Early speculation suggests the giant structure could have been the first building created at the Ness of Brodgar – an area where human remains, Neolithic seaweed and rock art have previously surfaced – or a chambered tomb.
It was covered by the largest Neolithic rubbish dump in Scotland, according to experts describing the stones as “perplexing”.
“The sheer size and scale of the stones unearthed are unprecedented on this site,” says Nick Card, the Site Director who oversaw the excavation of the round-edged stones.
“The way the stones are built into the construction is also unique to the Ness. This all suggests that they may have been re-used and taken from elsewhere.
“Perhaps they may be part of a stone circle that pre-dates the main Ness site. It is all a bit of mystery and we won’t know more until we do more work.”
The imposing nearby Watch Stone, described as “a great sentinel stone” by James Wilson in his account of the coasts and isles of Scotland in 1842, is a larger monument, standing more than 19 feet over the Brig o' Brodgar.
Archaeologists believe the newly-discovered behemoth could have been a twin to the stone, possibly as part of a largely-dismantled former stone circle.
The team plan to return to the site in pursuit of more certain conclusions next year.
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Three sites to find history in Scotland
National Museum of Scotland, Edinburgh The rare and exquisite jadeitite axeheads found around the Scottish countryside have long puzzled archaeologists. How did these beautiful Neolithic axeheads end up in Scotland, so far from their origins in the North Italian Alps around 6,000 years ago? Find out in the current exhibition, Stone Age Jade from the Alps. Until October 30 2016.
The Scottish Crannog Centre, Perthshire The Scottish Crannog Centre features a unique reconstruction of an early
Iron Age loch-dwelling, built by the Scottish Trust for Underwater
Archaeology. This authentic recreation is based on the excavation
evidence from the 2,600-year-old site of 'Oakbank Crannog', one of the
18 crannogs preserved in Loch Tay, Scotland.
Auchindrain Museum, Argyll Auchindrain was the last inhabited Highland farming township, a place
where people lived and worked from the medieval period up until the
1960s. In Highland farming townships, land was held and worked in
common, and the typically clustered layout of this type of settlement is
a reflection of the tight-knit nature of the communities that belonged
to them.