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Monday, 30 April 2018

The Waun Mawn quarrying complex

I spent a long time wandering around on Waun Mawn and Cnwc yr Hydd yesterday, looking at those quarries -- and I am as mystified as ever.  While I am implacably opposed to the idea that there is a "bluestone monolith quarry" at Rhosyfelin, these quarries are so obvious that nobody would seek to pretend that they were created by the forces of nature!

Grid ref:  SN 083345

First, the facts.  The geology map shows that the greater part of this area is underlain by Aber-mawr shales and mudstones of Ordovician age, with strips or sills or "microgabbro" (which is another word for dolerite, which is another word for diorite).   But the hill mass appears to me not to be made of sedimentary rocks at all, but of rhyolites and ashes similar to  the rocks to the east, in the Brynberian area.   I think there may even be foliated rhyolites similar to those at Rhosyfelin -- I have some samples, and proper geological identification is needed.  The rocks break up into slabs and flakes, and from the few rock exposures we can look at, there appears to be intense fracturing with multiple intersecting fractures.  Where the dolerite outcrops, to the west of the hill summit, we see occasional slabs and  rock projections up to 2m high, but the ground surface is covered for the most part with a dolerite blockfield, with thousands of weathered and abraded boulders and pillar-shaped stones lying about everywhere.

Foliated rhyolite outcropping in one of the quarry pits to the south of the summit

Part of the dolerite blockfield to the west of the summit.  Bedrock outcrops and loose blocks, some showing signs of glacial abrasion and others broken by frost action

There are scores of quarrying pits, for the most part less than 2m deep and 5m across.  In the most intensely quarried areas, the pits are so close together that the landscape has a "honeycombed" appearance, with intersecting pit edges, and with piles of spoil as well.  The quarries are strung out over a distance of more than 500m, from the edge of the common in the north to near the edge of the  "platform with stones" to the south.  Maybe the quarrying activity has been concentrated on the junction between dolerite to the west and rhyolites and other volcanic rocks to the east?

There are five groups of quarries, and some outliers as well, as shown on the Bing satellite image above.  Quarry 1 has an extent of 40m x 20m; Quarry 2 extends over an area of 30m x 30m; Quarry 3 is 100m x 50m in extent;  Quarry 4 (around the hill summit) is 30m x 30m in extent; and Quarry 5 extends over an area of 150m x 30m.  The total area covered by these quarrying activities is 12,100 square metres.   This is truly quarrying on an industrial scale.

This is what the quarries look like in close-up:

So are the quarries really prehistoric?  I really cannot tell.  Part of me thinks that maybe they have been exploited for rubble for the building of the farm track between Tafarn y Bwlch and the farm called Gernos-fach.  But if that was the case, one would expect more distinct trackways leading from the quarries  and maybe signs of heavy machinery use  -- but the only quarry that shows signs of "farmer exploitation" is quarry number 1, on the northern edge of the common.  So let's keep a question mark about the formation of that one.

But quarry complexes 2-5 are utterly convincing.  How old are they?  Neolithic?  Bronze Age?  Iron Age?  Medieval?  Might they even be related to the presence of a deer park in this area between 1550 and 1750?

And what on earth were the quarries for?  We can consider various options:

1.  Were the quarrymen digging for rhyolite bluestone monoliths for the putative "proto-Stonehenge" on the platform on the south side of the hill?  That's vanishingly unlikely, since the bedrock is flaky and heavily fractured, and does not break up into coherent large slabs or pillars.  Not one of the standing stones or cromlechs in this area is made of flaky rhyolite, let alone Ordovician shales or mudstones -- the stone of choice ALWAYS appears to have been dolerite, if it was available.  And here it was available, in great abundance.

2.  Were they hunting for a rock type suitable for axe manufacture?  That's a possibility, but the rock fragments visible at the surface today do not seem well suited since they are too flaky and do not fracture conchoidally.

3.  Were they digging for disposable scrapers and cutting tools?  Again, possible -- but could quarrying on this scale really have been undertaken for so mundane a purpose?

4.  Might the diggers have been following a quartz vein and digging out white quartz for ornamental use as at Newgrange?    Possible -- there are some rather spectacular quartz boulders in the area, including some projecting through the turf in the dolerite blockfield.

5.  Might the pits have been dug in the Bronze Age in the hunt for mineral ores -- copper or tin, for example?     Possible, but the diggers would probably not have found very much, since this area is not renowned for its rich mineral pickings.

