The bird struts across the henge as if its owns the place. It is quite a sight itself: large, confident, and long extinct. It was re-introduced here a decade ago but is rarely seen. Great bustards are shy, befitting the fact that they look very edible. This one, for a strange reason, had lost its fear of humans and it walks right up to us to have a better look. Its stony-coloured feathers mirror the stones behind it. The unexpected experience complements the historic atmosphere of Stonehenge. The many tourists walk in a large circle around the ancient, famous monument. Only the bustard is allowed inside the walk way. It really does own the stones.
After 5000 years, Stonehenge still fascinates. The massive stone ring stands in sharp contrast against the surrounding fields. Over time (lots of it), many of the stones have fallen and what now remains is an irregular echo of the original. Especially the innermost area is a jumble of fallen stones. Obviously, the British of 4000 years ago, just as their modern counterparts, did not believe in either building standards or maintenance. But the many tourists are captivated, pointing out detail to each other in a variety of languages, or posing for a selfie with the ancient stones. The path takes us around the stones but getting in among the stones is only for the privileged few, and strictly outside of normal opening hours. Too many people have tried to carve their initials into the stones: access has had to be limited just for safeguarding. If it has this effect over us, how much more would the visual power have been when the monument was at its prime? Stonehenge always was a magnet for the imagination. The site was never inhabited; instead people lived some miles away. There were no defensive fortifications: it never needed them. Just as China subsumed its invaders, so Stonehenge possessed each new possessor. Even the Romans adopted the site for their rituals.
But Stonehenge was always a work in progress, and was never finished. Generation after generation shaped it according to the new needs and fashion. It started out as a circular bank, which was build between 3000 and 2920 BC and which is still visible in the landscape. The material for the bank was dug out from the chalk underneath. This was a common technique, but normally the ditch was on the inside of the bank. At Stonehenge the ditch where the chalk was dug out was on the outside, an arrangement more commonly found in ancient meeting places or burial grounds. At the time the circle must have gleamed white in the landscape, visible from far away. Even at that time, this was a ceremonial site, not a fortification or habitation. There were circles of wooden poles inside the bank: the holes in the chalk are still there. (Others have suggested that even in the earliest times, the holes were for standing stones rather than wooden poles.)
A major upgrade followed 400 years later, when the inner area of standing stones was added. This included the ring (number 12 on the image below) of 30 standing stones, linked by the cross bars. 17 of these stones remain standing (the rest are fallen, broken, and some are completely missing!), and only 6 of the lintels are still in place. The ring is most complete on the northeast side. On the opposite side there is much less left. That is more than just the workings of time. The stones on the northeast side were well prepared, and the stone surfaces were worked to be flat. Even the lintels were shaped slightly curved, to follow the circle. The stones on the other side are much less regular, and one of the remaining stones here is much smaller than the others, and is too small to have carried a lintel. Perhaps the stones on this side fell more easily because they had not been put up with as much care! The outer ring may not even have been complete.
The care put into the stone work on the northeastern side shows that Stonehenge was build to be viewed from that direction, and in fact the ancient path towards it runs here. This path lines up with the solstice: people walking towards Stonehenge at mid-winter would have seen the sun set directly behind the monument. It works both ways: looking from within the stones along the path, it lines up with the rising sun of the summer solstice. But the asymmetry shows that Stonehenge was first designed for the winter solstice.
Just inside the circle of large stones, a second circle was build (it is number 13 on the drawing). This was made of much smaller stones. The inner circle contained perhaps 60 stones. They were not worked as well, Strangely the workings on two of these stones show that they had been used as lintels at an earlier point in time, but were re-used as standing stones in the circle. Only a few of these stones survive. It seems that originally, they may have been arranged in a double arc, and later (around 2200 BC) were relocated into a complete circle.
