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Saturday, 5 November 2011

The Vestrafiold "Quarry", Orkney

I'm rather intrigued by this.  According to Colin Richards, the standing stones at Stenness on Mainland have probably come from the NW, where there is a "quarry" of Stromness Flagstones which supposedly holds evidence of working and of stones being dragged away.  The quarry site is called Vestrafiold, and it is treated with due reverence. 


So far so good.  Now there would be quite convincing evidence if the standing stones could be genuinely provenanced to Vestrafiold, since this would involve the transport of large stones in a direction directly opposite to that of the last known direction of ice movement across the island:

The black blob marks the site of Stenness.  The ice flowed approx from the SE towards the NW.  So how convincing is this quarry, and the provenancing of the stones?  Looks a little dodgy to me.  What does the quarry look like?  Here is a picture of it from the air.


A scatter of stones scattered on a grassy slope.  A quarry, did you say?  Sorry Colin -- but we need more evidence than this.  Then things get more interesting still.  When we look at the geological map:

... we see that the Stromness Flagstones occur across the bulk of West Mainland.  Looking back at the map of ice movement directions, it therefore seems that the stones at Stenness, Brodgar and Maes Howe could be entirely local, or else they could be erratics transported from the SE towards the NW, and left lying around in the general area.

The stones lying around at Vestrafiold could be parts of a natural outcrop, or they could be a cluster of erratics, but they don't seem, on this evidence, to have anything at all to do with the famous archaeological sites to the SE.

16 comments:

Geo Cur said...

Until the stones are provenanced it is conjecture whether sourced from Vestra or elsewhere or glaciated . In favour of transport is the fact that there are slabs propped up by stones of the same shape and dimensions as the monuments at Loch Harray . Also worth considering the amount of effort to transport is minimal compared with digging the rock cut ditch at Brodgar which was 9 m x 3 with a diameter of 103 m . Against glaciation is the lack of any flagstones that are accepted as having been glaciated , and the general lack of erratic material that could be used in typical Orcadian setting .http://www.landforms.eu/orkney/glacial%20erratics.htm .
“It is clear that Orkney ,like Shetland , must have been far enough removed from the centres of the Pleistocene ice sheets to have been virtually unaffected by glacial “loading”.” Whittow Geology & Scenery in Scotland .

The exception is the Dwarfie stane on the adjacent island of Hoy which is considered to be a possible erratic but Hoy is quite a bit rockier than mainland Orkney . If Vestra Fiold isn’t the source of the Harray stones then it is more likely to be another local setting .BTW there is a quarry at Vestra Fiold a matter of metres from the site of propped stones .

BRIAN JOHN said...

Geo -- don't like this sort of argument: "Because they were capable of doing A, they certainly wouldn't have had a problem doing B." Too much of it about......

You misunderstand Whittow. If he was talking about loading he was certainly talking about isostatic depression -- not about ice action, or the capacity of ice to erode. There is plenty of till on West Mainland, and there are plenty of erratics.

If there is a burial chamber or something similar at Vestrafiold then of course we will see stones "propped up" -- that doesn't tell us anything about whether this was a quarry used for Stenness etc.

Geo Cur said...

Not capable of A ,did do A and capable of B because it was a lesser task in a similar vein and it is something that has clearly been done elsewhere . No worse than saying because glaciation may have taken place in one direction we should consider that it unlikely that human transport was involved in the opposite direction .
Where are the flagstone erratics on Orkney ? Most of the erratics seem to be small boulders and till from the east .
Propped stones like the ones at Vestra are not like the components of a burial mound but they are the same dimensions of the stones found in the other monuments .

Anonymous said...

Brian, you write

“The ice flowed approx from the SE towards the NW.”

How could glacier advance flow from SE to NW? How could this be if glaciers formed in the North and advanced towards the South? Is this a local direction that goes against the global trend? Please clarify.

A related question. Can there be other geological mechanisms for the transport of erratics, besides glaciers? And could these in places be in the opposite direction to the glacier advance?

Kostas

BRIAN JOHN said...

Kostas- As I have tried to explain over and again, glaciers form where snow accumulates in sufficient quantities to be transformed into glacier ice. Look at any map of the British and Irish Ice Sheet and you will see that ice flowed radially from the main centres -- north, south, east and west.

