The question of the ditch and bank
We know without a shadow of a doubt that the Ring o' Brodgar was originally surrounded by the deep circular ditch, typical of a henge monument. Sections of the ditch were excavated in 1973, and again in 2008,
This ditch remains clearly visible today, but having filled in over the millennia, is only a shadow of its former self. It has a diameter of around 123 metres (403 ft) and was originally three metres (9.8 ft) deep and seven metres wide.
Cut into bedrock, it has been estimated that around 4,700 cubic metres (11,000 tonnes) of rock had to be quarried to create the 380 metre long ditch - a momentous task previously estimated to have taken 80,000 man-hours.
This is the equivalent of 100 people working ten hours a day, for 80 days. Having seen the depth of the ditch, I wonder if that figure is slightly conservative.
Today, the ditch is much shallower than it was. Despite this, it is still possible, with a little imagination, to stand at the Ring's south-eastern entrance causeway to visualise how the site originally looked.
Bank, or no bank?
Strictly speaking, the Ring o' Brodgar is not a henge monument. Because there is no evidence of an external bank, the monument does not fit into the classification of a henge.
There is no trace of any bank today - something which would be unlikely if the bank was, as has been suggested by some, constructed from the rock quarried from the ditch.
In addition, the fact that the South Knowe was constructed so close to the ditch makes the presence of a bank unlikely. If there had been one there, the knowe would have been built on top of it.
However, given that both the Standing Stones o' Stenness and Maeshowe both had encircling banks, some contest that Brodgar was the same.
The purpose of the ditch - and bank, if it existed - is not completely understood, although it is commonly accepted that the monument had a ritual, or ceremonial, purpose. With this in mind, the ditch/bank probably marked the enclosed area as “special”, or sacred.
However, theories abound, with some suggesting that any bank kept the ceremonies held within the ring "private" while others feel that it may have had some function in the site's use as an astronomical observatory.