Below I have posted a short note written by Phil Morgan, in which he considers the question of whether Rhosyfelin really does have a man-made quarry there, or whether everything is natural...... I happen to disagree with many of the things that Phil says, but discussion is always good!
1. I quite accept that where quarrying is contemplated, it makes more sense to take stones from sloping walls where things will slide down under some degree of control, rather than from overhangs where there is great danger. So from that point of view the rock face at Rhosyfelin which looks down on the dig site is "suitable." This wall faces NW, as Phil says.
2. I don't agree that there would have been less freeze-thaw activity under a periglacial climate regime on the NW face than on the SE face. I think there may well have been more on that side, since with westerly winds predominating in this area, the SE face would have been relatively more protected by lee-side accumulations of snow, which inhibit freeze-thaw processes and frost shattering.
3. Orientation of big blocks. Phil mentions that there are 4 blocks lying at right-angles to the rock face -- I don't see that. If you look at all my photos on this site, and at the Gigapan, what you see is a jumble of fallen and broken blocks and scree with no preferred orientations. So I don't agree that this tells us anything at all about human involvement. I agree it would be interesting to know how many of the blocks have fallen directly down from the higher parts of the rock face, and how much downslope movement there may have been -- ie movement broadly parallel with the rock face. A factor which Phil doesn't consider at all is the influence of ice and snow-banks in all of this -- scree slopes in high-latitude or periglacial environments are very complex indeed, with snow, ice and running water all playing roles and with rocks falling down onto ice or snow and then sliding or settling later on. Believe me -- I have crawled about on such slopes many times in my wild youth!
4. The large block and the rails. Again, I don't agree with Phil. It could perfectly well have fallen and slid on ice or snow into its present position. No human agency needed -- and there are no rails either, as I have pointed out. Has anybody suggested that the longish stones beneath the big "orthostat" are made of mudstone? They look like perfectly ordinary local rhyolite to me.....
5. Heather as an indicator of quarrying activity? Sorry Phil, but there is heather all over the place in areas which have not been quarried. I don't believe a word of what you say here.
In short, I see nothing here or anywhere else to shift me from the view that this jumble of rocks, large and small, is entirely natural.
Rhosyfelin -- is it a quarry site?
Figure 1 – Stope
1). Quarry workers are concerned over where rocks fall, nature cares not.
Gold mining practice utilises ‘Stopes’, where large caverns are excavated to access the gold bearing veins. The ore veins seldom exist at a convenient angle for extraction, resulting in the sides of the stope forming steep angles with the ground, (Figure 1)
When mining operations form these steep angles the sides of the workings are named the ‘foot’ wall and the ‘hanging’ wall. The safer rock face to work is the foot wall side for the product slides down-slope to the floor, whereas when working the hanging wall there is always the danger of the side collapsing and falling vertically, which could cause injury.
The Neolithic stone gatherers would have disliked being struck by falling stones and they would have realised that working the Craig Rhos-Y-Felin outcrop from the ‘foot’ wall side, (north-west face), would be the safer option.
Figure 2 (below) – Craig Rhos-Y-Felin ‘Foot-wall’.
Figure 3 (below) – Craig Rhos-Y-Felin ‘hanging Wall’.
2). Considering the actions of ‘freeze-thaw’
The photo only shows the dig in the area of the north-west face of the outcrop, the face that would have been least exposed to the actions of freeze-thaw; whereas the ‘hanging’ wall, (south-east face), which would have been more susceptible does not appear to have the same smooth finish of the foot wall.
It may prove beneficial to place a small trench below the south-east face to examine any debris for similar large blocks of rock. If no such blocks are found then it would again support human activity.
3). Orientations of the larger stone blocks.
Gravity is unable to differentiate between human quarrying and natural quarrying; it is logical to think that the quarried material comes to rest in the same manner for both activities, and that the orientations would not favour either quarrying method.
However, it is unusual that there are four blocks lying at right angles to the rock face. I suggest that a search be made to see if the upper surface of each block correlates with the rock face immediately above it. If it does then it is more likely that it was wrenched from the solid with it rotating about its base as it fell, indicates human activity, (figure 4).
If the underside of each block correlates with the solid rock then it is more likely to have become detached by natural means and slid down the rock face, (Figure 5).
It is normal, and best, practice when removing rock from the solid, to work to a free face, which in this case would be to work from the top of the outcrop vertically downwards. Therefore, the above correlation test should initially be applied to the upper portions of the outcrop.
The method used to separate the blocks is unknown but the use of water to expand wooden wedges inserted in the natural joints of the rock would work, especially if combined with the use of levers and ropes.
4). The large block lying parallel to the rock face.
It is thought that this slab is too far from the rock face to have come to rest after falling by human or natural means. Therefore, it is reasonable to say that it has been moved.
Mention has been made of this stone resting on ‘rails’, however this could be purely by accident. It is suggested that the ‘rails’ be examined to verify whether they are made of the same rhyolite as the igneous outcrop, or of some foreign stone, particularly mudstone.
Mudstone becomes slippery when wet, especially when under load. The pressure breaks down the rock surface which forms a lubricating interface.
Craig Rhos-Y-Felin is situated in the Fishguard Volcanic Group of rocks, however, the mudstones of the Aber Mawr Formations are reasonably close and southwards, upstream of the Afon Brynberian.
If the rails are formed from mudstone this would again support the human activity principle.
5). Heather as an indicator of human quarrying activity.
A study has been made of the use of heather as an indicator that quarrying has been conducted by humans. Craig Rhos-Y-Felin formed a part of this study and proved to be an ideal candidate for human quarrying.
Briefly the study has shown that heather, which has an affinity for acidic soils, flourishes on man-made, acidic, scree slopes, while refusing to grow on identical, and adjacent, natural scree.
It seems the reason for this abnormal activity is that, generally, natural scree slopes have been formed by freeze-thaw during past ice ages, when no plant life could survive. However, all human quarrying activity has to have taken place after the last Ice Age when plant life could survive.
The heather at Craig Rhos-Y-Felin grows only on the igneous outcrop, (figure 6).
Figure 6 – Heather and gorse growth at Craig Rhos-Y-Felin.
It is thought that the combination of orientations of the four rock slabs, the changed direction of the fifth slab, the use of ‘rails’ possibly made of imported mudstone combined with the presence of heather indicates that this area has been quarried by human hand.
Phil Morgan, Inc. Eng.
18th September 2012