More stones were added inside the circle, probably as part of the major development of around 2500 BC. These stones were much taller, and they formed a horseshoe arrangement. The horseshoe consisted of five pairs of stones with a lintel, graded as to their height along the horseshoe (number 14 on the image). Each pair is called a trilithon. Three of these pairs still stand (one of these had fallen but was re-erected in the 1950’s). The largest of the pairs has suffered a collapse where one side and the lintel fell, but the other stone still stands: it is over 7 meters tall. There are axe blade marks on these stones, but this happened 700 years later. The horsehoe must have been erected at the same time or before the outer ring of stones was put up, as the gaps in the ring would be too small for the inner stones to pass through.
Inside this horsehoe was a second horseshoe, or perhaps semi-oval, made of much smaller pillars. As in the trilithons, these stones were graded in height with the tallest on the closed side of the horseshoe. These stones are the most carefully worked of all stones at Stonehenge. They did not have lintels, but it seems some had been attached to lintels before, at another location.
There are other stones at Stonehenge, which are not part of this general structure. One of these is at the centre of the horsehoe, but it is partly buried underneath the fallen stone and lintel of the largest trilithon and can’t be seen from the tourist trail. It is called the altar stone, but this is a made-up name as we have no idea what it was for. It was already lying on the ground when the trilithon fell on it. It may originally have stood upright, as do all other stones, but this is disputed. It was in some ways the centre piece of Stonehenge. The inner faces of the trilithons, facing the altar stone, are finely worked while the outer faces are more rough: this shows the importance of the central area. There are other individual stones scattered around the site, mostly close to the old circular bank: these include the evocatively (but incorrectly) named slaughter stone.
The current placements of the stones date from around 2200 BC. The most recent publications suggest that between 3000BC and 2200 BC there were 5 separate phases of building and rebuilding. After this, the re-workings ceased apart from the addition of two other rings of holes which apparently were never used. Up to that time, each generation left its mark.
An idealized view of Stonehenge is shown in the drawings, which show the closed ring, the five trilithons making up the horsehoe, the ring of smaller stones inside the closed ring and the smaller stones inside the horsehoe (click on the images to view in higher resolution). All these aspects were present, but Stonehenge never looked like this. This always was a building site.
Farmers finally moved in on the surrounding fields; by this time Stonehenge appeared to have fallen out of use. The Romans re-invigorated the site but they too were temporary. The first written descriptions of the ‘doorways’ appeared around 1130 AD. Archaeologist arrived a few hundred years ago and started digging around the site and speculated about how it was build and why. They were followed by the tourists, and finally by the druids, a modern re-invention of a pre-roman cult which had very little to do with Stonehenge. Everyone adopted Stonehenge to their own purposes. It still holds that power over us.
Monuments need maintenance. Since its development ended, Stonehenge has suffered a slow decay. A standing stone can be a liability: it can be prone to toppling. At least two stones have fallen in the past few hundred years. Stones may have disappeared entirely, perhaps use by a local farmer in need of of some stone. There are more holes on the site than there are stones. However, stones may also have occupied more than one hole, so that we don’t know accurately how much has been lost.
The site was last sold in 1915: both the head of the family that owned it and the inheritor had died on the fields of France. A local farmer bought it on impulse, but some years later donated the site to the nation. It earned him a well-deserved knighthood. Nowadays, Stonehenge attract a million visitors per year, each involuntarily donating their 21 pounds to English Heritage. The ticket buys you entrance to a fairly sparse exhibition centre, a model village, a bus to the site itself (or you can walk – it is the same price and you get to see more) – and the most monumental views in the world. Spend 6 pounds more (highly recommended) and you get an excellent booklet about Stonehenge.
Stonehenge is a monumental marvel, and is recognized as a world heritage site. But the stones are themselves also of interest. There are three main types: the chalk of the wall, the sandstone of the large stones, and the bluestone of the smaller stones. The altar stone is a sole example of a fourth type, a greenstone. All have history that goes far beyond the human presence. One type even holds a volcanic heritage. There is a story here – and it stretches across the most recent supercontinent on Earth.
