Between the years 1995 and 1998, coach tourists arriving at the busy, modern heritage centre of Clonmacnoise, or people travelling leisurely in pleasure cruisers down the River Shannon would also have witnessed an equally strange scene. However, now the sounds to be heard would have been the roar of outboard engines and the ear-jarring din of air compressors, as a team of Irish underwater archaeologists went about their own task. Over the last four years, we have been surveying, excavating and recording the remains of a large wooden structure preserved at a water depth of about 5m on the bed of the River Shannon. The task of investigation is now largely complete and the results have been startling to say the least. They include a full description and drawn record of a massive wooden structure, some 120m in length by 5m in width, as well as drawings and photographs of eleven dug-out boats which were preserved in the soft riverbed muds, some of which had tools lying within them. Other finds from the riverbed beside the wooden structure have included a rare decorated copper-alloy basin of eighth to ninth century date, a range of wooden and iron objects, large quantities of iron slag and butchered cattle, pig and red deer bone that was probably thrown into the river from the monastic site.
 
 

The Clonmacnoise bridge was first discovered in the summer of 1994, by two local divers who started an underwater survey in an attempt to locate riverine features associated with the neighbouring monastic site. The divers were also armed with an unusual medieval Irish annalistic reference which referred to an incident when local clergy were attacked by rebels in A.D. 1158, at the ‘joysts’ of Clonmacnoise, suggesting the presence of a bridge, jetty or wharf on the river. Indeed it was, that at a point several hundred metres downstream of the monastic archaeological complex, the divers encountered large upright posts forming two rows running out from the western riverbank. This discovery was then followed up in 1995, by a two-week underwater survey, carried out by a small team of divers, funded by the National Monuments Section of the Irish national Heritage Service. This underwater survey confirmed that there was a substantial wooden structure running out from the western bank, but whether or not it crossed the river was still unknown. The breakthrough came about during the underwater excavations carried out in 1997 and 1998 by the Management for Archaeology Underwater Ltd, funded again by the National Monuments Service. These excavations revealed the true scale and complexity of the structure and confirmed that it ran from bank to bank, with no interruptions.
 
 

The bridge was certainly built to some regular plan. Firstly a number of long, narrow hazel and alder saplings were sharpened and driven in a line into the riverbed. This line of poles probably marked out the bridge for the carpenters and the men in boats who erected the main structure. This wooden structure has been dated by dendrochronology to the year 804 A.D. and consists of two rows of about 25 pairs of large oak posts driven into the clays, stabilised on the riverbed by means of ingenious carpentry. The upstream and downstream post of each pair were about 4.5m apart, and each pair was spaced at intervals of 5-6m intervals running across the river. The vertical posts, each 30-35cm in diameter, were typically roughly hewn to remove bark and some sapwood, with their ends sharpened to a blunt tip. A single upright post was entirely exposed during the underwater excavations and removed from the river. It had been driven into the clays to a depth of 3.5m. Through its sharpened tip had been augured two narrow holes, presumably used with ropes to guide the posts into their final position. The posts were prevented from sinking further than required into the soft riverine clays by means of individual base-plates. By this it is meant that each post had its own base-plate, in contrast to later medieval bridges in Britain and Ireland which are often built from a foundation of lengthy wooden baseplates which hold two or more verticals.

Each vertical post had a mortise cut in its side some 3-5m from the sharpened tip. This mortise was used to hold a transverse beam, 1.5-2m in length. This beam itself was mortised either side of the vertical post and held two flat, oak planks measuring up to 2m in length, 30cm in width. The posts would have been easily driven into the soft riverbed clays, simple vigorous rapping on the sides of the posts may have been enough to have their own weight force them downwards through the mud. But as the post descended deeper in the clay, the beam and its two planks would have acted like breaks, halting the descent of the post as it joggled into position. On the shallower side of the river, a similar, though less complex technique was used. A roughly hewn branch was simply jammed into the vertical post’s mortise before it was driven into the river-bed. In one example, this rough branch broke across inside the mortise and both pieces were dragged down into the riverbed.

The upper structure of the bridge no longer survives, all we have left are the posts as they remain at the riverbed level. Rotting wood, riverine erosion and floods have all removed most other traces of the upper parts of the posts, and any other side-braces, transverse or longitudinal beams that supported the walkway. However there are sufficient annalistic and literary references to bridges in Early Medieval Ireland to conclude that the walkway could have been of cleft oak planks, tied-up poles or even woven post-and-wattle panels. Indeed, the use of post-and-wattle in the bridge walkway would have significantly reduced the vertical weight on the structure. The bridge measured at least 120m in length and was about 5m in width. Its original height is unknown, it may have been at the water level or could have stood 4-5m above its surface.