There are many mechanisms for the transport of big erratics -- storm waves, floating ice, turbidite flows, avalanches, landslides, fluvio-glacial torrents, solifluxion, etc etc etc......

BRIAN JOHN said...

Geo -- we are not going to get anywhere on this -- I suspect we are both speculating on the basis of inadequate field knowledge. All I am trying to do is encourage Colin and his ilk to be a bit more circumspect when talking about "quarries" when the evidence supporting the use of that label is, to put it mildly, inadequate.

Constantinos Ragazas said...

Brian,

I agree. Locally glaciers can form as snowfall accumulates over prolonged geological time. And locally the flow of such glaciers will be in every direction from the central glacier cap. I know this.

But isn't it also true the predominant 'polar cap' glaciers form in the North and are fueled by the absorption and freezing of seawater? Thus reducing sea levels? This is what I had in mind. Such glaciers would advance from North to South. Or considering the Earth's rotation, from NW to SE.

You forgot to include in your long list of erratic transport mechanisms stones carried on the surface of solid ice sheets.

Kostas

BRIAN JOHN said...

No Kostas -- sea ice formation does not evolve into ice caps or ice sheets, nor does it have any effect on sea level. The North Polar ice cover expands and contracts, but it does not thicken and become anything else.

Stones do not get carried about on the surface of "solid ice sheets" as you call them because there is no surface gradient. But stones from icebergs or cliff falls can of course land on sea ice and can then get carried about as the sea ice breaks up and drifts about under the influence of tides and currents.

Constantinos Ragazas said...

Brian,

Inadvertently, I may have 'invented' a new theory on the formation of polar glaciers!

I must confess I know little for sure on this! Sorry!

Kostas

Geo Cur said...

Thanks Kostas ,at last something concrete . Re. your hypothesis ,there is no evidence that the stones were brought to Stonehenge on an ice sheet , although human transport using frozen conditions is a possibility . I don’t know if the surface of the putative ice sheet would follow the contours of the ground surface that faithfully but why should it suddenly stop at the point when the trend from the NW –SE at the monument is downhill and approaching the monument from that direction is if anything slightly uphill . The “hill” to the west I assume is the point 121m OD ,a mile away and with rise of 19 m the slope is negligible and even so it to the west and wouldn’t impact on the SE trend . Even if the stones had been brought by an ice sheet it would hardly be that difficult for builders to move a few small bluestones to the empty section if necessary . Are we to assume that the sarsens came from the same direction and arrived by the same means ? I wouldn’t describe the siting of the monument as being in a basin .
It’s worth being a bit more precise than just saying there is an “empty quarter “ at the SW . There are gaps in the sarsen circle at various points where some of the sarsens are fallen . Between stone 16 and stone 19 there is a a gap of about 8 metres which would accommodate two sarsens at the spacing found in the rest of the sarsen circle but there are no fallen stones in the area that might be assumed to be like the other fallen sarsens . The area has never been excavated so we don’t know if there are any stone holes .That is the problem .
The shape of the sarsen circle as it is today isn’t that crescentic and even if it was it would hardly be justification for your suggestion of moon “worship” . To answer the “did they run out of stones ? “ might be answered in the negative if the stone holes are found by excavation , if the stone holes are not it doesn’t mean that they did run out of stones . We couldn’t answer the “did they get tired and give up ? “ tiredness and “giving up “ are unlikely features to retrieve from the archaeological record of the late Neolithic .

Geo Cur said...

ooops sorry folks ,the last post "wot I wrote "should have been sent to the "unfinished busines " thread .

Andre Preview .

Constantinos Ragazas said...

Geo,

I cannot say for sure how this ice cover I am hypothesizing resulted. But using it we are able to explain in minute detail all the 'facts on the ground'. This ice cover, I believe, may be very different from the glacier advance and consequent glacier transport of erratics Brian has been arguing. Or perhaps complements Brian's glacier transport theory. For one thing, such a 'local ice cover' may not leave behind the same trail of erratics expected to be found at Salsibury Plain, but is not found.

Two possible scenarios come to my mind.

1) Vast waterways were formed with the rapid glacier melting which then froze over during the sharp drop in global temperatures around 8500BC that lasted more than 1000 years. If it is also true that isostatic rebound was more pronounced during this period, with the south coast of the UK sinking (perhaps by 100m or more) while the northern coast rising (by an equal amount perhaps), that may have created enough gradient on the smooth flat frozen waterways to help transport erratics to the lowest points – along the coast and where geothermal hot spots created ice holes.

