W S Hanson & J G P Friell
For the complete text of this paper see PSAS, 125*, 1995, pp 499-520
The site (NN 8729 1458) was discovered from the air by J.K. St Joseph, and is the first tower known to the south of Strageath. It stands on a low ridge just west of the Roman road (fig 1)with an unobstructed view over Strathearn, to the north, and to Kaims Castle fortlet, on higher ground 2 km to the south. The site was ploughed flat, but had not been previously examined and thus offered an ideal opportunity to obtain a complete plan with minimum damage to surviving archaeological remains. All primary records from the excavation have been deposited in the NMRS and the single find in the Hunterian Museum, Glasgow.
The site consisted of a single-ditched penannular enclosure 14m in internal diameter (fig 2). A 1.2m wide entrance causeway faced the Roman road to the south-east and, as the butt-ends seemed the most likely places for the deposition of artefacts, two sections were removed on each side of the entrance. A fifth section was excavated on the north side of the enclosure where the ditch appeared to be widest. The sections showed the ditch to vary from 1.9-2.5m wide, and 0.6-1.0m deep (fig 3). The profile was of the ‘punic’ type, also noted at Roundlaw tower, with a gently sloping inner side and a sharply cut outer face. In some sections, notably on the north side of the entrance (AAA, AAB, AAC), the ditch bottom had a so-called ‘ankle-breaker’ or cleaning channel.
The ditch fill was broadly consistent in all the sections. Primary silt filled the bottom and sometimes continued a little way up the inner face. Above that was usually a layer of dirty gravel with a few larger stones, representing material which had collapsed in from the steeper outer face. The upper fill consisted of homogeneous orange-brown clay-silt admixed with stones, presumably the result of later plough infilling. The one fragment of pottery recovered came from low down in this upper fill in section AAA. There were no signs of deliberate backfilling, with the possible exception of some larger stones at the bottom of the channel in sections AAC and ABA.
There were no direct traces of an internal turf bank/rampart, as at Moss Side, Parkneuk, and Gask House, but this is unsurprising since plough damage had reached to the subsoil. Fragments of turf were detected on the inner side of the ditch in section AAA, as they were at Roundlaw, and although this might only represent material falling in from the original surface, the presence of turf at this point would also be consistent with slippage from the corner of a square enclosure bank. At both of the sites where internal banks have been recorded by modern excavations (Parkneuk and Gask House), the enclosures were square rather than circular in plan.
All four post-pits (fig 2) were located, but only the two facing the entrance were excavated. Only one of the pits was regular in shape, but all had their long axes running NW-SE . At the rear of the tower the more northerly pit (BAD) was approximately rectangular (1.6m by 0.8m), and was partly bounded on the north-east side by a large boulder. The pit was not completely excavated but the upper fill showed a hard-packed dirty-pink stony-clay filling giving way at the northern end to a dark orange-brown soft silt with some large stones. This clearly represented disturbance caused by the removal of a post, rather than the position of one which had rotted in situ, and indicates deliberate demolition of the tower, as also attested at both Shielhill North and South. A similar pattern was observed in the other rear post-pit (BAH) which was 2.4m long and pear-shaped, varying in width from 0.35m to 0.9m, with softer disturbed fill at its wider northern end. The narrow elongation of the southern end probably marks the position of a ramp to facilitate the erection of the post. Such ramps have been found in fort gateways, for example at Baginton and Carlisle, although they are sometimes misinterpreted as demolition features.
The two front pits showed similar characteristics but with the ramps at the northern end. The more southerly (BAG) was similar in shape to BAH, at 2.25m long and 0.5-0.85m wide. It was the most thoroughly investigated and appeared to have contained two posts. The setting for the main tower-support (BAC) was positioned to the north, not at the wider southern end. For half its depth this sloped at of 45o to form a ramp, before continuing down vertically. It was 0.3m in diameter, 0.75m deep and was filled with grey-brown silty loam. The irregularity of its profile hinted at the removal of the post. Towards the southern end of the pit, but separated from the post-setting by a baulk of undisturbed dirty-pink stony-clay pit fill, was an area of softer darker clay-silt 0.9m in diameter and 0.5m deep (BAE) (fig 3, D). Since this clearly cut the pit fill and the adjacent construction slot (see below), but does not impinge upon the setting BAC, it appears to represent the removal of a second post from the same pit. The thin layer of silt at the bottom of the trench may indicate the original post position. The adjacent front post-pit (BAK), though only half-sectioned, was similar. A central area of disturbance (BAI), 0.65m deep, (fig 3, E) marked the position of one post, although the existence of a second, more northerly, post was not tested, and the relationship with the adjacent construction slot (BAJ) was less clear.
