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Wednesday, 16 December 2015

On rockfalls from rock faces

 One of the excellent Aerial Cam photos taken from above the dig site at Rhosyfelin.  The big rectangular block (the "picnic table" as the locals now call it) looks to be gloriously isolated, but that is because other blocks and debris contained in slope deposits have selectively been removed from around it.  It has in other words been "prepared for presentation".......  Note that the block is located directly beneath the highest part of the crag, where other large blocks are now loose and are almost ready to fall too.....

"In September 2011 we started by digging a trial trench. We came down right on top of a prone 4m-long monolith, lying 4m from the edge of the outcrop. It seemed to have been manoeuvred into position by prehistoric quarry-workers and then left there. A large, jagged block had splintered off its underside, perhaps the reason why it was never moved out of the quarry."

Parker Pearson, M., Pollard, J., Richards, C., Schlee, D., and Welham, K. (2016).  "In search of the Stonehenge Quarries,"  British Archaeology, Jan/Feb 2016, pp 16-23.

Leaving aside the host of questions that are begged in that short extract, what intrigues me is the ongoing insistence on the part of the archaeologists that the pseudo-proto-othostat is in an un-natural position.  Let's explain yet again that there is nothing remotely problematical about the large rectangular block, given its position beneath the highest part of the crag. As I have asked before, if it is deemed to be lying in the "wrong" position, what is the "right" position supposed to be?

I bring this up now because today we went for a pleasant walk to Abereiddi and Porthgain.  At the latter place there is a dolerite quarry which worked till about 1930, with clean faces up to about 20m high.  If you think there is anything at all unusual or "man-made" about the rock face and rockfall bank at Rhosyfelin, just take a look at these pics:

Close-up of the rockfall debris bank at Rhosyfelin.  Much debris has been removed from the foreground.
Rockfall beneath a quarrying rock face in the Porthgain dolerite quarry -- abandoned around 1930.  Note the "random" arrangement of the blocks, some of which are almost 10m away from the rock face.

A rockfall from one of the lower faces at Porthgain dolerite quarry.  This fall has occurred within the last 85 years.  Here we see mostly large blocks.  One elongated block about 6m long is resting precariously, and will certainly move again when another rock fall occurs or when the bank settles.

Rockfall beneath a near-vertical abandoned face at Dinorwig slate quarry, North Wales

 Rockfall banks beneath another abandoned quarry face.  Here several "cones" have accumulated beneath gullies and have coalesced into an apron of scree incorporating very large blocks.

Two rockfall cones beneath an abandoned quarry face.  One consists of finer shattered debris in a discrete cone, and the other is a jagged pile of rubble incorporating very large blocks.

I still cannot understand that in the Brit Arch article, and in the Antiquity article, the authors (including two geologists) have not made ANY mention of the dramatic and prominent rockfall accumulations that mask the base of the Rhosyfelin rock face and which are interbedded with the radiocarbon dated horizons.  Have they really spent 5 seasons crawling about all over this rockfall bank, laboriously recording the position of every largish stone, without knowing what it is they have been looking at?


Myris of Alexandria said...

The proto-orthostat has been moved into that position
The photo you have used is dishonest there are plenty that show the orthostat with the quarry face directly behind it is not beneath the highest part of the outcrop.
Even were the proto orthostat not so but a detached joint block fallen from what was say 3 metres high then to end up in its present position it would take at least one and a half pikes and a rotating Jack knife to make it.
I have seen and worked in, probably not hundreds,many quarries and the scree looks like your examples. No horizontal tabular block lying at almost 90 degrees to the quarry face.
There is one amongst us whose experience in these matters far exceeds ours.
Let him speak, let he who is without spin cast the first stone.

BRIAN JOHN said...

Your confidence is touching, Myris. The stone's long axis is not aligned 90 deg away from the rock face. It is not even at 45 deg. Just look at the photos.

BRIAN JOHN said...

I agree that the highest part of the ridge is actually further to the SW. As the ridge crest rises irregularly, so does the adjacent valley floor. The bit adjacent to the big rectangular block is where the greatest relative height difference occurs -- ie where there is the greatest drop from crest to valley floor.

How unusual is it for a big rectangular block to land like this, with a nice flat face uppermost? It probably is unusual, but that does not mean it was manoevred into position by heroic quarrymen. The natural world is full of unusual occurrences which grab our attention, but in the end they are all explicable by reference to the laws of physics. If it helps, I can say that none of the geomorphologists who have visited the site have said to me that they thought the big block might have been put into position by human agency. Would I lie to you?

Myris said...