2) The ice cover could perhaps be the remnants of the glacier melting, caught in time during the cold snap around 8500BC that lasted for more than 1000 years. This ice cover will be smooth and hard and following somewhat the gradient of the land, but not completely as it would be thicker in areas and thinner in other areas, creating a flat gently sloping surface.

As to your specific points.

“I don’t know if the surface of the putative ice sheet would follow the contours of the ground surface that faithfully”

My sense is that it wont. Think of it as a frozen lake. Certainly the lake bed can be very uneven yet the frozen lake surface would be smooth and flat.

“ why should it suddenly stop at the point when the trend from the NW –SE at the monument is downhill and approaching the monument from that direction is if anything slightly uphill”

Reiterating my previous comment, the contours of the land need not be mimicked by the surface of an ice sheet.

“Even if the stones had been brought by an ice sheet it would hardly be that difficult for builders to move a few small bluestones to the empty section if necessary “

You are correct. It wont be at all difficult. But they would certainly start from the most accessible parts of the ice rim and work their way towards the least accessible parts. If there were more stones, they would probably have done that. But the bluestones from Wales brought to Stonehenge were limited and not enough to complete the circle. On this I agree with Brian.

“Are we to assume that the sarsens came from the same direction and arrived by the same means ?”

My understanding is the sarsens came from the North and were more locally sourced. Perhaps from outcrops above the ice falling onto the surface by weather, withering, earthquakes or hot lave sipping through bedrock strata. There is evidence of volcanic ash in the strata seams!

“I wouldn’t describe the siting of the monument as being in a basin .”

The meltwater retaining basin I am referring to is made of ice. The meltwater debris and deposits in this basin collecting at the bottom is what I claim created the Stonehenge layer. The Avenue is the drain channel of the meltwater collecting in the retaining basin.

“It’s worth being a bit more precise than just saying there is an “empty quarter “ at the SW . There are gaps in the sarsen circle at various points where some of the sarsens are fallen .”

Kostas
(continued next)

Constantinos Ragazas said...

(continued from above)

I am comfortable with whatever description of this part of Stonehenge you like to use. Certainly it is not as 'developed' as the other parts. As for the irregular spacing between sarsens, certainly dropping these from above and for no particular reason (perhaps just having fun, or part of a spring ritual) would adequately explain such gaps. As also why some sarsen are fallen. As not all stones dropped from above would stick and be embedded into the thick chalk putty – especially if these were more irregular in shape, like the Slaughter and Alter Stones.

“... isn’t that crescentic and even if it was it would hardly be justification for your suggestion of moon “worship” “


This is not what I believe! This is how others try to explain these facts on the ground. To me, it's pure fantasy.

“To answer the “did they run out of stones ? “ might be answered in the negative if the stone holes are found by excavation “


There are many more holes in the bedrock than stones (standing, buried or lying). I think it is a mistake to think of these as belonging to stones or as stones being reset from place to place. The photos of the rugged chalk bedrock does not suggest this. What in my view explains this unusual texture in the chalk bedrock under the Stonehenge layer is that along with block of stones falling over the ice rim, blocks of ice certainly could have likewise been brought and fallen over the ice rim. When the ice blocks melted, the empty pits remained behind.

I can go on explaining many more details Geo, but space (and Brian) may not permit. Ask instead!

Kostas

BRIAN JOHN said...

OK Geo -- you rose to the bait and asked for that! You now know Kostas's theory in all its bizarre glory. I thought I'd better allow Kostas the space here, since you seemed unaware of the fact that we have been over it all many times before. As I have said before, Kostas is assuming a set of wondrous processes that have never been observed in nature or considered by geomorphologists -- and which are underpinned by a whole series of misunderstandings of the nature of the terrain and the published literature.

So I'm stopping this thread here. If I don't, we will all be wasting each other's time ad infinitum.

Anonymous said...

Has Kostas met Garry and his Stonehenge coal mining university theory they would been well-suited.
Hey ho only Monday morning.
Thomas the Rhymer

BRIAN JOHN said...

No more, please!! I can't bear it......