Wednesday, 23 April 2014
This photo shows Maiden Castle and Lion Rock, near the Trefgarn Gorge (which carries the A40 road) between Haverfordwest and Fishguard. The rocks are very fragile and very precarious -- and it is doubtful, in this geographical situation, that they could have survived over-riding by the Irish Sea Glacier during the Devensian. So the suggestion is that central Pembrokeshire remained ice-free around 20,000 years ago, as shown on my latest ice edge map.
In contrast, the monadnocks or upstanding rocky hill masses of the St David's Peninsula are much more heavily eroded, as we see here (the two photos are of Carn Llidi, near St David's):
Up on the summit of Carn Llidi, and on nearby Penbiri, and on St David's Head itself, there are extensive ice-smoothed rock surfaces, as we see in the lower photo. So the ice streaming across this area must have been thick and quite active.
The northern tip of the Pencaer peninsula was also moulded by streaming ice, as we see in this photo from near Carregwastad (where the French landed during the "last invasion" of 1797). Notice the generally scoured appearance of the cliffs and the landscape inland, and the rounded off or smoothed profiles of the hills:
Closer to the ice edge, as for example in the Dinas - Newport - Carningli area, things are much more complex, with some ice-smoothed surfaces and some quite delicate features that look too fragile to have survived very active glaciation. Here is one of the glaciated slabs on Garn Fawr, on Dinas Mountain -- as fine a feature as you will find anywhere in the world:
And here is another, from Carningli above Newport:
And here is the area of glaciated slabs at Carn Meini, adjacent to what used to be called "the bluestone quarry":
Are these slabs in these widely spaced locations all of the same age? Cosmogenic dating is seriously needed........
Another fascinating difference between Carn Llidi and Carningli is the lack of a big scree bank at the former and a very substantial scree bank on the lee (south) side of the latter:
So the explanation must be that there was a prolonged period of ice erosion at the former site, buy just a short one at the latter site, maybe with marginal ice melting more or less balancing the rate of ice advance. maybe there was a windscoop here, within which periglacial slope processes were able to operate at or near the peak of the glacial episode.
So we have three Devensian landscape types in Pembrokeshire:
1. An area beyond the ice edge where fragile landforms were able to survive -- but where there will have been extensive snowfields and where periglacial processes operated for many thousands of years. (For example, the Trefgarn Gorge area)
2. A zone maybe 5 kms wide across which the Devensian ice edge oscillated around 20,000 years ago, with some fragile features surviving, some glacial erosional features created or freshened up, and banks of scree able to accumulate in places. (For example, the area around Carningli and Carnedd Meibion Owen)
3. An area well inside the maximum position of the ice edge, where erosional processes were able to operate, where fresh scree was not able to accumulate, and where the landscape was rounded off or smoothed. (For example, the hill masses at the NW corner of the St David's Peninsula)
Gradually it is all coming together. Work in progress.....
Tuesday, 22 April 2014
There are several big blocks of quartz low down in the sedimentary sequence at Rhosyfelin, which I have previously interpreted as erratics transported into the area along with other foreign stones and boulders. Well, I have had a change of opinion on this. Now, following the completion of the 2013 dig, one can see that the biggest block actually seems to be sitting on an outcrop of quartz bedrock. So there must be a big mass of it incorporated into the rhyolites. Perhaps Rob or Richard will tell us if this is typical -- maybe it is a secondary feature, formed some time after the rhyolites were laid down?
I must admit to having been a bit worried about the "fresh" appearance of the quartz blocks, with few signs of abrasion and many sharp edges. This would be consistent with the blocks having travelled hardly at all from their place of origin.
A bit more digging in 2014 will reveal whether this is really the bedrock floor in the digging area, and how thick the sediments are beneath the lowest digging surface from 2013, as shown in my posts of last September.
The Craig Rhosyfelin area, showing the grid of sampled points referred to by Ixer and Bevins
Having been out to look at Rhosyfelin again yesterday, it's not a bad idea to look again at this statement:
Quote from Ixer and Bevins 2011
"This is the first time that any lithics from Stonehenge have been unequivocally assigned to an area of a few square metres, namely to within a very small single outcrop or couple of outcrops......"
