Observations from a flying visit on 7th June 2014
Colin Richards: The end product -- the circle or the stone setting -- may not be the prime focus. We should concentrate more on "the immediate and socially significant acts of quarrying and dragging individual stones" (In "Rethinking the great stone circles of Northwest Britain.") Richards does not like the idea that monoliths are always obtained from the most convenient source (because the final form was assumed to be of prime concern). Rather, he wants the emphasis to be on the ritual acts of quarrying and transportation, and argues that there would be a preference for the use of stones from many different "special" sources. Quote: "This importantly shifts attention away from the circles and back to the location of the megalithic quarry sites." Quote: ".... construction was a social process as much as a feat of engineering......." Quote: "........the discovery and investigation of quarries may reveal them to be special places, possibly already having sacred qualities."
The Lewisian Gneiss complex of NW Scotland
"The crust of NW Scotland, together with parts of Greenland and North America that make up the ancient continent of Laurentia, was built up mainly from igneous rocks that crystallized around 2900 to 2700 million years ago. At that time, the rocks we now see were deep in the Earth's crust. They were deformed and metamorphosed at very high temperatures, producing gneisses with a folded layering. Actually, there were two periods of deformation and metamorphism (named the Scourian and Laxfordian events), separated by a stable period when the crust fractured and allowed in basic magma that crystallized as a set of dykes."
"Lewisian Gneiss is metamorphic, in that volcanic heat and pressure has altered its structure somewhat, originally the rocks were like granite which changed as the Earth's crust became molten and they solidified, which is the reason you can see great variations in the way the layers are displayed, ranging from the white, to pale grey and even then the really dark grey.The rock is mainly grey with coarse bands of white and dark minerals through it. The pale bands contain quartz and feldspar, whilst the darker bands are dense minerals , like maybe biotite mica and hornblend."
For the most part the rocks exposed at the ground surface and in the cliffs of NW Lewis are felsic gneisses, which are rich in light-coloured minerals like feldspar and quartz. There are abundant masses of white quartz exposed in and near the coastline. In places the whitish rocks could be mistaken, from a distance, for limestones......
The Callanish landscape
The landscape around Callanish is deeply scoured by overriding ice, with abundant small knolls of Lewisian gneiss (and other ancient rocks including altered granites, pegmatites, dolerites, gabbros, schists and basalts) exposing many glaciated slabs, whaleback / streamlined forms and roches moutonnes. It appears that the last direction of powerful ice movement might have been from the south-east towards the north-west, but there is little evidence of striations at Callanish to support this since the local rock is unsuitable -- so many of the glaciated forms might be inherited from earlier glacial phases. There are, however, fresh striations on the outer coast and on slabs of suitable rock including basalt -- suggesting the the last ice flowed more or less S-N. These observations are in line with the latest ice modelling which shows that there might have been a local ice cap centred in the highlands of Harris around 18,000 -16,000 years ago. (Clark et al, 2010) It's reasonable to hypothesise that the ice from this ice cap flowed northwards across the site of Callanish, leaving behind striated bedrock and fresh glacial deposits.
The landscape of smoothed rocky knolls and intervening depressions is very beautiful, having been determined above all else by the highly convoluted and fractured nature of the Lewisian gneiss which has allowed deep erosion in the most fractured areas and the survival of hillocks in areas where rocks are less fractured or more resistant on account of lithological differences. In many areas it appears that meltwater has had a powerful role in landscape formation, with survivals of short sections of meltwater channels and in some cases longer deeply cut channels which appear to have been major meltwater routes. Again, it is almost inevitable that these channels are of many different ages, with the freshest dating from the Devensian and others being of great age, periodically re-used and freshened up during the Ice Age. In the depressions there are accumulations of till and mixed deposits incorporating fluvio-glacial materials and some periglacial or slope deposits. Scree and rockfall deposits are rare, except beneath crags and short lengths of inland cliffs. At the coast there is abundant evidence of rockfalls in a highly complex "high energy" coastal environment which includes collapsed caves, stacks, arches and coves containing storm beaches.
One feature makes the environment around Callanish very suitable for "stone gathering" -- and that is the extent to which the bedrock surface has been washed by meltwater derived from the dissipating Devensian ice and maybe also from extensive snowfields at the end of the last glacial episode. I would guess that there were many centuries of heavy snowfalls and rapid thaws in this highly dynamic western environment characterised by strong seasonal variations. The washed surfaces are almost as spectacular as those of the Stockholm Archipelago where the coast is still recovering isostatically and is emerging from the sea; but this comparison cannot be taken too far, since there is no trace of raised shorelines here on Harris. (There must have been considerable isostatic uplift here, so glacier ice must have survived until a late stage in the isostatic recovery process.)