From the east the bridge could have been approached dry-shod. However, on the west side there stretched literally miles of impenetrable raised bogs. However it seems likely that there was a bog trackway built to cross them and the Clonmacnoise bridge can be linked in with its discovery. Buried several metres under the peats of this bog are traces of an ancient trackway, built up by dumping loads of gravel and clay into the wetlands. This trackway measured about 3-5m in width and at least 30cm in depth and can be traced over a distance of 1.5 miles, before it is lost in modern farmland. Interestingly, the various archaeological sightings of this roadway made in the 1980s can be linked up, to trace a line that leads directly to the western end of the Clonmacnoise bridge. Presumably this trackway was built at the same time as the bridge, to enable both pedestrians and carts to reach the river-side.

Underwater archaeological surveys and excavations have also revealed the presence of eleven dug-out boats lying lightly buried in the riverbed’s silts and muds. The dug-outs probably range in date, but their proximity to the bridge suggests that some are contemporary with it, while others were used as ferries before or after its existence. Although all of these dug-out boats have been much reduced by erosion (the bed of the river is covered in millions of fragments of broken freshwater mussel shells, which act as a kind of liquid sand-paper), its is possible to trace some variation in their form and construction. There are a number of long, narrow craft with round bottoms which were probably designed for rapid travel. There are also squarish, flat-bottomed craft, and a single dug-out which had separate frames inserted into its floor. These were carved out of natural trunk-branch junctions from ash and alder, in the manner of ‘knees’ from later plank-built craft. All of the dug-out boats are made of oak, which alone provided the necessary girth and length of trunk in early Ireland’s woodlands. Astonishingly, three of the dug-out boats still had tools lying on the floors of the vessels. One had a whetstone for sharpening axes, while another had an Early Medieval woodworking axe lying in the middle. One of the best-preserved of the dug-outs had two Early Medieval woodworking axes lying on its floor, a felling axe and a general purpose carpentry axe. One can only speculate that a tragic accident ensued as a timber beam for the bridge was being manoevred, the unstable river craft suddenly flooded and brought its occupants and their tools into the deep.

The most important find from the Clonmacnoise bridge underwater excavations is a decorated copper-alloy basin, found lying upside-down beside one of the vertical posts. This large finely-made vessel of hammered metal, was spun and polished on a lathe and was decorated by means of hammered-out ridges. It can be typologically dated to the eighth or ninth century AD and was probably used in the early Irish liturgy, probably for the washing of the hands during the mass. The basin was undoubtedly part of the collection of rich, ecclesiastical metalwork that would have been used in the monastery above the bridge. Copper-alloy basins of this type are rare in Ireland (there are three other examples), most have been found in Norwegian Viking graves, persumably many taken from Ireland during the raids of the ninth and tenth centuries AD. Indeed, the Clonmacnoise basin has a large puncture mark in one side, where it appears to have been violently struck by a sharp object. Perhaps it was lost from the bridge during a Viking or Irish raid on the monastery?

The bridge must ultimately be interpreted both in terms of its role in the urban economy of the monastic town and in terms of local and regional politics. Clonmacnoise, founded in the sixth century AD by St. Ciarán, was strategically located at a crucial junction of two major natural routeways; the River Shannon (Ireland’s largest river, which flows southwards past the monastic site) and the Slige Mor (‘great way’) which crossed the islands central bogs and wetlands from east to west, running along an elevated natural esker ridge. Undoubtedly this location was one of the reasons why the monastery had emerged as an important political and ecclesiastical centre by the early ninth century A.D. Perhaps the bridge was built to improve communications and to copper-fasten the monastic community’s control of this crucial cross-roads.

However, Clonmacnoise was also located on a political frontier in 804 A.D., between the territory of the increasingly aggressive kings of Connacht and the growing power represented in the midlands by the Clann Cholmáin kings of the southern Uí Néill dynasty. It is possible that the bridge was a military venture, thrown up rapidly by an army moving swiftly across the river during a raid or campaign. In any case, there is no evidence that the bridge was ever reconstructed or repaired. After twenty or thirty years, it must surely have looked treacherous, not long after that, its upper structure would have fallen entirely into the water. But there the vertical timber posts themselves stayed for centuries, until out of the shadows and gloom of the peaty waters of the River Shannon, Irish archaeologists have brought the story of Ireland’s oldest bridge. Or at least, the oldest one yet known!

• Acknowledgements

The Clonmacnoise Bridge Project was funded by the National Monumnets Service, Duchas - the Heritage Service. This article was originally published in the exciting new world archaeology magazine, Discovering Archaeology Vol. 1, pp. 32-37 and we gratefully acknowledge the contribution of the editorial and production team to this article. The photographs are by the authors, but have been scanned from that original article. The reconstruction drawing was drawn by Mark Offutt, of Discovering Archaeology. The website is at www.discoveringarchaeology.com