Article: "CRAIG RHOS-Y-FELIN, PONT SAESON IS THE DOMINANT SOURCE OF THE STONEHENGE RHYOLITIC ‘DEBITAGE’
Rob Ixer and Richard Bevins, Archaeology in Wales 50, 2011, pp 21-31
This implies that Locality 8 on the air photo is actually the source of the "rhyolite with fabric" found in the Stonehenge debitage. It also explains why Mike Parker Pearson and his colleagues have become so obsessed with the idea of the "Rhosyfelin Quarry" -- in the conviction that the geologists have given them the "all clear."
However, the geology is not that simple, and I have previously questioned the reliability of that statement made by Rob Ixer and Richard Bevins.
I have argued that without a rather dense sampling grid, it would be a mistake to argue that the "foliated rhyolite with Jovian fabric" matched up with Stonehenge samples is restricted to the one site (locality 8) at the NE tip of the Rhosyfelin rocky ridge. The geologists don't know where else this particular rock type might outcrop -- or if they do know they haven't given the evidence to support the contention.
Strong planar foliation shown on the side of a detached block in the Rhosyfelin "litter" beneath the rock face. Some fractures run parallel with this foliation, and others cut across it
One problem I have with the evidence as presented is that we do not see (in the Archaeology in Wales paper) the thin section slides for the other localities, so we have nothing to compare with locality 8 or with the samples from Stonehenge. What we do have are these 3 illustrations:
Jovian fabric -- locality 8
Jovian fabric -- Stonehenge debitage
Jovian fabric -- Stonehenge debitage near Heelstone
Bear in mind that one of these illustrations is at a slighly different level of magnification than the other two. In the Heelstone sample, the black blobs look like little fishes or tadpoles -- and they do not look like this in the other slides. In the other Stonehenge sample there are fewer black blobs and they are less elongated, and the other components in the slide look almost liquid rather than crystalline. The locality 8 sample is different again, with a greater density of black blobs and with much more irregular shapes. To my untrained eye, these three fabrics do not look identical, but they are similar, suggesting that they are part of a continuum, and in my view it is quite possible that the Stonehenge samples have NOT come from the few square metres around locality number 8.
I would be more convinced if we had at our disposal the thin section slides showing us the fabric at all the other sampling points identified on the air photo. How different, or how similar, are they to the three slides shown above? At one point I recall Rob referring to locality 9 as the possible source of some of the Stonehenge debitage.......... was that a mistake, or was that at one time a working hypothesis, later changed when the slide from locality 8 had been examined?
So if the area of foliated rhyolite with Jovian fabric is more extensive, where might other outcrops be located? Here is a photo of the rock face:
The Rhosyfelin rock face, after clearing by the archaeology team. According to the grid of geological sampling points, no samples have been taken from anywhere along this face.
Rob tells us that the crack featured in my last post is 90 degrees away from the direction of the foliations. We have to work in three dimensions here -- and this means that either the foliated band sampled at point 8 runs as a very thin band all the way along this face and might outcrop in other localities in the Rhosyfelin - Pont Saeson area, or else it runs more or less parallel with the rock face, either along the cliffline itself, or some distance outside it or some distance within it. Such a "sheet" (we cannot call it a bed) could run deep into the ground and could have been exposed much higher up than the current crest of the ridge. In other words, bits and pieces of it could have been carted away from this general area by overriding ice at a time when the landscape had a rather different appearance from the Rhosyfelin of today.
We need more geological info here, chaps........ is it in the publication pipeline?
I have just found this thin section slide from one of the other geology papers by Rob and Richard. It's from Point 10, quite a way to the south of the rock ridge where the archaeologists have been working and over 200m from Point 8. Is this also a part of the same "rhyolite with fabric" continuum? There are strong similarities with the slides shown above, except for the absence of the prominent black blobs.
Monday, 21 April 2014
There are lots of photos flying around which show clusters of earnest ladies and gentlemen gazing intently at this crack in the rock face at Rhosyfelin, having things explained to them by either Richard Bevins or Mike Parket Pearson. I assume, therefore, that this is where the samples came from which have been matched up accurately with the debitage collection from Stonehenge.
