Underwater excavations at Loch Arthur Crannog, New Abbey, Dumfries and Galloway
Dr Jon Henderson and Dr Graeme Cavers
In 2003 the Underwater Archaeology Research Centre undertook excavations at Loch Arthur Crannog as part of the South West Scotland Crannog Survey. The site was found to have been subject to multi-period occupation. Radiocarbon dates obtained during the excavation suggest that the main packwerk mound at Loch Arthur was constructed in one event at some point during the 4th to 3rd centuries BC. After a period of abandonment, represented by the deposition of natural loch silts, a boulder capping was placed on the original packwerk mound sometime in or after the 15th century AD.
Figure 1: General view of Loch Arthur Crannog
The crannog in Loch Arthur appears above water as a tree covered island, some 30 metres in diameter, which is connected to the shore by a muddy reed bed (Figure 1). Small scale exploratory terrestrial excavations were carried out on the surface of the island in 1966-7 revealing the footings of drystone/clay packed walls that the excavator interpreted as the stone undercroft of a wooden framed building (Williams 1971, 123).
Despite a complete absence of finds, Williams offered a 15th to 16th century AD date for the crannog based on two bronze tripod cooking pots which had been recovered from the loch in the 19th century. Loch Arthur thus entered the general literature as an example of a late medieval crannog. However, in 1989 a vertical birch pile off the northern side of the site was sampled for radiocarbon dating and provided evidence of Iron Age activity at the site (GU-2463, 2260 ± 50 BP, calibrating at 400-200 BC; GU 2644 2240 ± 60 BP, calibrating at 410-160 BC).
Loch Arthur was examined as part of the current South West Crannog Survey (Henderson et al. 2003, 89-90; Henderson 2004; this volume). Diving on the site revealed that the majority of the structure of this crannog lies underwater as the island visible from the shore appeared to sit on a much larger fully submerged structure. Detailed survey of the site demonstrated that the dry island sits on a long promontory, some 81m long and 46m wide, running north-west/south-east from the shore which may be, if not entirely, then at least partly artificial.
Figure 2: 2004 survey of Loch Arthur Crannog, New Abbey, Dumfries and Galloway.
Contours at 0.1m increments,
These two features could be distinguished by their composition, the upper mound (the island) being built primarily of large boulders within a well-developed soil, and the lower submerged structure of timber (alder and oak), organic deposits and stones (Figure 2).
Evidence of artificial construction (horizontal alder timbers and organic deposits) could only be seen on the southern and eastern sides of the promontory where active erosion is occurring presumably due to wave fetch.
The western and northern sides of the promontory are completely obscured by soft silts making it difficult to say with certainty that the whole promontory is artificial. The upper surface of the submerged structure lies only 0.3m below the water level while the base of the artificial levels occurs at a depth of 3.5m after which no other timbers can be traced in the soft loch silts. Oak and birch vertical piles can be found in the silt along the northern edge of the promontory providing more evidence for artificial construction but these cannot be directly linked to the main structure of the mound.
Nothing could be traced of a stone causeway to the upper island observed in 1874 (Gillespie 1874, 23) but apparently obscured by 1968 (Williams 1971, 123). It seems most likely that this causeway lies underneath mud and reeds in a deposit that has built up between the closest point of the island to the shore and the shore itself. Indeed it this build up which makes it possible to walk to the island today, something that was certainly not possible c.1860 from the evidence of the First Edition Ordnance Survey map of Loch Arthur where the crannog can be clearly seen to be an island.
The build up of silt along the western and northern margins of the site may be the result of the re-deposition of silt carried by wave fetch and currents (see also the discussion on Dorman's Island in Henderson this volume). With this in mind it is possible that the promontory revealed in the digital terrain survey may not be entirely artificial but rather a large oval artificial island which is now connected to the shore due to the build up of silt.
Figure 3: Underwater excavations at
Loch Arthur Crannog using a
Small scale excavations were carried out in 2003 to both stabilize and sample eroding submerged deposits and to attempt to clarify the dating and structure of the site (Figure 3). Examination of the submerged exposed sections (Trenches 1 and 2) suggested that the lower mound was constructed through the deposition of layers of alder timbers, laid horizontally and weighed down with comminuted plant material, brushwood and stones. Both exposed sections were located to the south-east of the upper island; one on the southern edge of the submerged promontory in 2.5m to 3.5m of water (Trench 1) and the other on the flatter top of the submerged mound in 0.75m to 1m of water (Trench 2).
Over 50 alder timbers were exposed and recorded in Trench 1 (see Figure 4 and 5). These timbers were arranged in horizontal layers, radially into the centre of the mound. Each layer of timbers was laid at approximately 30 - 60 degrees to the layer below, in a matrix of twigs and comminuted organic material containing hazel nuts and woodchips as well as many fire cracked stones (most likely used in cooking and dumped onto the site from occupation above). Trench 2 revealed a very similar structure in existence on the top of the submerged mound (Figure 6).