Stonehenge is build on chalk. Chalk underlies much of southern England, surfacing in various places, most spectacularly on the white cliffs of Dover. On the map below, the surface chalk is shown as lime green, a nominatively suitable colour. Around London a basin formed and here sediment has covered the chalk. Of course Dover is not the termination of the chalk, but is just an interruption. The chalk continues in France.
The Latin word for chalk is creta, and this word gives a clue to its age. Indeed, the local chalk formed in the Cretaceous, 80-100 million years ago, when a tropical sea covered much of Europe. The sea was full of cocoliths, minute marine algae with shells; these shells dropped to the sea floor and formed the chalk. The chalk consists of carbonate, and the whiteness shows that the water was crystal clear. In the chalk one can see thin layers of chert and flint, which are especially clear at Dover. This is quartz. Typically, these layers indicate where a layer of mud had formed. The mud came from organisms in the sea water such as diatoms, which secreted silica, and the silica formed the flints.
But why was there so much water over Europe? This was a time of high sea level: the sea had come in and covered all of the continental shelf and quite a bit of the coastal areas. Sea level may have been 200 meters higher than now. It was a warm period without sea ice, and the warm water expanded and rose. The sea had become unstoppable. But there was a second reason. This was a time of rifting. The supercontinent of Pangaea was breaking up, and the Atlantic Ocean was forming. The rifts were busily forming new sea floor, and did so at a high rate. But this sea floor consisted of warm rock, replacing the cold ocean crust which was subducting elsewhere to make space for the new crust. Warm rocks expand, are lighter, and float higher. The new sea floor was therefore much less deep than the old crust. The water had nowhere else to go but up.
The good times (for sea creatures) did not last. The new crust cooled and the sea level dropped. The fragmented continents drifted apart, but elsewhere began to collide and push up new mountains. One of these fragments was Italy, which in a moment of careless drifting crashed and caused the Alps. As a distant ripple of the Italian collision, the British-French chalk region was also pushed up. The elevation of this ripple reached as high as 1 kilometer in the Weald. The peak elevation eroded and this removed the chalk in the Weald, whilst elsewhere the chalk survived, now well above sea level.
The chalk has also been subject to the stress of the ice age. The glaciers left deep striations in the chalk around Stonehenge, running roughly southwest-northeast. They are nowadays difficult to see, but these striations may have been a reason to build Stonehenge here, as they approximately lined up with the solstice.
Chalk is a soft stone. The earliest bank of Stonehenge was made from chalk which was dug out using deer antlers. Here, unusually, bone is harder than stone. Try to imagine what it would be like to dig with antlers, and it becomes understandable that the ditch eventually became quite irregular.
The largest stones of Stonehenge are so-called sarsen stones. These were used for the outer ring and for the trilithons of the horsehoe. The individual stones elsewhere at Stonehenge also are sarsen stones. In total, 53 sarsen stones remain, out of perhaps 80 which were present in the heyday of the original monument. The stones are huge: the weights range from 4 to 35 tons each.
Sarsen stones are a sandstone. They are a silcrete, formed when sand became cemented by silica. It is similar to concrete, and like concrete it forms where there is an abundance of water. The silica is dissolved in the water, and is redeposited when the water dries out. Sarsen stones are found across much of southern England. They formed during a time of tropical conditions, after the sea had withdrawn, between 23 and 65 million years ago. Because of the need for water, this probably happened along water courses. Even nowadays, the sarsen stones can be found lying on the ground in many places. The stone is hard and resistant to erosion. Much of the stone has been used for building. The sandstone is a poor material for houses (it lets too much water in) but it was used for paving, stepping stones, and graveyards.