Both front post-pits had an associated feature unique amongst the Roman timber towers examined so far. On the inner side of each pit, beginning about half-way along its length, was a straight sided, flat bottomed slot (BAF & BAJ) some 0.3-0.35m wide and 0.1-0.18m deep. These ran parallel, c.1.4m apart from centre to centre, to between 0.8m and 1.3m beyond the ends of the post-pits towards the entrance of the enclosure (they presumably relate to access to the tower). Both were filled with grey-brown clay-silt and showed no signs of post-impressions; suggesting that they may have contained sill-beams.
Only two other features were found in the enclosure. C.1.5m to the west of post-pit BAG were two shallow depressions 1m apart. The larger (BAA) was 0.6m in diameter and 0.1m deep. In the centre was a patch of charcoal-rich soil some 0.4m in diameter, extending to the full depth of the feature and surrounded by light orange-brown clay-silt. The second was similar but only 0.4m in diameter and 60mm deep, and the charcoal extended for only half its depth. Though in diameter and location these features would make sense as part of some contemporary structure, they are rather too shallow, compared to the tower post-pits, to have taken much weight. The charcoal in their fill is perhaps more likely to have been derived from a hearth, but there was no indication of the subsoil being affected by heat.
The Roman Road
The general line of the road is marked on air photographs by lines of pits (fig 1), the result of quarrying for surfacing material. These indicate a SW-NE alignment, though the exact position of the road line between them is uncertain. The tower faced south-east towards the road, which probably ran only a few metres away. If the most visible line of quarry pits to the north of the tower continued southwards they would have intersected the line of the enclosure ditch. Indeed, a pit-like feature, which appeared to merge into the outer edge of the ditch, was detected just north of the entrance (fig 2) which may have been a small quarry pit. No chronological relationship was apparent.
The excavation produced only a fragment of heavily degraded pottery, from the upper fill of the ditch on the north side of the entrance (AAA). It had the general characteristics of a body sherd of mortarium and Mrs K Hartley concluded that, if it was mortarium, the fabric was of Flavian date.
Construction and Reconstruction
Exact dimensions for the tower cannot be provided since the position of only one post was determined with precision. However, it seems to have been rectangular and approximately 2.5m by 3.8m. The sizes of the post-pits are broadly commensurate with those of fort gateway and tower structures, and may reasonably be taken to suggest that the tower would have risen three storeys high.
The presence of a second post in at least one of the front post-pits is best explained by the need to provide support for an external balcony such as those shown, apparently running around all four sides, on the towers at the beginning of Trajan’s Column. At Westerton, however, a balcony may only have been provided at the front to allow entry at first-floor level, for the two shallow slots facing the entrance seem best interpreted as the supports for wooden steps. Entry at first-floor level is generally assumed for watchtowers on the German frontier as a security measure, and seems certain in the case of examples on the Odenwald Limes which have solid bases of dry-stone and timber.
Little can be said about the superstructure. There was no sign of construction trenches linking the posts, to support wattle infilling or sleeper beams (as at Moss Side). Nailed planking would seem the simplest method of forming connecting walls, but daub was attested at Shielhill North and timber appropriate for wattle construction was recovered from Raith. As suggested above, there may have been a turf bank surrounding the tower, but there were no indications of post-holes to serve as a revetment or to support any entrance structure, so any such bank is likely to have been too slight to have served a major defensive function. Similarly, the ditch may have been more for drainage than defence. The upcast is likely to have been thrown to the outside, for counterscarp banks are attested at several Gask towers.
