Everyone's familiar with the great henge at the Ring of Brodgar in the West Mainland, just up the Brodgar peninsula from the ongoing excavation at the Neolithic site of the Ness of Brodgar. The 'henge' part of the Ring of Brodgar is not the standing stones, it's the ditch around it, and it's been about 4,000 years since anyone saw this in anything like all its glory, since the ditch has badly silted up over the millennia. The ditch is however the one bit of Brodgar that's been subject to modern excavation: first in 1973, when the great archaeologist Colin Renfrew - in those days Professor Renfrew, nowadays Lord Renfrew of Kaimsthorn - excavated two trenches across the ditch on the north and south sides of the Ring, and more recently in 2008, when Renfrew's trenches were reopened and enlarged by a team from Orkney College UHI.
The scale of the Neolithic undertaking at Brodgar is immediately apparent - that's bedrock they've cut through more-or-less from the top, using only antler picks and cobbles stone tools, maybe wooden or antler wedges, and an awful lot of sweat and determination!
You don't really appreciate the scale of the ditch at Brodgar until you're standing in the bottom of it looking up, and then it's difficult to get your head round the fact that this ditch goes all the way around the Ring of Brodgar and is 122m in diameter (measuring to the centre of the ditch), compared to the 104m diameter of the ring of stones.
Interestingly at the base of the ditch the Neolithic stonemasons went with the natural lie of the stone,they didn't try to shape it to give a completely smooth profile from the top, so you get a very angular finish, following the natural bedding planes of the rock:
Sadly, after the end of the 2008 excavation, the trenches were backfilled and so are no longer visible,although if you know where to look you can see where they were, because the heather hasn't grown back over that area. A great shame - I always think that when the Ring of Brodgar was new it wouldn't have been the stones themselves that were particularly awe-inspiring, but rather the immense ditch and the thousands of tonnes of rock that must have been removed to dig that ditch through the bedrock.
Renfrew's excavation of a number of Orcadian sites, including Maeshowe and Quanterness chambered tombs, along with the Ring of Brodgar, was published as a monograph in 1979, and this can be sometimes be picked up second hand - it's well worth it if you can get hold of a copy:
Renfrew, A C 1979 Investigations in Orkney London: Society of Antiquaries, Reports of the Research Committee of the Society of Antiquaries of London 38