So the sarsen stones are a memory of an age where southern England was a fluctuating mix of sand and water. After the Cretaceous, the area to the west and northwest, from Brittany to Wales, was uplifted. There was a sea arm south of England and the North Sea basin already existed, both associated with the continuing formation of the Atlantic ocean. In between was a delta area which was at times submerged under a shallow sea. Rivers brought the eroded material to the south and deposited the sand. Later, the sand became cemented by silica. It was this combination that formed the sarsens. Some argued that the sarsen stone formed a cap over much of the area, but it is perhaps more likely that it mainly formed along the river courses.
There are sarsen stones near Stonehenge but they are small and are unlike the ones used in its structure. Already in the 18th century, the area around Marlborough was identified as the most likely origin of the Stonehenge sarsens. It is about 30 km north of Stonehenge. The sarsen stones here are sufficiently large and thick (1.5 meters) to fit the Stonehenge requirement.
The most likely source is at the top of the ‘downs’ (hills) above Clatford where a sarsen quarry may have existed. The stones here were tabular and much better shaped than those in the valley. Old reports states that near Clatford, there once were 12 stones lying which had been partly prepared, and were in the right location for transport to Stonehenge. That site no longer exists: it is now covered by the A4 road. Stukely, in the 18th century, wrote ‘Perhaps those were stones going to Stonehenge for they seem to have been brought from the top of the hill northward thence where upon a long ridge there are many stones of Vast bulk besides what lye in all the valleys round in great quantity.’ The ridge is near Totterdown Wood. But by 1812, the sarsen stones here had been destroyed for building works.
The stones were transported to Stonehenge where the dressing of the stones took place. A working area has been located about 100 meters north of Stonehenge, next to the avenue. This shows that the stones came from the north. Stone chips show that the stones were worked in this area, before being erected.
The solitary stones are Stonehenge are also sarsen stones, which unlike the main monument were not dressed. Among these is the slaughter stone, which lies on the ground next to the tourist trail. It is a rough sarsen stone. When it rains, water collects in the hollows and can become red-tainted, giving the impression of blood – thus the name. Of course, it is as historically incorrect as the Stonehenge druids. The stone originally stood upright: the hole where it resided is still there. There is a second hole, and there may once have been two stones (or the stone was moved, or the first hole was dug but rejected by the architect: archaeology requires a lot of conjecture). The sarsen stones contain whatever elements and minerals were present in the sand and water from which they formed. Many contain some iron, and this is what stains the water. There is no connection to blood, unless one considers that blood also gets its red colour from oxidized iron!
The altar stone differs from all other stones at Stonehenge. It too is a sandstone, but it is a 6-ton greenish sandstone with mica. As it is buried underneath the huge fallen sarsen in the heart of the monument, no real excavation has been possible. However, the difference with the other stones is clear. There is no similar natural stone in the general area and it must have come from a considerable distance.
The precise origin is not known, but the nearest similar stone is found in the Senni formation. This is a layer of sandstone in south Wales, running in a narrow strip from Llanelli to Herefordshire. This puts the source of the altar stone in the Brecon Beacons or the Black Mountains. The Senni formation dates to 400 million years ago, making the altar stone much older than the sarsen stones which now cover it.
In fact, the Devonian deposits of which the senni formation is part, are among the oldest parts of England and Wales which formed above water. (Scotland has much older rocks, though). They formed in a similar way to the sarsens, along long river beds which deposited sand. The sand was became cemented in places, but here the cementing was done by calcium rather than silica: this is called calcrete.
The image shows the lay of the land in the early Devonian. This was just after the closure of an ancient ocean, the Iapetus. The Caledonian mountains formed along the closure: they can still be seen in Scotland, the Lake district and North Wales. South of the suture was a flat, wide delta created by sediment from the mountain range. The senni formation is a layer within this delta, which is exposed in places south Wales.
So we know the approximate area the altar stone came from but we cannot be fully certain of the precise origin. We do not have certified fragments of the stone available for analysis. What we have are chipped fragments taken from the location where the stones were worked, and although the chips we have are likely to come from the altar stone, this is not proven.
