Watching Brief

Date October 2005

Event ID 551091

Category Recording

Type Watching Brief

Permalink http://canmore.org.uk/event/551091

HY 3182 1277 Further to exploratory work earlier in 2005 (DES 2005, 100, ) a second phase of works was carried out in October 2005. This involved the excavation of a trench c 10 x 10m to fully reveal the concrete raft covering the roof of the tomb. The aim of the exercise was to allow the relaying of its bitumen upper surface, as well as the laying of a waterproof membrane around its edges and the reinstatement of the drain around the edge of the raft. This drain was assumed to run off the summit of the mound in its SE corner (its lowest point), and run into the ditch. In the event the drain only just ran off the edge of the concrete raft before petering out, and a third phase of work was undertaken in December 2005 to find a suitable course for this. Excavation began with the removal of turf from the summit, over an area determined by the limits of the raft as identified in the earlier test trenches. This comprised an initial area roughly 10m square, although as the work progressed this was slightly extended.

The excavations on the summit demonstrated that the early 20th-century roof comprises a square raft of concrete, domed towards the centre, some 9m across, with a bitumen coating. A drain was formed around the edge of this by infilling the 'lip' at the limits of the raft with sand and gravel, and laying slates over this. The concrete raft was then covered by a substantial deposit of silty clay, which was remarkably homogeneous and free of finds. This was generally 650mm thick, but thinned to 350mm at the edge of the trench, continuing beyond the edges of excavation. This upper area of early modern landscaping appeared, prior to the excavation, as a more rounded, gentle shape than might have been expected from the steepness of the sides of the mound.

The material against which the concrete was cut was variable in colour, but consistently very compact silty clay, rich in stones. Very little of this material was excavated, and no finds recovered, so its date is unknown, but it was thought that it was very unlikely to represent undisturbed Neolithic mound material. It is more likely that it represents the remains of earlier disturbance against which the roof was cut. The absence of any drain to take water collected by the concrete roof made it necessary to install a pipe running from the SE corner of the newly bitumen-covered concrete roof down the S slope of the mound towards a stone-lined drain in the base of the ditch, c 25m to the S of the base of the mound.

Initially, after the line of the trench excavated by Childe in the 1950s (PSAS 1956) was found towards the top of the mound, it was possible to follow the E edge of his trench down the slope to create a channel through previously disturbed ground. Unfortunately this was not possible further down the slope, as it seemed that Childe's excavations had not penetrated to a sufficient depth to leave a clear edge. A shallow trench was opened over a larger area in an attempt to re-establish the line of Childe's trench. The complicated sequence found in the sondage in the E side of this trench included compact deposits, unlikely to be recent backfill, indicating that significant archaeological material survived in this area. The works indicated the survival of stratified deposits, but given how little had been seen in the side of Childe's trench further up the slope, these were of uncertain date and significance. The clay-flecked deposits found along the S side of the trench at the base of the excavation lay as little as 100mm below the turf and seem likely to represent undisturbed Neolithic layers. As these seem to lie within the course of Childe's trench, it must be assumed that he did not fully excavate all the deposits he encountered. For these reasons, it was decided to use a narrower diameter plastic pipe, buried in the top 20mm of topsoil, although this would require more frequent maintenance. From the base of the slope of the mound across the level platform and into the ditch to the S, only a very shallow trench of maximum depth 80mm was dug to accommodate this narrower pipe. Across the platform the upper surface of presumed Neolithic layers was found only 70-80mm below the modern turf.

Excavation in the ditch at the far S end of the trench revealed a stone-lined and capped drain to sit in a tight cut, and to be in apparent working order. Although not capped by Orkney roof slates, this style of construction, with a smaller slab over the join between two larger slabs, is identical to that used in the drain on top of the concrete raft, perhaps indicating that these are contemporary.

Almost all deposits encountered during this project were silty clays, merely varying in colour, compaction and inclusions. Much of this material (from both recent and ancient work) is probably locally derived, meaning that the definition of archaeological layers on this site is technically very challenging. It is recommended that any future ground disturbance work is done under strict archaeological controls, and in an area of sufficient size to allow a reasonable understanding of what is found.

Archive deposited with Orkney SMR held at Orkney College, Kirkwall, Orkney Archive to be deposited in NMRS.

Sponsor: Historic Scotland.

David Murray and Andrew Hollinrake, 2006.