how the bluestones came to be included at Stonehenge and that there are several theories.
So they are not saying the dragging them there from Wales theory is right!"

Caroline says that there is now a better balance in the way that EH is openly discussing the issue and candidly stating there are several theories but no conclusions. Videos also state this quite openly.

This is after all what one might hope for from the custodians of the site, who have a responsibility to offer context and summarise the latest findings and theories.

Contrast this with the recent British Museum exhibition which suggested that the word "dispute" did not exist in the English language........

Carreg Samson

 

Thanks to Alun Roach for posting these pics on Facebook.  Carreg Samson is a rather splendid cromlech near the north Pembrokeshire coast -- in the most spectacular of settings.  It is supposed to have been created in the Neolithic, about 5,500 years ago.  There are six uprights and a capstone.  As we can see, the stones used in the construction are varied -- all erratics, and all used (as usual) more or less where found.  The burial chamber was constructed in a pit -- thought by the archaeologists to be the excavation pit from which the capstone was lifted. So this cromlech was built exactly where a potential capstone was found; the uprights might then have been imported from a short distance away.

That's a rather utilitarian conclusion relating to the cromlech's location -- unlike other suggestions which are a good deal more romantic and imaginative..... 

Here is a photo from Alan Hughes, showing the capstone in close-up.

The geology has not been well described -- but at least  three of the stones are made of Ordovician dolerite, and the capstone, like one of the uprights, is made of what appears to be a rough volcanic agglomerate or ignimbrite, probably derived from one of the local igneous outcrops.  This might be the rock referred to by BGS as a pyroclastic "crystal tuff" belonging to the Llanrian Volcanic Formation.

Grid ref:  SM 848335

The Porthlysgi picrite erratic

This is one of the most famous glacial erratics in Pembrokeshire, close to the cliff edge near Porthlysgi.  Once upon a time it was in a cage, to stop people from collecting chunks of it; then it fell over the cliff edge onto the beach below; and later still it was rescued with the aid of a helicopter and replaced on the clifftop, to the north of the footpath. 

It is reputed to have come from North Wales.  Here is an excellent description of it, on the web:

ht;tps://historypoints.org/index.php?page=glacial-erratic-boulder-porthlysgi-st-davids

The boulder is about 30m N of the coast path, at grid ref SM 73199 23361.

Correction:  This Google location point is in the wrong position.  The boulder is further to the west, north of the coast path at grid ref SM 73108 23383