There is some colour confusion apparent in the naming. The altar stone is a greenstone, coloured by glaucite. But the Senni bed belongs to the old red sandstone formation. To make things worse, the generic name for Welsh stones and Stonehenge is bluestone. This humble, buried centre piece is red, green and blue all at the same time.
Bluestones and the Stonehenge volcanics
The other stones of Stonehenge are found in the inner circle and the inner horsehoe. They are much smaller, and are called the bluestones, a colour which is only (just) seen when they are wet. There are much bluer rocks in the UK, especially the blue liam of Lyme Regis on the Jurassic Coast. But you can’t make monuments from them. Instead, the Stonehenge builders sources the most impressive rocks they could find. But from where?
The answer to that has been known since quite a long time. The type of stone is unique to the Preseli Hills, an area in Pembrokeshire on the west coast of Wales. It is a range of hills, up to 500 meters high, which dominate the lower land around them. There are burial sites, hill forts, bank and circles on the hills: it was a popular place in the neolithic. An ancient pathway, called the golden road, runs over the ridge of the hills. The hills are some 200 kilometers from Stonehenge.
There are several potential neolithic quarries here from where the stones could have been taken. For a long time, the southern side of the hills was favoured for the origin of the bluestones. That was in part because of the possibility of moving the stones by sea. In recent years, attention has shifted to the northern side of the hills, where two specific sites were identified, the outcrops of Carn Goedog and Craig Rhos-y-felin. The outcroppings are a few kilometers apart. In both places, the rock cracks naturally to form large slabs wich are easily removed from the mountain. Carbon dating on the sites shows that quarrying took place around 3200-3400 BC, which is a bit older than Stonehenge.
The validity of this source is still discussed, but it seems clear that the stones came either from here or from a very nearby region. However, although from such a specific location, the bluestones are not uniform in composition. The main type found among the fragments of bluestones scattered around Stonehenge is dolerite. Dolerite is an intrusive rock type: it forms when magma intrudes underground but solidifies in situ. However, fragments of rhyolite and tuffs have also been found.
The Preseli hills are Ordovician in age, around 450 million years old. At the time, this area was below water: the hills consists of marine mudstone. The magma intrusion happened a bit later. Erosion left the harder dolerite as outcrops above the softer mudstone. The sites identified above are for the dolerite bluestones, and these come from the intrusions.
During the Ordovician the Iapetus ocean still existed but it was already in the process of closing. Britain (without Scotland) was some 40 degrees south of the equator. Originally, it had been part of Gondwana, as a volcanic arc. During a rifting episode, the area broke off from Gondwana (an early brexit) and the microcontinent of Avalonia, including Britain, began to move north. To the northwest was the subduction zone that pulled Avalonia forward. On the far side of the closing ocean was Laurentia (effectively the early North America, including Scotland). To the northeast was another microcontinent, Baltica. The collisions followed in the Silurian, the period after the Ordovician. First, Avalonia joined America, and later both joined Baltica and Siberia. Now the northern continent of Laurasia had formed. After the Silurian came the Devonian, when the old red sandstone formed which gave rise to the Stonehenge altar stone. Some 300 million years ago, Gondwana caught up with escaped Britain, and the merger of Gondwana and Laurasia form the Pangea supercontinent. And much later, the break-up of Pangea led to the events that formed the chalk and the sarsen stones.
During the Ordovician, before the closure of the Iapetus, the area around Preseli was a back-arc basin, oceanic but without sea floor spreading. The rocks of Preseli were deposited in this basin. Other rocks of this age are found across West Wales and in the Lake District. The mixture of soft mudstone and harder rocks have left a dramatic landscape.