In addition to the evidence of deep scouring, we see signs of cliffed sections on the western flanks of many of the knolls, where broken boulders and slabs have accumulated. Some of these rocks are elongated and flattened (ie similar shapes to the elongated slabs built into the Callanish monument) and are already detached from the bedrock. Other slabs are exposed via elongated fractures parallel (and in some cases not parallel) with the foliations in the gneiss; they are still part of the bedrock, but could be separated from it without too much difficulty through the use of wedges and levers by just a few men. One of these cliffed faces is within 50m of the standing stones, on the W side of the rocky knoll called Cnoc an Tursa, between the standing stones and the visitor centre. Some of the standing stones are so thin that they cannot have survived long distance transport either by ice or human agency. So they must have come from the immediate locality -- probably within 50m - 100m of the sites where they were used. This "stone gathering" of very local stones is assumed in the bulk of the literature about Callanish. (I prefer the term "stone gathering" to the term "quarrying" since in truth virtually no technology would have been involved in the sourcing of the utilised monoliths.)
What was the natural mechanism for block removal from the bedrock? In some cases (as on the down-glacier flanks of roche moutonnees) there must have been direct plucking or dragging away of blocks by thin active glacier ice -- probably around 20,000 - 18,000 years ago. Other blocks and elongated slabs on the flanks of glaciated ridges and knolls might have occurred by rock bursting or pressure release, possibly assisted by periglacial processes following ice wastage. This probably loosened elongated slabs and made them easily available.
The rocks used in the standing stone settings all appear to have come from the "native rock" -- all within 100m of the places where they were set into the ground as vertical monoliths. The coarse foliated Lewisian gneisses -- white and grey in colour -- may in some cases have been moved by ice, but not very far. So it would not be a good idea to call them glacial erratics. This is true of Callanish 2 and 3 as well -- in each case the stones have come for the most part from nearby crags with broken detached blocks and slabs lying about beneath them. (Mostly the source cliffs are west-facing, suggesting an easterly flow of ice maybe close to the Devensian glacial maximum?) Some "suitable" stones are still to be seen lying around partly covered by turf. Colin Richards has suggested that many of the flattened elongated slabs used at Callanish have one face that is more weathered than the other, suggested that the slabs were recumbent and that some faces were exposed (and affected by overriding ice) while the lower faces were protected until the slabs were levered upward by Neolithic quarrymen. On my visit I did not notice any great difference between "fresh" and "old" stone faces, given that for the past 5,000 years or so the west-facing stone surfaces of the standing stones have received a much greater battering from the weather than the east-facing surfaces, and that north-facing surfaces have spent most of their time in shadow while south-facing surfaces have been drier and sunnier.
At Callanish 3 all the stones except two are "normal" Lewisian gneiss. These other two are of a darker rock type-- more like dolerite. There seems to be an exposure of it near the road about 100m away -- in the place where a boat is parked. The outcrops here have a quite different colour from all the other rocks in the area, and a different surface patina. (I did not have time to investigate properly.) One of the darker standing stones has a groove on its surface which looks as if it might have resulted from glacial erosion -- but I would not stake my reputation on it. Another source of confusion arises from the fact that for the greater part of 5,000 years the Callanish stones have had their lower parts buried in an extensive "protective" cover of blanket bog. When the stones were excavated, this layer of peat was stripped away, and records show that it was in places 5 feet thick.
The 3 Callanish monuments were built here because this is where elongated slabs were available in some abundance. For the most part the stones would have simply been available as litter on the flanks of some of the glaciated knolls in the Callanish area. One cannot discount the theory that some slabs might have been forced from the rock face, but this sort of human intervention would generally have been unnecessary, since there must have been an abundant litter of slabs available on a relatively clean rock surface in Neolithic times. Indeed, many slabs and pillars are still available, lying about in the landscape, and either unused or rejected as being unsuitable (too heavy, not elongated enough etc). Many of these remaining stones are buried or partly buried in the turf. I can see no evidence of serious quarrying. Neither do I see any evidence that the stone settings were put up in predetermined "auspicious places" (related to astronomical alignments etc) and that the builders travelled far to find stones to fit what they were intent upon doing. Callanish 1 is in quite a prominent position, on a ridge, so it might have been put there as a sign of status, or to impress neighbouring tribes etc. But to claim anything else would be fanciful. Callanish 2 and 3 are not in very prominent positions at all -- but they are very close to potential rock sources.
So was there a huge and deliberate quarrying exercise designed to provide stones for a group of stone monuments in predetermined auspicious locations related to astronomical alignments? No -- the evidence on the ground does not support this.
(My thanks to my colleague Prof David Sugden, in whose company I visited the site and whose observations and suggestions are incorporated in these notes.)