Rhosyfelin -- some inconvenient truths

A few days ago I sent a message to MPP asking if I could meet him and the team down at Rhosyfelin to have a chat about the sediments appearing in the current digging stage.  I didn't get a reply -- not complaining, since he is probably incommunicado just now, being somewhat involved in two big digs in Pembrokeshire..........  So after tea I ambled down to Rhosyfelin to see if there was still any action down there, and the guys and gals had all gone home.  So I had a mooch about, taking great care not to disturb anything.  Very interesting.  Here are some inconvenient truths:

1.  Now that the dig has been extended for 70m or more, right to the inner end of the rocky spur, we can see that the dig site is on the floor of a small meltwater channel, similar to many of the features we can see on the flanks of the Gwaun-Jordanston meltwater channel system.  It's difficult to assess how many times this channel might have been used, but its use by meltwater (probably subglacial, under very high pressure) might go some way towards explaining the long, smooth, regular wall which others tend to refer to as "the quarry face."  It's probable that the channel was used both in the Anglian and Devensian Glaciations.

 View of the dig site, looking along the rock face of the spur towards the top end of the meltwater channel.  In the far distance the channel loops round to the left and rejoins the main meltwater channel now occupied by the Brynberian River.

2.  If you look carefully at the photo above you can see that the big "orthostat" resting just to the right of centre in the photo has fallen from the highest pinnacle on the ridge.  I see no problem at all with a slab of this size crashing down the rock face, either in the Late Glacial or at some stage during the Holocene, and ending up exactly where it is today, some distance from the rock face.  It might have slid, might have tumbled down end over end, or might have rolled.  No human agency required in the process.

3.  In his Moylgrove lecture, MPP placed great emphasis on the "killer fact" that a big transverse stone just downslope of the "proto-orthostat" has striations or erosional grooves on it, proving beyond doubt that other large stones have been hauled across it during earlier stone removal operations.  It's lucky that I managed to have a look at it today, or this myth might have been perpetrated until the end of time -- and there would have been no way of checking, since the spoil is about to be thrown back into the dig location at any moment now..............  Anyway, they are not striations, scratches or erosional grooves.  They are outcropping foliations on the rock surface, no different from those on scores of other stones to be found throughout the dig site.  They follow the strike of these micro-structures.  If you look at the side of the rock you can see how the foliations or "pseudo-layers" run within the rock, downwards towards the bottom left of the photo.  I am 100% confident that any geologist or geomorphologist who looks at this rock would agree with me on this.  Here are some pictures:

Above:  Close-up of the "grooves" supposedly caused by heavy orthostats being dragged across the stone in question.  There are indeed grooves, but they coincide exactly with the outcropping foliations on the stone surface.  They are perfectly normal weathering phenomena, of no significance whatsoever to the quarrying debate.

Above:  Another grooved block of rhyolite embedded in till in the lower part of the dig site.  The grooves on this one are seriously interesting, since they are slightly curving and since they don't seem to be related to the pattern of outcropping foliations.  This one has "glacial erosion" written all over it.

Above:  More grooves on the end of another slab of local rhyolite, not far from the "proto-orthostat."  These grooves are very prominent, and are clearly related to the pattern of outcropping foliations.

Even more interesting -- and something you don't see very often in Pembrokeshire.  This is a heavily glaciated slab of rock on the lower part of the dig site - look at all the smoothed off corners.  But look even more carefully at the crescentic gouges or fractures.  If you are a geologist you might call them conchoidal fractures.  They show that a glacier with very hard tools has been at work on this surface.

4.  In the Moylgrove lecture, MPP also dismissed the idea that the excavation had reached a layer of till. He said that if there was till present, it must be much deeper down.  I have news for him.  In the lower part of the dig at the moment we can see some of the best till I have seen in a long time.  In fact, they hit it last year, as I mentioned on this blog.  (Why don't these people listen when they are given solid information?)  It is probably Devensian, around 20,000 years old.  Its upper surface -- as always in Pembrokeshire -- is a foxy red colour.  When I saw that I was pretty convinced that I would see a rather sticky clay-rich or silty till underneath the reddish layer, and indeed, there it is for all to see:

The lower part of the dig site.  In the foreground we see a cluster of rhyolite boulders which have been embedded in the Devensian till.   The foxy red colour is typical of the weathered upper surface of the till everywhere in Pembrokeshire.  Deeper down, the colour changes, and there are signs of gleying, with streaks of buff, grey and black colouration.  I'm not sure whether the black colouring is due to manganese oxide or organic material.  We can see this if we look carefully at the section beneath the white labels -- bottom left.  There is not a huge amount of clay in this till -- it is more sandy and gravelly.  Again that is typical of the inland tills in Pembrokeshire.  On the north coast, where the ice has come in from the sea, the clay percentage in the Irish Sea till is substantially higher.

5.  I may be wrong, but I get the impression that when archaeologists look at erratics, they see hammer stones.  I'm not denying that there may be the odd hammer stone lying about on this site, but what is indisputable is that we have a splendid collection of rounded, sub-rounded and shaped glacial erratics representing many different lithologies.  I counted five or six different rock types very quickly, but did not wish to disturb any of them for fear of messing up the survey work.  The most prominent are the blocks of quartz (very rough and jagged, since quartz is almost impossible to smooth off nicely except on a beach or in a very turbulent river), several types of dolerite, and several types of volcanic ash.  Probably they have all come from the NW, where the rocks of the Fishguard Volcanics are exposed at the surface.  Some of these are small enough to use as hammer stones, if you were determined enough to bash something, but many of the boulders are far too large to be lifted, let alone used for percussion or stone shaping.  You can see some of these stones in the photos posted by Chris:  http://chrisjohnsonnl.smugmug.com/Rhosyfelin-2013-dig

Many of them are still embedded in the till, and others have been lifted by the digging team and slung onto the spoil heaps.  Here are a few of them:

Large erratic boulder embedded in the till in the lower part of the dig site.  This is probably a dolerite.

Another erratic boulder resting on the surface of the till.  This also appears to be a dolerite.

A large quartz erratic boulder, upslope of the "proto-orthostat."  This one could have come from almost anywhere in North Pembrokeshire........

How many more erratics do you want to see?  I won't bore you with any more close-ups, but here are a few that the diggers have extracted and pushed to one side.

So there we are then.  If anybody tells you that there are no traces of glacial action at Rhosyfelin, do not believe them.  Trust me.  I know what I am talking about.  Always happy to help.  And by the way, I'm still waiting to see some evidence that this site was used as a quarry.