This story begins with the dolerite bluestones, but how about the rhyolite and tuff? Subduction is associated with volcanism. That happened here too in the Ordovician. Andesitic volcanoes formed: the eruptions formed the so-called Fishguard Volcanic Group. This was of course related to the dolerite: one formed below the surface, the other above it. The lava and tuff deposits are found especially on the eastern and northern side of the Preseli hills. Pillow lavas are common, showing that this was a marine basin. The Fishguard group has been dated to 460 million years ago, and this age agrees with those of crystals in the Stonehenge bluestone fragments. The Craig Rhos-y-felin quarry is within the region of the Fishguard volcanic deposits.
There is also evidence for a rhyolitic component to four of the Stonehenge bluestones. Rhyolitic rocks are found only in small area of basaltic pillow lavas. The precise origin is not known, but a relation to rhyolitic rocks exposed near Pont Saeson has been suggested. This is close to Craig Rhos-y-felin.
The route from here to Stonehenge must have gone over land as there are no harbours within reach of the northern slopes. The southern route is prohibited because of the steep ridge between the two sites and the sea. However, there is a fairly flat route from the north over land, which follows several river valleys. It does require that the river Severn was forded. The transport is now considered as quite doable for a group of determined people: the bluestones are much smaller than the sarsens, and typically weigh about 2 tons.
(It has also been suggested that the stones were carried to the Stonehenge area by the ice age, as erratics. This proposal suffers from a lack of evidence, and it has difficulty explaining why the ice took only stones from Preseli and took them only to the Salisbury plains 200 km away and nowhere else. Human transport seems far more likely.)
The bluestones appear to have been moved to Stonehenge quite early. Based on bluestone shards found nearby, it may been around 2900 BC. This is still several hundred years after the quarrying at Preseli took place. One suggestion is that the original Stonehenge monument was not at Stonehenge but in Wales, and that this older stone circle was moved en mass to a new location and a new use. This would explain the several hundred year delay. Why was this done is something we may never know. Was it a vanity project by a powerful leader? Such things are well known in England: the useless but celebrated monuments build by people with too much money are called follies.
The bluestones may thus have been the first stones to arrive at Stonehenge, when it transformed from a circular bank to something altogether more impressive. This would fit the suggestion that the holes inside the bank were not used for wooden poles, but for standing bluestones. They may have been the same stones, later re-used time and again during subsequent redesigns of the site. A 2-ton stone is easier to move, after all than a 20-ton one! The stones were not dressed in Wales: the shaping took place only at Stonehenge.
Stone of ages
The stones of Stonehenge are an eclectic mix. The oldest are twenty times as old as the youngest. All are somehow related to Pangaea, the most recent supercontinent to exist on Earth. The oldest rocks are associated with the collision of Avalonia with Laurentia, which was a crucial step in the formation of Pangaea. The chalk stone and the sarsens are associated with last stages of the break-up of the supercontinent, and the completion of the Atlantic ocean. Perhaps Stonehenge should be called The March (or Rise and Fall) of Pangaea. Humans did have to move the heavy rocks – but plate tectonics did much of the work for them.
Our time at Stonehenge has come to an end and we have to head back to begin the drive home. Joining the queue for the bus, we have a final look at the old ruin, and the circle of barrows on the surrounding ridges. Stonehenge is a very human monument. It is a ruin from our distant past. Once this was the place to be seen, or at least to be buried. In a way, it still is. So much effort has gone into building it, nameless people dragging many tons of stone across 200 kilometer of undulating, spectacular but dangerous countryside. They too were under the pull of Stonehenge. But it is more than just human history. Stonehenge is also the result of the blood, sweat and tears of the Earth itself. The stones that made Stonehenge once build a continent. Stonehenge is also a monument to the Earth itself. It really does bring the past to life, both human and geological. Even that great bustard is a memory of the past. Once it roamed the plains, and now it has come back from extinction to reclaim ownership. It too belongs to Stonehenge.
Albert, July 2019
(This post is closely related to a previous post on another stone monument in the UK: The stones of Calanais)