1997:447 - CLONMACNOISE, Offaly

Underwater archaeological excavations were carried out over a six-week period in August and September 1997 on an early medieval wooden bridge crossing the River Shannon at Clonmacnoise, Co. Offaly. These excavations were funded by the National Monuments Service, 51 St Stephen’s Green, Dublin 2. The bridge is located between Clonmacnoise townland, Co. Offaly, and Coolumber, Co. Roscommon, crossing a narrow part of the river west of the Anglo-Norman masonry castle. Underwater archaeological excavation has revealed that this structure was built using a complex method of sharpened and through-mortised posts with individual base-plates of beams and planks. There are also hints of the superstructure of the bridge in the occasional well-preserved vertical timber now lying on the riverbed.

The bridge was clearly sited to provide access across the river to the monastic town of Clonmacnoise. It was also sited at a location where natural and man-made routeways converged at a relatively shallow and narrow part of the river. A gravel and stone bog trackway in Coolumber approached the site of the bridge from the west bank and is probably contemporary with the structure. In the early medieval period the River Shannon flowed slowly along this channel and was probably slightly wider than its present size, but was unlikely to have been much deeper. The riverbed was of soft, alluvial clay and, although it would have been easy to drive posts into the bed, there would have been problems of subsidence and stability.

The bridge measures 120m in length by 5m in width and probably stood to a height of 10–13m. It crossed the entire span of the river and probably survives as a timber trackway or causeway in the callows fields on the west bank. The bridge was built to some regularity, with an occasional variation according to the topography of the riverbed. A line of narrow hazel and alder poles were first driven into the riverbed to mark out the line of the bridge. It was built of about 25 pairs (17 pairs of which survive today) of vertical oak posts, the upstream and downstream posts being 4–5m apart. Each pair was spaced at 5–6m intervals running across the river. The vertical posts were typically hewn to a squared cross-section, measuring 0.25–0.35m in width. A single post was excavated and survives to a length of 3.5m. It had been crudely sharpened to a blunt point and had augur-holes drilled through the tip. In all there would have been at least 50 vertical posts in the bridge. There is no evidence for the repair or reconstruction of the bridge at this location; it is a single- phase structure which hardly stood longer than 40–50 years.

The posts were prevented from sinking by means of individual base plates. The post was sharpened and had a through-mortise cut through its side. This held a transverse beam, 1.5–2m long, mortised at either end; these held cleft oak planks 2m long and 0.35–0.4m wide. The posts were then driven by some means into the riverbed. As the post descended, the flat planks struck the riverbed silts and halted the descent of the post. There is no other evidence for bracing and it seems that this was the only means of holding up the posts in the river. This technique was used right across the river, but on the west side only trimmed branches were inserted through the vertical posts. These were only partly successful in holding up the posts, although they certainly did not fall over.

The superstructure of the bridge has been destroyed by collapse and riverine erosion, and most of the timbers were possibly robbed out soon after its abandonment. Two horizontal timbers lying beside the verticals may actually be collapsed posts. These are highly eroded, but there are traces of joints which enable a reconstruction of the upper part of the bridge. They measure 11–13m in length, indicating the probable height of the original bridge. This means that the walkway was probably 6m above the summer water-levels, somewhat less in winter. Cross-beams would have been laid across the tops of the timbers, and there may also have been beams between the verticals halfway up their height to tie the structure together. It is not known what walkway surface was used, but to judge from early medieval historical references to bridges on the River Shannon either planks or wattle panels were used.

The oak trees used in the bridge construction were small, narrow in girth and heavy with side-branches. They were slow-grown and were probably located on the margins of the bogs or along the river valley. There may have been a scarcity of good-quality oaks in the early 9th century AD. The bridge is highly informative about carpentry and woodworking in early medieval Ireland. There is evidence for hewing, cleaving and joinery and the use of narrow axes, broad axes, adzes and augurs. The variable quality of the wood technology indicates that a team of unskilled labourers supervised by experienced carpenters were responsible for the construction of the bridge.

Finds from the Clonmacnoise bridge site include nine wooden dugout boats, four iron woodworking axes, an iron dish, quantities of iron slag, a large decorated copper-alloy basin, a whetstone, iron slag, a prehistoric chert core, and a small but interesting assemblage of animal bone. Apart from the chert core, which may be of Mesolithic or Neolithic date, most of the finds would appear to date from the early medieval period. The basin, datable by typological studies to the 8th or 9th century AD, is an important contribution to early medieval metalwork studies as these basins are rare in Ireland. It is very similar to one from the Derreen hoard, Co. Clare (on the Shannon estuary), that may even have been made in the same workshop.

The dugouts vary in size and form. In some cases they can definitely be associated with the wooden bridge, probably reflecting the activities of the labourers and carpenters during its construction. Indeed, four of the dugouts produced finds (three iron axes and a whetstone). It seems likely that these vessels were lost during the construction of the bridge. Animal bone from the bridge site included cattle, pig, sheep/goat and red deer, some of which showed evidence for butchery.

The early medieval bridge would have been one of the largest wooden structures of its time. The bridge, boats and metalwork can be interpreted in terms of the regional politics and urban economy of the monastic town of Clonmacnoise. The bridge contributes to recent debates about the origins of the town in early Ireland, as it should be seen as the act of a town population who both required and desired the benefits of the improved communications to Clonmacnoise that the bridge provided. Clonmacnoise is situated at the junction of the River Shannon, itself a major nautical routeway through the country, with the Eiscir Riada, which was the east–west routeway through the country. The bridge should be viewed as part of this routeway across the country. Building this structure across the river meant that the Clonmacnoise monastery could control, or at least had access to, a major communications network across the midlands. Incidentally, there are also annalistic references to early medieval bridges elsewhere on the River Shannon, particularly at Athlone.

In 804 Clonmacnoise was under the patronage of the kings of Connacht. Later it came under the hegemony of the Clann Cholmáin kings of the Southern Uí Néill. Clonmacnoise was strategically placed on the river, which acted as a border between provinces. However, it was primarily a Connacht monastery, being perceived as the entrance to that region. A bridge was an obvious feature for Clonmacnoise, given that it is across the River Shannon and technically in the territory of Mide. The bridge could only have survived for about 50 years and was not reconstructed at this location. Was it a once-off, politically inspired construction? In the 8th and 9th centuries the monastic town was expanding. Domestic structures and industrial working areas have been found in the vicinity of the modern graveyard by Heather King and elsewhere by Con Manning. By the 10th and 11th centuries, the monastery was to become a centre for learning and art, with the production of manuscripts, the high crosses, church buildings and fine metalwork. The bridge may have been constructed at the beginning of this confident expansion in town population and works, with an added important role in physically linking the monastery with its political hinterland.

Arden Road, Tullamore, Co. Offaly