However, to extend that geological work into an assumption that this is where at least one of the Stonehenge orthostats came from is taking things too far. For a start, this crack is no more than 30 cms wide, and there is no way that a viable orthostat could have come out of that crack -- or "alcove" as it is described by some people. Possibly small lumps of stone might have been taken for here, for the manufacture of tools.
But if an orthostat (or two) did travel from here to Stonehenge, by some means or other, it must have come from a slab of rock closer to the camera, which has now been entirely removed, or else from a slab of rock higher up, above the top of the current rock face. The fractures here -- and the foliation -- run almost vertically. So could the rock that found its way to Stonehenge have come from ten feet higher up? Twenty feet higher? Thirty feet higher? It would be good to get a geological view on this -- leaving archaeology entirely to one side -- from either Rob or Richard. Over to you, chaps.....
I've done a previous post on the occurrence of rounded pebbles at Craig Rhosyfelin, and today I wandered along the river bank for a few minutes, looking for things that archaeologists, in their infinite wisdom, might refer to as "hammerstones." I found four very quickly, on the river bed and upstream of the Rhosyfelin dig site. The pebbles have all come from the till and fluvio-glacial gravels exposed in the river banks. I am sure that similar pebbles -- and many rounded stones that are larger and smaller then the ones in the photos -- also occur in the till and in the overlying deposits investigated by the archaeologists.
Let's think straight here. The occurrence of stones like these in the sediments exposed during "The Rhosyfelin Quarry Hunt" does not prove that there was a quarry here. However, some of the stones could well have been discovered by past inhabitants of the site and used for breaking or shaping stone. Also, we cannot preclude the possibility that past inhabitants might have collected rounded stones from the banks of the river and used these as hammerstones at the camp site being investigated.
All the "convenient" stones I have seen and collected are entirely natural, and require no human intervention at all when it comes to explaining why they are where they are.
Even if some of the "hammerstones" have genuine percussion marks on them, that does not prove that they were used in quarrying operations designed to obtain large stones for transport to Stonehenge. They are much more likely to have been used in local tool-making, or for crushing animal bones, beating fibrous plants or crushing fruit.
At great personal risk, in the cause of science, I climbed up through the gorse bushes this afternoon and took a look at the crag immediately above the 'abandoned orthostat'. This is how it looks from the other side of the ridge.
Immediately to the left of the highest crag, near the left edge of the photo, you can see the detached blocks which haven't quite got round to crashing down on top of the hard-working archaeologists thus far. But they are a tough lot, and very dedicated to the task in hand -- rumour has it that they will be back in September. I hope they wear their hard hats............
I spent a very pleasant Bank Holiday afternoon at Craig Rhosyfelin, "doin' a bit o' geomorfin'." (That's how the Royal Navy personnel on HMS Protector referred to the work being done by David Sugden and myself in the South Shetlands in 1965-66.)
Plenty of images to share, and I'll probably do a number of posts about my new observations. But to start with, here is a rather nice image of the rock face -- taken straight into a late afternoon sun. Excellent shadows which bring out the edges of the slabs of rhyolite. You can see the "abandoned orthostat" in its black plastic shroud, awaiting the return of the diggers in September 2014. Not exactly beautiful -- and in fact rather spooky......
But you can see the prominent crag about 35 feet above where the big stone finally came to rest. I have always thought that this crag is the source of the big stone -- so I climbed up and fought my way through gorse bushes to have a look at it. This confirmed my belief, and I was interested to find five other big stones up there, in quite precarious positions, ready to crash down, when the time is right, in the general direction of the black plastic monstrosity.
So the crag is now in an advanced state of decay, and it seems entirely reasonable to conclude that during the thousands of years of periglacial climate at the end of the Devensian glacial episode, frost action did the job of loosening these blocks. Other loosened blocks actually fell down the rock face. Now, with the advent of a full interglacial climate, biological processes have taken over, and it may be that the roots of gorse bushes and small trees will widen cracks and contribute to the next set of rockfalls. Maybe archaeologists and JCBs will accelerate the process.......