Excavations at the Cromlech Tumulus on Slievemore Mountain 2014-2016


View across Achill from the site of the Cromlech Tumulus on Slievemore.

In 2014 work began at a very intriguing site on Slievemore known as the ‘Cromlech Tumulus’. The site was originally recorded in the 19th century, first by the Ordnance Survey and later by W.G. Wood Martin, the famous Sligo Antiquarian. It has caused considerable debate throughout the 20th century as different archaeologists tried their best to interpret the confusing remains visible at the site. Wood Martin thought it was part of a complex “sepulchral group” and clearly felt it was a megalithic site of some sort. Thomas Westropp thought it was an ancient multiple hut site, an opinion shared by Michael O’Kelly. Stuart Piggott and Thomas Powell thought it was a megalithic tomb with perhaps some later interference whilst Ruaidhrí de Valéra and Seán Ó Nualláin thought the site was best regarded as “one or more houses with lamb shelters built into the eastern end” with a possible hut or kiln at the west, and imply a recent origin. The Sites and Monuments Record currently lists the site as a possible megalithic structure.

Work began at the site with an intensive survey, using high precision GPS units to produce the first accurate plan of the site and a manual survey with a 50cm resolution allowed for the production of an accurate 3D topographic model of the sites form prior to excavation. In 2014 three large trenches were excavated over the western half of the site and during the first half of the 2015 season a fourth trench was excavated to complete work on the western half of the site. We will return to the site during the first half of the summer of 2016 to begin work on the eastern part of the site.

The western part of the site contains two main phases of activity. The initial period of activity at the site revolved around the construction and prolonged occupation of a large oval shaped building during the Middle Bronze Age period. The building is similar in size to the Middle Bronze Age Roundhouse 1 and Roundhouse 2 that Achill Field School had previously excavated on Slievemore, which are located almost 2km to the west. However there are some important differences between this building and the two Roundhouses. Firstly the construction of the perimeter wall is quite different, the use of stone facing to retain a thick core of earth is quite different to the regularly coursed dry stone walls utilised at the Roundhouses. Secondly the entrance is located at the south south-east of the building, rather than at the east south east, and instead of it being a long stone lined passage it appears to be a simple gap in the wall flanked by two large orthostats. It is worth noting that although the overall layout is different the entrance passage of Roundhouse 1 also terminated at two large orthostats.

The third major difference is that the interior of the building contained a tremendous number of pits and postholes. Whilst the interior areas of Roundhouses 1 and 2 both contained pits and postholes they were not in the profusion found at this site. At the very least this strongly suggests a longer and more complicated period of occupation, but it may potentially also relate to a difference in function. Finally whilst the finds assemblage from the Roundhouses and this building at the Cromlech Tumulus site are largely similar, consisting mainly of small lithic flakes, diminutive lithic tools, hammerstones and rounded pebbles of currently unknown purpose, the building at the Cromlech Tumulus site also produced two sherds of Bronze Age pottery and a unique sculpture of a head carved out of a small pebble. Again it is not clear whether this relates to a functional difference or simply that there was a longer period of occupation that led to more occasions for artefacts to be deposited in recoverable contexts.

The ‘Cromlech Tumulus’ and ‘Danish Ditch’ as shown on the first-edition OS map.

A pre-bog field wall identified as ‘the Danish Ditch’ on the early Ordnance Survey maps links this site to a Neolithic Court Tomb located about 130m to the east. We have examined two sections of this wall in small additional trenches located 30m and 60m east of the main excavation. We suspect this is part of a Bronze Age field system surrounding the site, but its date has not yet been firmly established. The wall seems to connect to the southeastern side of the Middle Bronze Age building but this has yet to be confirmed through excavation. A second field wall was discovered during the excavation of the northwestern part of the site in 2015. This wall actually seems to overlay the edge of the Middle Bronze Age building, which suggests it was built sometime after it, although it may have been constructed whilst the building was still in use.

The second phase of the activity at the sites relates to a pair of small huts that were built over the ruins of the large Middle Bronze Age building. The earlier of these structures was a small oval building defined by a simple stone foundation, which reused the pair of orthostats from the underlying building as its entrance. The building measured 6.1m E-W and 3.6m N-S externally. A series of three overlying hearths were found in the centre of this small structure and a Radiocarbon date of 1435 -1618 AD was obtained from the middle hearth in the sequence. This date spans the Late Medieval period and the very start of the Post Medieval period which is a very interesting result given the general dearth of low status rural buildings known from this period in Ireland.

Plan of the ‘Cromlech’ excavation site. The later medieval and post-medieval hut sites are marked in blue. Note the thick walls of the roundhouse and the dense cluster of post holes and pits in the floor of the roundhouse.

Additionally Achill Archaeological Field School has already recorded two similar buildings elsewhere on Slievemore, which whilst currently undated overlay well-dated Early Medieval buildings and so potentially these may be of a similar date to the structure at the Cromlech Tumulus. The function of this small oval building is difficult to ascertain but certainly it would conform to the size range expected from transhumant seasonally occupied Booley Houses, and this is an exciting possibility we are currently exploring. The oval building was in turn overlain by a smaller circular building, defined by a simple turf bank that must surely date to the Post Medieval period. This building had an external diameter of just 2.85m and certainly could not have been any sort of full time residence. Whether it represents a much smaller Booley house, an emergency shelter or some sort of agricultural structure has yet to be determined, but this is a subject that we are actively investigating at present.

Students excavating at the Cromlech Tumulus in 2016.

In 2016 we returned to the Cromlech to excavate an additional trench on the eastern side of the monument. An elaborate curving entrance to the building was found on the north side of the trench. More evidence for the later use of the site was revealed in the presence of a small corbelled hut of probable medieval or post-medieval date. Some of the most exciting material from our 2016 season at Slievemore came from excavations at the Danish Ditch. The trenches at the site uncovered what seems to be a terraced road running along the 110m contour. In 2017 we will return to finish the excavation of the Cromlech and conduct further investigations at the Danish Ditch.


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