Analogies and parallels
The closest analogies for Westerton come, not surprisingly, from other towers on the Gask. The single-ditched enclosure is paralleled in all examples to the north of the site. The sites between Glenbank and Kaims Castle have a second ditch, but are otherwise comparable, and similar enclosures are well attested elsewhere. The simple four-post timber tower is also well paralleled. All the excavated Gask towers show this arrangement, as do numerous others in Britain and elsewhere. These sites do, however, show a surprising diversity. The Gask towers are not of a consistent size and shape. Westerton is the narrowest and one of the most markedly rectangular (some of the towers are square), but such variation is typical of Roman military timber buildings, reflecting their ad hoc mode of construction. The expansion at the front of the Westerton tower (see above), is, however, currently unique on the system, although a tower with similar front projections is known at Welzheim, on the Outer Limes in Germany (9/134), albeit this is a stone tower of significantly later date.
Westerton is clearly part of a more extensive system of military posts: towers, forts and fortlets, which, as currently understood, runs from the fortlet at Glenbank in the south to the fort at Bertha in the north and it seems increasingly likely that the system may extend even farther south, at least as far as the crossing of the Forth west of Stirling.
The Roman road from the Forth, though not known in its entirety, may not extend north of the Tay. Along it lie four auxiliary forts: Doune, Ardoch, Strageath and Bertha, spaced at intervals of 6.1 – 13.6 miles. Between them, further reducing the spacing to 2.3-5.9 miles, are fortlets at Glenbank and Kaims Castle and a third such fortlet now seems likely at Midgate, although the recent suggestion, by Woolliscroft, of a further fortlet surrounding the tower at Raith is open to doubt. The towers are interspersed between these garrison posts and currently total 17 or 18, depending on the long-standing, but dubious, identification of a tower at Midgate (see below). Their spacing varies between 800m and 4200m, but distances of 800-960m and 1400-1520m recur, suggesting an underlying regularity.
It is the spacing of the towers which provides the main indication of their function. They have often been interpreted as signal towers. But the primary factor in the location of elements of an arterial signal system is intervisibility. Regularity of spacing would be surprising, and very close spacing potentially counter-productive in terms of the speed and accuracy with which messages might be conveyed. Thus the primary function of the posts was as watchtowers, albeit that part of this role was to keep the nearest garrison post informed, which might involve signalling. Yet the Gask towers generally do not take best advantage of the topography to ensure maximum all-round visibility as, for example at Westmuir, where a position only 300m to the north would have put the tower on the highest point on the Gask Ridge. Such positioning contrasts with that of single towers elsewhere (e.g. Eildon Hill North) which presumably served as look-out posts for adjacent forts. Thus, it is the emphasis on close lateral visibility which provides the key to understanding the Gask towers, not their potential for long-range surveillance and signalling. There can be little doubt that the arrangement constitutes a demarcated line within a frontier system; the towers serving to oversee, monitor and, with the aid of the nearby garrisons, control movement across the strip of land through which the road ran. Indeed, if the arguments below for the date of the system are correct, it is the earliest such line yet known in the Empire. Accordingly, it represents the beginning of a development which culminated in the construction of Hadrian’s Wall and other such barriers, with their elaborate provision for the close control of movement across demarcated frontier lines. This in turn suggests that other towers remain to be discovered to fill the larger gaps, particularly at each end of the system and between Westerton and Strageath.
Whenever a date has been ascribed to the Gask it has been regarded as Flavian. The parallels for the timber towers are predominantly first-century and the system would make little strategic sense in the context of the campaigns of Severus. An occupation in the Antonine period might be a possibility, but seems unlikely given the existence of the Antonine Wall only a few miles to the south, whilst what little direct dating we have from the line tends to support the Flavian hypothesis. Its precise context within the Flavian period is more contentious, however. Its construction could be associated with any of four events: the halt on the Forth/Clyde isthmus in Agricola’s fourth campaign (Agricola 23); the consolidation of territory overrun by the end of his sixth campaign; the construction of the legionary fortress at Inchtuthil and its associated auxiliary forts in Perthshire and Angus, by Agricola, at the end of his campaigns, or his unnamed successor; or as a stage in the withdrawal from Scotland in the late 80s.
The least likely is the third option, as there would seem to be little strategic sense in constructing a closely controlled frontier in Strathearn whilst consolidation was taking place up to 45 miles further north. The so-called glen-blocking forts could be construed as outpost forts beyond the road line, but such an interpretation does not fit happily with the inclusion of a legionary base amongst them. Similarly, the second option, with its implication of successive linear defence systems, does not readily fit the context of continuing advance. Indeed, the phraseology used by Tacitus in reference to the halt on the Forth/Clyde isthmus strongly implies that such a defensive approach was not favoured (Agricola 23).
