After more than a century of searching, researchers may be getting closer to understanding the origin of the altar stone (weighing more than six tons and almost five meters long) that lies at the center of Stonehenge. The age and chemistry of the minerals that make up the red sandstone block They point to an area of Scotlandabout 750 kilometers from the monument, researchers reported today in Nature.
“It’s amazing,” he says. Susan Greaneyan archaeologist at the University of Exeter in England who was not involved in the work. It is “really exciting” that the team has identified a site in the far northeast of Scotland (possibly in the Orkney Islands) that appears to have been a hotspot of Neolithic culture and activity. Stonehenge is on Salisbury Plain in southern England, and construction began about 5,000 years ago, around the same time. The find, Greaney says, “highlights links between those two areas that have been, until now, kind of hypothesized.”
Questions about how and why ancient people built this stone circle have long puzzled researchers. And within this string of huge questions, even the origin of the stones is a mystery. Recent research work have allowed to locate the origin of the sarsen stones which form the iconic outer ring of Stonehenge about 25 kilometres north of Salisbury Plain. The monument’s bluestones have been associated with Wales since the 1920s, and Bevins and his colleagues have located some of the stones in outcrops in south-west Wales, about 225 kilometres from Stonehenge.
But the so-called altar stone has remained an enigma, despite efforts to determine its origin since the 1870s and 1880s, says Richard Bevins, an Earth scientist at Aberystwyth University in Wales and one of the authors of the study. The stone’s true use remains unknown, but its placement evokes an altar, hence its name. “This stone is different from the bluestones because of its weight, its size, the type of rock it is and its position in the monument,” he explains.
Bevins has been searching for the altar stone’s origin for 15 years. His team has ruled out dozens of possible sites by comparing its chemical composition to outcrops across Wales and parts of England. And now, at last, they have found a match. “It’s extraordinary. You have to pinch yourself every now and then,” he says.
This time, Bevins teamed up with Anthony Clarke, a graduate student in earth sciences at Curtin University in Perth, Australia, to borrow techniques from geology. Working from a fragment of the altar stone cut in 1844 and verified as matching the chemical composition of the block, the researchers first identified the ages of the various minerals that had bonded together to form the sandstone, then compared the results from the altar stone to data from outcrops of sedimentary rocks in Britain and Ireland.
“The only area where there was a match was in the northeast of Scotland,” says the study’s co-author. Nick Pearcea geologist and geochemist also from Aberystwyth University (Wales). “These footprints are very distinctive. I don’t think there is anywhere else where they match up,” he says.
The site, called the Orcadian Basin, covers several thousand square kilometres and is up to eight kilometres thick in places. However, the exact source of the stone remains unknown.
It is also unclear how the Neolithic people managed to transport the massive rock over such a vast distance. Some have suggested that the monument’s stones might even have been transported by glaciers. Based on what is known about the movement of glaciers in the British Isles, “it is virtually impossible that a block of sandstone of that size could have been transported by ice from northern Scotland to Stonehenge,” he says. David Nasha geomorphologist at the University of Brighton in England who was not involved in the study. However, Nash acknowledges that glacial transport would have been possible for part of the route.
Another hypothesis is that the ancient builders may have transported the altar stone overland (although this is also unlikely). Scotland is “incredibly mountainous” and Britain had a lot of forests at the time, Greaney says.
Instead, the authors suggest, the ancients may have transported the block by sea. They probably sailed along the coast, and then perhaps inland by river before carrying the stone overland to its resting place, they say. Jim Learyan archaeologist at the University of York in England who was not involved in the work. The exact time of the block’s passage and its arrival at Stonehenge are unknown. But there is evidence that other heavy objects, such as livestock, were transported by sea at the time, and that sea vessels were available to travel between islands.
The stone must have been of great importance because of how far the Neolithic people moved it, Leary says. It’s possible the megalith was used in other stone circles or monuments along the way, he says. “You can imagine that stone may have had a very long and winding route over perhaps a period of more than 100 years before it ended up at Stonehenge.”
The altar stone’s long journey highlights the connections between different communities in what is now the British Isles, Greaney says. Archaeologists have drawn links during the Neolithic between sites in Scotland, Wales, England and Ireland through similarities in tools, pottery and monuments. Evidence of prehistoric pig roasts at sites near Stonehenge They also suggest that the area united disparate communities.
People apparently travelled from place to place carrying ideas and ritual practices with them, and they probably shared religious beliefs and had similarities in their language and in the way they organised their societies, says Greaney. Even the central position of the altar stone at Stonehenge tells us about the importance of these ties. “It’s an unusual stone, lying flat in the centre, with the rest of the monument arranged around it,” he explains. These connections, he says, “must have been very important to those who built Stonehenge, otherwise they wouldn’t have placed it in such a prominent place.”