The jury is still out on all of this -- all suggestions welcome!  In the meantime, here are a few photos of what the pits and cuts look like on the ground:

PS.  Another possibility comes to mind:

6.  Might the diggers simply have been taking slabs of rock for building and / or roofing purposes?  Did Bronze Age and Iron Age people use manageable slabs on the roofs of their structures to keep the weather out?  Much more solid timbers would have been used to support the extra weight of slate or shale slabs, as compared with the structure needed to support thatch, but the extra investment in time and effort might well have been deemed worthwhile?


GCU:In two minds said...

Dolerite is now called microgabbro. It is not the same as diorite that is a totally different rock with a totally different chemistry and mineralogy. You probably meant to say diabase a term not used in British geology for nigh on a century.
Diorites are uncommon but pretty rocks.

Note that at the dolerite country rock junction at Carn Menyn there are small quarries in the metamudstone. Sounds similar.
In one of the SPACES reports.

You should have a professional geologist identify the rocks, these are not easy rocks.

GCU:In two minds.

BRIAN JOHN said...

Sorry Rob -- yes, my mistake. I should have said diabase.........

I really need you to have a look at the rocks from this area -- in trying to work out what they are, I flip between thinking they are light-coloured mudstones (very unlike the black ones found in other parts of the Ordovician), or meta-mudstones, partly converted to slate by proximity to dolerite intrusions, or volcanic ashes, or rhyolites. Maybe all of these are present -- I would not be surprised..........

I shall send some fragments for you to have a look at! As you say, these are not easy rocks.

GCU:In two minds said...

My guess is they are meta-mudstones but I await the bits of rock.
Often a thin section is needed and then sometimes that is not enough and geochem is required.
GCU:In two minds

GCU:In two minds said...

See Carn Menyn Report 2007-2008 on R.Ixer's page; discussion and description of meta-mudstones.

Gordon said...

Was there much shipbuilding in Fishguard or Newport? Ballast?

BRIAN JOHN said...

Plenty of shipbuilding at both places. But if they had wanted ballast, they could have got it easily in the immediate neighbourhood.

BRIAN JOHN said...

Re the meta-mudstones at Carn Meini, and the supposed "Mesolithic quarrying", as I recall Darvill and Wainwright always were a bit coy on the subject of "Why would they have wanted to dig out rather ordinary flaky slabs of rock from a hole in the ground? Dod they ever express an opinion on that?

BRIAN JOHN said...

Hey diddy doddy doo -- I meant "did".......

Gordon said...

If they were building ships they would most certainly have needed rocks for ballast.The Mary Rose had over 100 tons of ballast.The town wall of Kings Lynn has been constructed from re used ballast and contains many types of rock even dolerite.

BRIAN JOHN said...

But the Pembs coast is not short of rocks, Gordon! They could have obtained -- both in Lower Town Fishguard and in Newport -- thousands of tonnes of broken shale or even boulders and cobbles for ballast, if they had wanted it. Why would they have gone off up into the Preseli Hills to collect rock from the top of a hill?

GCU:In two minds said...

No les frere Grimme were always quiet on the reason for the metamudstone quarrying and still are.
Axe production is an obvious first suggestion but there is no axe group that fits although there are many 'fit' ungrouped axes.
It remains an enigma. Perhaps.
GCU:In two minds.

GCU:In two minds said...

No les frere Grimme were always quiet on the reason for the metamudstone quarrying and still are.
Axe production is an obvious first suggestion but there is no axe group that fits although there are many 'fit' ungrouped axes.
It remains an enigma. Perhaps.
GCU:In two minds.

BRIAN JOHN said...

It's interesting that lots of people seem to be reading this post -- 242 views so far. Well, that's great -- maybe those of us that have info can share it with each other. I'm quite open to whatever turns up here and whatever it shows -- and just hope that the evidence, when it gets to be unearthed in the autumn, is not all forced into a "proto-Stonehenge" ruling hypothesis..... but MPP's pronouncements thus far do not inspire confidence on that score......

Rob Ixer said...

I have taken a look at the 6 very small!!! bits of rock.

The little surface pebble is probably a little bit of a dolerite chilled margin.

The other five are all (I think)meta-mudstone meta-very fg siltstone.

I can see why you wanted them to be banded rhyolite but they are too green-grey indicative of chlorite. They are similar macroscopically to the meta-mudstones quarried at Carn Menyn.

My guess is they are close to the contact zone of a dolerite body.

BRIAN JOHN said...

Thank you Rob -- that's very helpful. As you said, difficult rock. Next time I go up there I'll try to collect some bigger bits for you!