The Ring o'Brodgar is 340 ft in diameter, and originally contained 60 stones, of which 27 still stand today. The stones - which range in height from about 7 feet to a maximum of a little over 15ft - are set within a circular ditch up to 10 deep, 30 ft wide and 1,200 ft in circumference that was carved out of the solid sandstone bedrock.
It is unknown when the site was built, by whom, or for what purpose (though there are many speculations of course: see, for example, this book by Christopher Knight and Alan Butler, that connects sidereal days, pendulums, the "Minoan foot" - an ancient unit of measure used for the construction of palaces in Crete c.2000 BC - and the planet Venus). Current best estimates place its origin at between 2500 BC and 2000 BC.
More details about the Ring o'Brodgar, and the other monuments making up the Heart of Neolithic Orkney, can be found in this report, published by Historic Scotland.
Personal Note. My wife and I visited the Ring o'Brodgar several times during our stay in Orkney. We were both drawn to its mystery, and enchanted by its timeless aura. As I wandered around with my camera, looking for angles and compositions, dodging the inevitable tourists (such as ourselves) to get clear shots of the stones alone, I felt myself drift in and out of the time of the "here and now" into a more ancient, and ineffable, time; a time that lurks somewhere in the shadows, and is a part of the very fabric of the megaliths themselves.
Mindful observers are seduced with glimpses of a parallel world that coexists with ours, but whose essence transcends the "normal" dimensions perceivable via our physical senses alone.
The Ring o'Brodgar is - for me - a physical symbol of timelessness and transcendence. It is a place for serious contemplation and meditation. A boundary between all that has been forgotten and the just as mysterious unknown future history that is yet to be written.
Through it all - immersed in time (and succumbing to time's inexorable gift of entropy), yet strangely unaffected by it (since its secrets are too old for even time to recall their true origins) - the Ring o'Brodgar's eerie silence beckons with its magical siren call.
I've posted a gallery of shots from our Scotland trip here.