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Monday, 21 March 2016

Who needs quarries anyway?





Following on from my recent post, I came across these two photos in my collection.  They both show the extent to stone litter on uncleared land; I suspect that there are hundreds of acres of this sort of terrain on Preseli and Carningli, except that over large tracts the stone litter or erratic assemblage has been buried beneath peat, windblown sediments, colluvium and stratified slope deposits.

The top photo is from the western flank of Carningli, and the lower one is from a morainic mound near Gernos Fawr -- the surface has been exposed following foraging by pigs.   The stone litter in the foreground of the top photo has been described by Roger Worsley as a "chevaux de frise"  similar to the features at Carn Alw and Castell Henllys.  (The feature is a defensive one consisting large numbers of sharp stones set into the ground at an angle, to deter Iron Age tribal groups from mounting cavalry charges towards the entrance of a fortified settlement.)   In this case, I beg to differ -- I can see no trace here of any "arrangements" of the stones, and no evidence that pointed stones have been deliberately placed in one particular direction in order to protect the settlement's main entrance.  There archaeologists have very vivid imaginations.......but we knew that already.........)

16 comments:

Dave Maynard said...

Brian,

Are you saying there is a suggestion of a chevaux de frise on part of Carn Ingli? I know there are field boundaries and enclosures formed of stones dragged into lines, but I was not aware that stronger suggestions had been made.

Sounds like another place I must go and visit, I've never got to the top, always preferring the Carn Ffoi area.

Dave

BRIAN JOHN said...

Yes, Roger Worsley suggested it a couple of times in assorted non-specialist publications. Roger (who sadly died some years ago) belonged to the more colourful end of the archaeological non-establishment........

PeteG said...

so if I was to walk from Stonehenge to Wales where is the nearest source of bluestones I would find and how far away from the 'quarries' is it?

PeteG

BRIAN JOHN said...

Which route were you thinking of taking, Pete? And what sort of bluestones might you be interested in? And how big should they be? And does shape matter?

PeteG said...

I'd take the shortest possible land route and any type of bluestone.
Where would be the nearest ourcrop?
PeteG

BRIAN JOHN said...

If you are looking for outcrops, and any sort of bluestone, including the Altar Stone, you would head for the Senni Beds outcrops (Devonian) around the fringes of the South Wales Coalfield. Don't have my geology maps handy at the moment, but I suppose the nearest would be in Monmouthshire, just to the west of the Severn estuary. Of course there are igneous outcrops in parts of the Mendips too, so if you really wanted dolerite you could probably get it from there......

I would hazard a guess that if you were interested in "orthostats" made of spotted dolerite, unspotted dolerite or rhyolite, and easy to collect, the greatest concentration of scattered boulders would be on the south side of Preseli, around Maenclochog and Mynachlogddu.

BRIAN JOHN said...

Rob reminds me that we can all check out where all the outcrops are by looking at the free geology of Britain viewer:
http://mapapps.bgs.ac.uk/geologyofbritain/home.html

Had forgotten about that!

Alex Gee said...

Your comment about Dolerite in the Mendips most interesting! Perhaps the Sarsen stones at Stonehenge were found in situ and the Dolerite bluestones were transported by glacial ice from outcrops to the north of Weston Super Mare?
Has Mr Ixer sampled these?

Alex Gee said...

Another consideration is whether the Altar Stone is Pennant Sandstone? Outcrops of which are plentiful in the South Bristol Coalfield! Another rock type that could well have been transported to the vicinity of Stonehenge from the vicinity of the Mendips by a surge the Anglian Ice sheet? Has this local rock type been sampled also?

BRIAN JOHN said...

Well, there are dolerites and dolerites. I recall that the old geologists eliminated the "local" ones because they were petrologically different from those in the Stonehenge collection. The same with the sandstones -- Rob Ixer and colleagues were (are) pretty convinced that the Altar Stone is from the Senni Beds, and I thought their paper was convincing too!

Dave Maynard said...

If the rock face at Craig Rhos-y-Felin was subjected to heavy glacial activity, where would any erratics end up? Could they have arrived at the Tafarn y Bwlch moraine you've reported? Only suggesting this as the closest potential location.

Could a I suggest a 'hybrid' interpretation, that allows for the deficiencies in the quarry location and for collection of rocks in the open landscape of Preseli by Neolithic man. If the stones had not traveled very far, they might be seen as a coherent group with similar charcteristics.

Dave

chris johnson said...

I would think it is relatively easy to determine whether stones were removed from Rhosyfelin by glaciation. As Brians photos show, there are trails of what looks like bluestone (narrow definition) leading in a s/se direction. Very few of these are protected as monuments and an active geologist could make a survey on his or her summer holiday.

Dave Maynard said...

How easy is it to determine visually the similarity between rock groups?

A full petrographic sampling of Rhosyfelin (say every metre across the rock face) might be needed to show the variability in the parent material. To carry out that survey to a similar level of intensity would be very time and resource heavy for a potential morraine, especially as I guess there would be a lot of material from other sources mixed in with it.

Spreads of stone look to be very interesting, but how can we tell if someone has selectively removed stones of a certain colour and size, and at different times in history? The only ones we can really identify are the current collectors of bluestones. As a side comment, are they collecting or quarrying? They just smash up a few surface stones because that is easier.

BRIAN JOHN said...

I doubt that any erratics from Rhosyfelin would have ended up at Tafarn y Bwlch, since that would have involved ice movement NE >> SW -- all the evidence suggests ice movement from the N or NW. If the ice crossed the crag and entrained erratic debris, it would have been carried up and over the Preseli ridge in the vicinity of Carn Goedog and the other big tors. But my theory -- still to be tested -- is that any entrained material was carried in shear planes up into the glacier under a compressive flow regime -- so we would not necessarily expect a "train" or erratics down-glacier. I have explained that in various past posts.

Chris, none of my photos show trails of bluestone heading S or SE. Some of my diagrams show that as the direction of most likely ice movement, as do the field maps of Griffiths and others.

BRIAN JOHN said...

Dave -- yes, visual identification of a rock type from just looking at a weathered surface on a boulder is very difficult. Spotted dolerites sometimes stand out, as do some rhyolites and ashes, but other rock types are very difficult to be sure about without taking hand specimens or even thin sections. Rob never ceases to remind us of the dangers of "quick" identifications.......

Even after all the work on Rhosyfelin samples, we still do not know how much variability there is across the rock face. Perhaps Rob can tell us whether there are many more analysed samples which are still to be published?

chris johnson said...

Sorry Brian, careless of me. The s/se direction contains similar stone groups to the photos you have shown, and is also in the direction of likely ice movement.