Over 30 alder timbers were uncovered within a matrix of comminuted plant matter as well as broken down twigs, wood chips and fire cracked stones. All of these timbers were laid in parallel, horizontal layers, with each layer laid at approximately 30 - 60 degrees (in plan) to the layer above and below. Several of the large timbers had mortise joints cut through them, though there was no evidence of any tenon or other structural purpose of these joints (similar to the joints in two timbers recently uncovered at Oakbank Crannog in Loch Tay, Perthshire; Dixon this volume).
Figure 4: Loch Arthur crannog, Trench 1 plan
Figure 6: Loch Arthur crannog, Trench 2 plan
It is possible given the rough nature of the mortise joints, which are rather weak and lack any tenon or obvious structural purpose in their current position that these holes were cut into the ends of timbers simply for the purpose of dragging them on to the site [USOA1] . Rather surprisingly, undisturbed natural loch bed marl was encountered in Trench 2 at a depth of just 1.1m from the top of the trench suggesting that the artificial structure may have capped an existing natural feature. Anthropogenic deposits on the southern and eastern margins of the site, however, are considerably thicker, occurring at depths down to 3.5m (as seen in Trench 1), suggesting that the sides of the site may have been built up into the water. Whatever the case, at 46m across, the submerged artificial structure is quite substantial.
Figure 5: Photo mosaic of exposed section, Trench 1, at Loch Arthur crannog (photo mosaic work progress.
The deposits revealed in the exposed sections suggest that only foundational constructional material is present underwater. No artefacts or in-situ occupational material were uncovered in Trench 1 or 2. Consequently, it seems likely, from the evidence of Trench 2, that if any occupational deposits existed on top of the submerged mound they have since been eroded away leaving only foundation deposits. There were no vertical piles in the sections examined and such piles could only be traced off-site in the surrounding silts (particularly along the northern margins). Thus the possibility of external structures such as breakwaters, boat nausts or a walkway exists but certainly as far as the main structure of the mound is concerned it must be considered to be of packwerk construction.
A small 2m by 1m trench (Trench 3) was opened on the dry area of the crannog to determine the relationship between the upper dry island and the submerged mound. The structure of the island was seen to be entirely artificial. Beneath a 0.8m thick layer of rounded boulders, horizontal alder timbers were encountered which had been laid in horizontal lines resembling the foundations for a floor. These two contexts were separated from each other by a thin (0.12-0.14m) grey layer of re-deposited loch silts indicating a period of inundation and abandonment between the two constructional phases. Significantly a sherd of medieval green glazed pottery was recovered sitting within the matrix of the upper boulder layer providing the first definitive evidence for medieval activity on the site. On comparison with other Galloway wares, the fabric, form and well fired nature of the sherd suggested a 15th century AD date (Laing, pers. comm.).
Timber samples were taken for radiocarbon dating from contexts in all three trenches to provide the basis for a secure chronological framework for the site. These are presented in Table 1 (below) alongside the two dates obtained in 1989.
The dates taken at various levels throughout the submerged mound suggest that it was probably constructed in one event sometime between 400 and 200 BC; the limitations of the radiocarbon calibration curve for this period prevent closer dating. The date of 2240 ± 35 BP (GU-12173) obtained from a timber located at the base of Trench 1 (Timber 19, Figure 2) is identical to that obtained from the top of the timber mound, sampled in Trench 3, which provided a date of 2215 ± 35 BP (GU-12175). Significantly, the date from Trench 3 came from a constructional layer of alder timbers which underlay the upper stone mound, and was separated from it by a deposit of grey, inorganic loch silts.
The medieval green glazed sherd recovered from within the boulder matrix of the upper stone mound suggests that it was built sometime in or after the 15th century AD on a pre-existing artificial mound that was built in one event during the Pre-Roman Iron Age, while the silt layer that separates the two contexts is concurrent with a period of abandonment in between these two phases of use. The dates recovered in 1989 come from a birch pile located during the 2003 survey just off the northern margins of the site and may indicate that there was an external pile driven element to the Iron Age use of the site.
Henderson, J.C., 2007. Recognising complexity and realizing the potential of Scottish Crannogs. In: Barber, J., Clarke, C., Crone, A., Hale, A., Henderson, J., Housley, R., Sands, R., Sheridan, A., eds. Archaeology From The Wetlands: Recent Perspectives. Edinburgh: Society of Antiquaries of Scotland. pp. 231-241.
Henderson, J.C., Cavers, M. G., and Crone, B. A., 2006. The South West Crannog Survey: Recent work on the lake dwellings of Dumfries and Galloway. Transactions of the Dumfriesshire and Galloway Natural History and Antiquarian Society, 80, pp. 29-52.
Henderson, J.C., Crone, B.A., and Cavers, M.G., 2003. A condition survey of selected crannogs in south west Scotland. Transactions of the Dumfriesshire and Galloway Natural History and Antiquarian Society, 77, 79-102.
[USOA1]: Note large oak timber from Coolure with mortice at its tip, note also that one of the OB timbers has notch (or mortice) at its point.
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