Thursday, October 23, 2008

To the north of the dun territory, in Orkney and Shetland, Caithness and Skye, appeared a series of tall, tapering, stone towers, clearly designed as fortifications and known as brochs. As Euan MacKie has observed, these were the only advanced architectural buildings ever to be created entirely within prehistoric Britain, apart from Stonehenge. About 500 of them are known in northern Scotland where some of them once stood 9 m. (10 yd) or more in height and over 20 m. (22 yd) in diameter (plate 51). Each circular tower consists of an inner and outer layer of dry-stone walling tied together with a series of horizontal lintels which bridge the gap between them. In this space, which is about 1 m. (1 yd) wide, are a series of galleries superimposed one above the other and a slab-built staircase which climbs clock-wise to the top of the tower. The galleries were probably intended only to lighten the weight of the tower wall, thus allowing it to be built higher. The total thickness of the wall at its base was about 4.5 m. (5 yd).

The outer wall of the broch has no windows and is unbroken, except for an entrance, and rises upwards for two storeys or more, with a gentle, inward batter, like an electricity cooling-tower. The inner wall face is sometimes broken by vertical openings called voids which allowed light and air into the staircase and relieved some of the weight of the walls, especially above the entrance. The long entrance passages needed to be strongly built to withstand attack. They averaged 1.5 m. (1.6 yd) high and 0.75 m. (0.8 yd) wide. About half-way along the passage were stone door jambs and a slot into which a bar could be placed to secure a heavy wooden door, to which was attached a strong iron ring handle, like the one found at Dun Ardtreck (Skye). (Confusingly duns and brochs are both called duns or duins in Gaelic.) Behind the door was a corbelled guard chamber in the thickness of the wall.

Around the inside of the broch ran a ledge or scarcement, varying between 1.5 m. and 3.5 m. (1.6 yd–3.8 yd) above the floor, which seems to have supported a wooden gallery which jutted forward on to a ring of posts. Higher up, a second scarcement probably carried a roof over the gallery. It seems likely that the central part of the tower was open to the sky which must have provided problems in bad weather for the central cooking hearths that were often present, though storage tanks in the same area could have been filled with rain water from the gallery roof. Drinking water was often provided by a well.

Some brochs have outer defensive walls or ditches added, like the bailey of a Norman castle, which contain small villages of domestic buildings. At the Broch of Gurness (Orkney) there were sufficient houses for 30 to 40 families enclosed within three stone-faced ramparts and ditches. Enclosed buildings at the broch of Howe (Orkney) would have accommodated about 250 people.

Although there is considerable uniformity in broch design, on one point there is a major variation between those in the Hebrides and those in north Scotland and the Northern Isles. This shows in the ground plans which are usually ground-galleried in the Western Isles and normally solid-based in the rest of Scotland. As the names imply the ground-galleried brochs have cells and galleries starting at ground level, whilst the solid-based examples have an almost continuous solid wall of masonry at ground level, broken only by the entrance passage and two or three corbelled cells.

The classic broch is Mousa on Shetland, standing on a rocky headland guarding Mousa Sound (fig. 61). The tower is almost complete, standing to a height of 13 m. (14.2 yd). Although its external diameter is 15 m. (16.4 yd) at the base, the thick walls leave room for an interior only 5.5 m. (6 yd) across. The entrance passage leads directly into the interior with no guard chambers. Inside are three large cells with tiny wall-cupboards all set in the thickness of the wall. The continuous staircase passes up through five floor levels or galleries in the wall before reaching the walk-way at the top of the tower. The galleries were lit by three voids. Two ledges or scarcements projecting from the inner wall face at heights of 2.1 m. (2.3 yd) and 3.7 m. (4 yd) above the floor of the broch probably represent the floor and roof height of a wooden gallery, which could be reached from an entrance on the stairway. The interior of Mousa is now partially blocked by a wheelhouse- type structure of perhaps the third or fourth centuries AD, though recent work in Orkney may suggest that this is contemporary with the main broch occupation.

Many brochs, though not all, stand on the seashore, and this has led to speculation that they were erected against seaborne attack by people united by maritime interests. It is likely that over many centuries they guarded the small tracts of good farming land that were dispersed in an inhospitable terrain, hemmed in by mountains and moorland. It is clear that in the closing centuries BC life in northern Britain was ever more turbulent and attack was always a possibility. The loss of livestock or personal belongings or capture for a southern slave market must have been a constant fear. The brochs probably acted as temporary refuges into which the local community could retreat to withstand a short attack or siege. It would be difficult for an enemy to overcome the great height of the broch tower which would prove impregnable to spear or slingstone. Scaling would be incredibly dangerous and battering of the door difficult due to the narrowness of the passage and the presence of guard chambers and armed guards. The excavation of Dun Mor Vaul (Tiree) showed that although the broch was in use for about 250 years, occupation was only sporadic and for short periods.

There have been years of debate regarding the origin of the brochs. It now seems likely that they developed in the Orkneys as prestigious fortified towers. Recent study has begun to show various round houses that could be claimed as ancestral to them there. That they can have originated in the mid-first millennium BC is shown by a radiocarbon date around 600 BC that has been obtained for the demolished broch site at Bu, near Stromness (Orkney). To the south-west, in the Hebrides, Dun Ardtreck on Skye has given a radiocarbon date of 115 BC. As has already been suggested, the great similarity between the brochs reflects a certain unity amongst the people of north-western Scotland, which may have led to a specialist group of local architects responsible for their design, who travelled from one community to another and passed on their knowledge from one generation to another. It is clear, however, that not all of them were built with the same degree of sophistication.


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