It was the main element of Stonehenge complex which remained to be identified. Its construction began just under 5,000 years ago. The outermost megaliths, called sarsen stones, come from a nearby quarry, about 25 kilometers to the north. The inner circle, that of the bluestones, comes from much further away, from the Preseli Mountains (Wales), about 250 km away. If we consider that what the Romans would later call Great Britain was in the middle of the Neolithic, even with many technological limitations, the distance is enormous. In the center, at some point during its erection, its creators placed a huge bluish rock as an altar. For years, there has been debate about its origin. Today, by combining sophisticated scientific and mining techniques, they have been able to date it and, even more, highlight its origin: in the Orkney Basin, in the far north of present-day Scotland. How and why did they go there to look for a huge stone? How and why was it transported to the south of what is now England? The lack of definitive answers adds even more mystery to Stonehenge.
Richard Bevins, an honorary professor at Aberystwyth University in the United Kingdom, and his former student, geologist Nick Pearce, had been studying the Stonehenge altar for years. As researchers at a Welsh institution, they sought to confirm that the central element of the main English monument was also Welsh. The last attempt was made in 2018. “The altar stone is located in the centre of the monument and is a bluestone, although very different from the others. It is about twice the size of the smallest igneous bluestones, weighs six tonnes and is five metres long, the others weigh about three tonnes at most,” Pearce described during an online press conference. “It is a type of rock, a grey-green sandstone that everyone in the UK seemed lost for,” he adds. Eventually, they had to give up and to acknowledge in a scientific work that the altar was not Welsh. They then decided to look for the quarry in England and the south of Scotland. They were there when they received an email from Anthony Clarke, a Welsh PhD student at Curtin University (Australia).
“It's a type of rock, a grey-green sandstone, that everyone in the UK seemed lost for.”
Nick Pearce, geologist at Aberystwyth University (UK)
“My PhD was on the dating of Welsh rocks [no relacionadas con las de Stonehenge] using the isotopes in them. Nick asked me if you wanted to try analysing the altar stone? Before I knew it, they were sending me samples to Australia for analysis,” Clarke recalls. “Why Australia, why send material halfway around the world for analysis? This is because of the diversity of equipment we have at Curtin University, partly because of the mining industry in Western Australia. “If you want to understand where your next iron ore deposit might be, you’re going to use the same tools to figure out where the altar stone came from,” he adds.
The stone is a sandstone, which means that it is composed of many small microscopic grains of minerals. The geologist of the Spanish Oceanographic Institute (IEO-CSIC) José Antonio Lozano, not linked to the study, recalls that for “past societies it was a very good rock, because it could be cut; In fact, many cathedrals and Renaissance and medieval buildings in the south of the Iberian Peninsula are made of sandstone. These microscopic grains, their presence, their distribution, their age… give a certain stone its own fingerprint that provides information about its origin. “It is vital to map its territory. We geologists went into the field with a hammer, a magnifying glass, a compass, a map and good boots and you determine on the map which materials are everywhere. Then very detailed studies are carried out on the age of each of these materials, how it was formed, the genesis. These maps are essential to the strategic resources of each country, to know where to find gas, rocks for a quarry, aggregates for roads, etc.,” explains Lozano.
British scientists obtained the petrographic fingerprint of the altar and compared it to that of different regions of Great Britain. To complete it, they used the presence in the rock of small amounts of three minerals, zircon, apatite and rutile, which, as Clarke, the first author of the research, recalls, “fortunately for us geologists, they contain uranium,” that is, they emit detectable radiation. “Over time, the uranium disintegrates into lead and acts as a miniature atomic clock,” he adds, concluding: “If we analysed enough grains in the stone of the altar, we could establish a fingerprint of its age, by comparing it to rock outcrops. Britain and Ireland and even northern Europe. The place where rocks like those of the altar exist is the Orkney Basin, in the far north of Scotland.
The Spaniard Lozano highlights here the main strength, but at the same time a limitation, of these results: “This fingerprint is a statistic. That is to say, what they have seen is that there are zircons that give a peak in a billion years, another peak in one thousand five hundred million years, another in one thousand six hundred… and this leads them to say that these age peaks make the sandstone of the altar more similar to the sandstone of the Orkneys than to any other. To solve the problem, they would have to locate the exact quarry, which they are going to try, although it will not be easy after almost 5,000 years.
The rest of the work, published in the prestigious review Nature This is necessarily more speculative. The history of the stones is easier to reconstruct than that of their transport or that of the motivations which led the British Neolithic to move them from one end of the island to the other, more than 700 kilometres if they had gone in a straight line. As for the first thing, transporting a stone of this weight and size by means of a freight transport seems hardly credible. At this time, the inhabitants of the island still had no riding horses. The sarsen and the bluestones of the rest of the complex could have been moved with the help of logs, the first, and perhaps the most expensive, of Welsh origin. The possibility that the one on the altar was brought down from Scotland, taking advantage of the large expanses of ice remaining from the last ice age has been excluded. It may have worked in the north, but by then the glaciers of England had almost disappeared. By process of elimination, they only have the sea route left.
The researcher from the Archaeology Institute of Mérida Elías López Romero recalls that, although the British Neolithic is later than the continental Neolithic, and even later than the Iberian Neolithic, “long distances have already been traveled”. No remains of ships have been preserved, although there are remains of river canoes, but references to these have been found in the megaliths themselves.
“The alignments of Carnac [Francia]The dolmen of Antequera, Stonehenge… they functioned as a social medicine. When the Bronze Age arrived, they disappeared and were replaced by violence.”
Leonardo García Sanjuan, archaeologist at the University of Seville
The other big question is why. What leads to a pile of stone being pulled from the north and transported to the south? “We are once again entering the realm of interpretations,” recalls López Romero. There are several levels of symbolism here. “There is the symbolism of color, which many archaeologists have spoken about; the color green, the color blue, in this case they also have importance,” explains the Spanish scientist, who is not related to the research. There is also the origin. In the region where the stone was extracted was the colony of Orkney, in the northernmost part of Scotland. The entire region is full of megalithic monuments, but Orkney is considered at that time the most important Neolithic city of the islands, being the only one with stone foundations.
And finally, there is the destination, Stonehenge. The archaeologist from the University of Seville, Leonardo García Sanjuan, recognizes that the Orkney region was the most advanced of this culture. “But like the rest of the island, they were isolated communities that formed temporary aggregations once or twice a year in special places to satisfy many needs, commercial, political, spiritual, biological …”, he says. “At Stonehenge, as in Antequera [Málaga] Thousands of people would gather for days, especially around the solstices, and carry large stones that they considered special. The mobility of these rocks allows us to trace the movements of human communities,” he adds and concludes: “In the Neolithic period, the Carnac alignments [Francia]the dolmen of Antequera, Stonehenge… they functioned as a social medicine. When the Bronze Age arrived, they disappeared and were replaced by violence.
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