The 6-tonne stone at the centre of Stonehenge may have come from a location in Scotland more than 700km away from its final placement, new research has suggested.
The Altar Stone was long believed to have come from Wales, but may have been transported via sea, according to research led by Curtin University and published in science journal Nature.
The peer-reviewed study, funded by an Australian Research Council Discovery Project, was a collaboration between Curtin, the University of Adelaide, Aberystwyth University and University College London.
Stonehenge, a UNESCO World Heritage Site, is located on the Salisbury Plain in Wiltshire, England, and is believed to have been constructed between 3100 BC and 1600 BC.
Previous research has identified two types of stone used to make its inner and outer rings.
The outer ring is made up of standing sarsen stones originating from a location about 25km away, while the inner ring is made up of smaller "bluestones", of which the Altar Stone is the largest.
Lead researcher and Curtin University PhD candidate Anthony Clarke said their team had analysed two fragments of the stone.
"As of last year, the ultimate source of the Altar Stone was an open question," he said.
"We've had [work] ongoing on the Altar Stone … [and it] has a provenance all the way in the Orcadian Basin of north-east Scotland, some 700 to 800 kilometres away.
"This is a completely unexpected result, given that we thought it was going to originate in Wales."
Professor Chris Kirkland of Curtin University's School of Earth and Planetary Sciences said they had examined the Altar Stone's "fingerprint" using the "little crystals" in the sandstone.
"What we can do is look at those little crystals … [they] contain small amounts of uranium, and we know the rate of change [from] uranium to lead," he said.
"By measuring the … uranium and lead within these little crystals, we have miniature clocks [that] tell us the original age of the material.
"We can [then] build up an age fingerprint, much like DNA for a rock. Then we can compare that to a large database of other rocks around the United Kingdom [and] compare the age."
The team also investigated several ways the stone could have moved or been transported such a long distance to its final location in the centre of the monument.
University of Adelaide associate professor Stijn Glorie, a co-supervisor of Mr Clarke's PhD studies, leads the Australian Research Council Project, which funded the research.
"We collected age data on minerals inside this [sample of the Altar Stone] to understand the fingerprints of the rocks and then link that with rocks in Britain," he said.
"What this study shows us is that there was a long, long transport mechanism involved for those people to bring the rocks from Scotland.
"We first looked at the possibility that these rocks were just transported by glaciers, and we could rule this out because of the flow directions of these glaciers at the time were the opposite direction.
"So it was kind of impossible. The other option would then be that these humans have transported it themselves.
"Over land that becomes tricky, because of huge obstacles along the way, so we quite quickly get to the conclusion that the only possible way really to bring that rock from Scotland to Stonehenge is over [the sea]."
Many questions surrounding Stonehenge, including its original purpose, remain unanswered.
"It's been something that's been, particularly in Britain's history, quite important," Dr Glorie said.
"So it's something that is in people's minds and understanding how it was built and how it was formed has clear importance for understanding history.
"One key question, and I don't think we can answer it at all, to be honest, is the why?
"Why would those people go all the way to Scotland to bring a rather unremarkable rock all the way to Stonehenge?
"Obviously there is no written record, so it's hard to work out why that would be so.
"We can only speculate on this, it's a very hard research question. Why would they have come all the way to Scotland [for] a rock that looks not much different to rocks that are locally sourced?
"Maybe it has some kind of spiritual meaning to them."
