Building the stone circles

Although we can only speculate as to the purpose of stone circles, such as Brodgar and the Standing Stones o' Stenness, there is one thing we can say for certain.

Their existence shows, without a shadow of a doubt, that the society responsible for the monuments was sufficiently organised and well-ordered to carry out building projects on this scale.

An incredible amount of work went into erecting each stone circle, with recent estimates putting the number of man-hours for both the Brodgar and Stenness rings at between 85,000 and 200,000.

This figure would imply that the monuments were regarded as significant enough to warrant such an incredible outlay of manpower and time.

Selecting and preparing the site

Before we consider the effort required to quarry and transport the stones themselves, we should first look at the preparation of the site.

We can't say for sure what the builders of the Brodgar and Stenness rings were looking for when they set out to find a location for their monuments, but it seems very likely that the chosen sites were significant in some way.

The reasoning behind this is simple. There were undoubtedly easier places to erect a stone circle - sites, for example, that did not require the builders to mine through bedrock as part of the construction. The 123-metre diameter ditch that surrounds the Ring o' Brodgar is cut from solid rock.

Three metres deep and five metres wide, around 4,700 cubic metres of rock had to cut and removed from the Brodgar ditch, using nothing but muscle power and stone tools. Modern estimates have put this task at around 80,000 man-hours - the equivalent of 20 men, working five hours a day, every day for 2.2 years.

Measuring the circle

Once the ditch digging was under way, or complete, work probably began stripping away the turf and vegetation covering the enclosure, ready for the outline of the stone ring to be measured and marked out.

Being a perfect circle, Brodgar would have been relatively easy to mark out, presumably using a single length of rope, pegged at the circle's centre.

Then, moving along the circumference of the circle, a mark would have been made every six degrees - the site of each individual stone in the ring.

Quarrying the stones

Figure 1: Vestrafiold in relation to the Harray and Stenness lochs

The fact that the lowest part of Brodgar's ditch was found to have vertical sides could indicate that some of the stones within the circle were quarried from the ditch.

However, from geological studies of the megaliths, we now know that not all were.

There are a number of traditional sites in Sandwick that are thought to have been the sources for the Stenness and Brodgar megaliths.

Of these, the best known is Vestrafiold, a hill north of the Bay o' Skaill in Sandwick. Prone stones can still be seen lying on Vestrafiold's hillside to this day and recent work at the site has confirmed that megaliths were indeed quarried there.

Comparatively little effort would have had to have gone into shaping the stones as the Vestrafiold rock splits easily into slabs.

After being quarried from the hillside, the megaliths were transported the 7.5 miles to the Ness o' Brodgar.

Transporting the megaliths

The two prone megaliths by the side of the Stenness loch, a short distance from the Ring o' Brodgar

To haul the stones overland from Vestrafiold to the Ness o' Brodgar would also have required considerable manpower and effort.

Again, we cannot say for certain how the freshly-quarried stones were moved, but the Brodgar megaliths were undoubtedly easier to transport than their larger Stenness cousins.

The widespread notion that the megaliths were hauled over wooden rollers is possible - but this might not have been the best way. Aside from the problem of a lack of wood, the method is not particularly efficient over rough or uneven land.

Instead, could it be that some form sled apparatus, possibly using sections of 'track', was employed to drag the stones slowly across the countryside?

A theory that the stones were transported by water, floated down the Stenness loch, was dealt a blow in April 2008, with the results of environmental coring work in the loch.

It showed that prior to around 1500BC, the Stenness loch didn't exist. Instead the area was wet marshy bog, surrounding pools of water or lochans. Not the best landscape to be dragging massive megaliths through.

Erecting the stones

With the momentous task of quarrying and transporting the megaliths complete, the task of erecting on site would probably have been comparatively easy.

The method used by the prehistoric engineers is open to speculation, but it seems likely that a system of platforms, levers, ropes and pulleys were used raise the stones. Systems that continued to be used in the islands for thousands of years afterwards.

Calculations based on size and weight would indicate that the largest stone in the Brodgar ring would have taken around 20 men to raise, with the same number employed to ensure the monolith remained standing.

Given the estimated population at the time, and the close proximity of settlements, scholar Aubrey Burl suggested that a workforce of around 300 labourers would have been available for the construction of Brodgar.

"Working an unlikely unionised eight-hour day with no breaks for weekend, hauliers could have dragged the stones to the site, put them up, dug out the ditch, built up the bank and returned to domestic drudgery within a month."
Aubrey Burl - A Guide to the Stone Circles of Britain, Ireland and Brittany

But although based on solid mathematics, Burl summarises the situation perfectly:

"Mathematically, the computation is immaculate. In terms of prehistoric life, it may be no more than fantasy. And what happened inside the completed arena may rest forever unknown."

Brodgar's ditch and bank