An impression of how the ring may have originally looked with the rock-cut ditch, and possibly an earthen bank, surrounding the stones.

The ditch and bank

The Ring o' Brodgar was originally surrounded by the deep circular ditch typical of a henge monument.

This ditch remains clearly visible today, but is only a shadow of its former self.

The ditch has a diameter of 123 metres (403.6 ft) and was originally three metres (9.8 ft) deep and five metres (16.4 ft) wide.

Cut into bedrock, around 4,700 cubic metres (11,000 tonnes) of rock had to be quarried to create the ditch - a momentous task estimated to have taken 80,000 man-hours.

These days the ditch is wider than it was, erosion at the edges widening it to around ten metres, twice the original width. 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?

Brodgar's ditch today. The picture shows the ditch by the south-eastern entrance. The mound known as South Knowe is in the distance.

Although opinions differ, it is thought that the monument was also surrounded by a high earthen bank - although no actual evidence for this has been found.

However, given that both the Standing Stones o' Stenness and Maeshowe both had encircling banks, why would Brodgar have been any different?

If it existed, in its heyday, the Brodgar bank would have stood approximately three metres high (10 ft). Its size would have meant that from the outside, the ring's 60 stones would have been practically hidden from view.

However, no trace of any bank remains today. If there ever was one, soil and stone contained within bank was probably carted away by farmers over the millennia.

The purpose of the ditch and bank is not completely understood, although it is commonly accepted that the monument had a ritual or ceremonial purpose. With this in mind, the ditch and bank probably marked the enclosed area as special or sacred.

However, theories abound, with some suggesting that the 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.

Archaeological sites around the Ring o' Brodgar