The latest context, first fully argued by Breeze & Dobson, has been the most widely supported. The Gask is seen as a preliminary stage in the withdrawal from northern Scotland following the abandonment of lnchtuthil and the glen-blockers. But recent analysis of the numismatic evidence makes this interpretation virtually untenable. The abandonment of the northern forts is dated by reference to the latest coins recovered. Consistently these have been issues of AD 86, their mint or near mint condition taken to be indicative of a Roman withdrawal not long after. But extensive excavations at Camelon (information from Dr V A Maxfield) and Elginhaugh in recent years has produced the same pattern, suggesting that the abandonment of all the Roman forts north of Newstead was synchronous. Moreover, Hobley’s analysis of the circulation of Domitianic coins in Britain shows two peaks in the years 86 and 87. Thus the absence from the northern Scottish sites of coins of 87 becomes crucial, for it indicates their abandonment before such coins were in circulation. If, as is widely assumed, these coins were shipped straight from the mints to pay the troops, the withdrawal from the whole of northern Scotland must have occurred by the middle of AD 88 at the very latest, and probably before, which leaves no time for it to have taken place in phases.
The only remaining context for the Gask is the earliest, which would see it as part of the halt on the isthmus in Agricola’s fourth campaign. Tacitus’ unusual geographical precision in describing that location need not carry the implication that associated structures did not extend farther north. Indeed, he makes clear that Agricola’s army had penetrated as far as the Tay in the preceding year (Agricola 22). Furthermore, when the isthmus was utilised as a frontier again by the Antonine Wall, in the second century, it is clear that Roman occupation, in the form of auxiliary forts (on the same sites as those of the Flavian period) extended as far as the Tay.
It has been suggested that the Gask system shows two phases of use, primarily on the basis of the morphological difference between the enclosures to the north and south of Kaims Castle, now supported by the discovery of two structural phases in a number of recently excavated sites (e.g. Greenloaning tower). This is too strong an inference to draw from the presence of a second ditch around some of the towers, particularly given the minor variations in structural detail apparent between the sites. Moreover, the paucity of finds from the excavated examples suggests a very short period of use. Thus it seems much more likely that the structural variations reflect simply the activities of different building parties, a principle which is familiar from both Hadrian’s Wall and the Antonine Wall. Indeed, the historical context proposed would require only a very short life-span between the halt on the Forth/Clyde isthmus in Agricola’s fourth campaign and the resumption of the advance north in his sixth.
The close proximity of the postulated tower and fortlet at Midgate, lying only some 13m apart, has prompted the suggestion that the towers and fortlets may not be contemporary. But, although long accepted, there are major problems with the identification of the postulated tower there. It is the only enclosure in the system which is markedly oval rather than circular; more importantly, Christison failed to find any post-pits in his complete excavation of the interior, but did discover the remains of a small stone base bounded by upright kerbs in the centre, a feature unparalleled in any other tower and reminiscent of a prehistoric burial monument. The continued identification of the site as a Roman tower may thus seem untenable, removing any need to challenge the chronological integrity of the frontier.
Evidence from Shielhill North and now Westerton indicates that on their abandonment the towers were deliberately demolished. This seems to have been a common practice on timber-built forts and has been attested at other watchtowers in Scotland. The reasons for this may be debated. It might reflect the systematic thoroughness of the Roman army; a desire to prevent subsequent use of the site, or the massive timbers involved, by an enemy; or a combination of these. What is clear, however, is that the process of abandonment was both measured and deliberate.
In conclusion, the identification of a demarcated frontier line stretching from the Forth to the Tay has important implications. Though a natural routeway, the line followed is not an obvious topographical frontier and was presumably determined, therefore, by the political geography of the area. This in turn suggests that it may have corresponded to a pre-existing tribal division. The line chosen is ideally situated to control communications across Strathearn between the tribes of Fife and those to the north and west. More specifically it seems to imply a desire to protect the more southerly area from the Caledonians. This would lend support to the suggestion that the Venicones, assuming that they are correctly identified as the occupants of the Fife peninsula, were in some form of treaty relationship with Rome.