It would be easy to produce a rose diagram to show the orientation of the long axes of the tabular joint blocks with respect to the quarry face and then see just how very 'unusual' the proto-orthostat is.
All the archaeologists and a couple of geologists who have visited the site have said that it was moved intentionally into that aberrant position.
A BA post-BA date also is suggestive.
We need to hear Phil's thoughts as a mining engineer his views will be respected by all.

Nimlet said...

Out of interest, is it possible to work out which of the narrow ends was "up" on the block? (Apologies if this one has already been covered before).

BRIAN JOHN said...

I think the up-valley end is a bit narrower -- suggest you do a search on the blog. Use the search facility..... try 'orthostat' or 'Rhosyfelin' -- lots of pics of the stone.

Phil Morgan said...

Hello to Myris,
Phil’s thoughts as a mining engineer --- (retired, thank God).
Last month I submitted several photos of a quarry in the Merthyr Valley, where a stone weighing about one tonne had recently fallen near vertically, from a height in excess of fifty feet. For those who wish to view the item it was posted on the 29th November 2015. This stone demonstrated that a falling block of high mass, landing on a relatively flat surface doesn’t bounce -- where it lands is where it stays.
Brian’s photos in this post show:
1). Craig Rhos-y-Felin.
If we look at the larger undisturbed stones, we see that each major axis is at, or close to, right angles to the direction of the rock face, this would indicate that they have probably slid down the face. However, the eight tonne rock which causes disagreement has its major axis at an angle of 30 degrees to the direction of the rock face. In my opinion this would be commensurate with it originally resting at the same angle as the other large rocks and then being extracted from the rubble and turned to its present position. Whether it fell or was pulled from the rock face could provide a discussion in its own right, but the rotation, the final angle, and possible use of ‘rails’ to manipulate the block, favours human endeavour.

2). Craig Rhos-y-Felin #2
As for photo 1

3). Porthgain dolerite quarry #1.
Note the "random" arrangement of the blocks, some of which are almost 10m away from the rock face.
Where industrial quarrying takes place, only a specified size and/or quality of rock are targeted, the random arrangement of rock can be attributed to sorting discard from the product.

4). Porthgain dolerite quarry #2.
I assume that the rock fall has occurred naturally, sometime after the quarry ceased operations. The photo demonstrates the reluctance of large falling blocks to move any distance from where they land, with the 6m one even failing to reach ground level.

5). Dinorwig slate quarry.
This image is typical of man-made discard; it should be kept in mind that the material has been removed by the use of explosives which eject the rock making it land a greater distance from the foot of the face than naturally falling material.
6). What is evident in this photo is that the remaining large blocks are intermixed with the scree at varying levels; there is also no indication of any large blocks of stone coming to rest beyond the perimeter of the scree slope.
7). As for photo #6 above.

Brian rightly points out that the ‘proto-orthostat’ is about four times the size of the average bluestone at Stonehenge; however, making an eight tonne monolith from a two tonne stone is extremely difficult, whereas, forming a two tonne monolith from an eight tonne block would only take skill and time, perhaps it was intended to alter its dimensions.
I am of the opinion that the quarry that supplied the majority of the bluestones for Stonehenge has yet to be located, and I predict that it will not be found at any of the outcrops so far put forward; namely Craig Rhos-y-Felin, Carn Goedog or Carn Meini, none of which lend themselves to sensible quarrying activities.

BRIAN JOHN said...

Thanks Phil -- useful information. You won't be expecting me to agree with you about the "human endeavour" bit, so we'll just have to go our separate ways on that. Yes, I think the bog stone has come crashing down the face at some stage, but as I have suggested, it could have moved to its "final" resting place in several episodes -- as that big rectangular block at Porthgain will do eventually. As you suggest, large pillar-shaped stones falling off a rock face might land approx at right angles to the rock face if they are falling onto a convenient debris slope with nothing to deflect them. Two biggish stones near the "picnic table" do indeed lie like this, as we can see from all the photos. But there are many other large elongated stones that do not lie with this "ideal" alignment -- and we can also see this in the photos. They lie at all sorts of angles to the rock face -- some of them almost parallel to it. It;s not exactly random, but it is pretty chaotic. And further up the valley there are five or six very large elongated stones embedded in the turf which seem to be almost parallel to the face and which lie some way away from it. I did a post on these "other orthostats" some months ago.

Another reason why I do not think that the "picnic table" is enough of an aberration or anomaly to make us think of human intervention is that what we see now is not what was there to start with. This "proto-orthostat" has been very carefully prepared for presentation to the world, and we do not know what other stones and sediments have been taken away from its vicinity as part of this public relatione exercise. Some time ago I bemoaned the fact that the stratified sediments that surrounded it might have have given us clues both as to its emplacement and as to what happened subsequently. Alas, it is all now too late for proper geomorphological / engineering analysis.

BRIAN JOHN said...

Bog stone? Sorry -- big stone......