The bridge strengthening works were carried out during January 1996 by North Yorkshire County Council Highways and Transportation Department on behalf of the Ministry of Defence. Catterick Bridge is a scheduled monument (NY50) and a grade II* listed building. The archaeological monitoring and recording by Northern Archaeological Associates (NAA) was required as part of the scheduled monument consent for these works.

This report also provides a brief summary of the first phase of archaeological assessment and evaluation (Cardwell and Simpson 1995; Young 1995). It also presents observations on the bases of the two southerly bridge piers after removal of falsework erected in advance of the strengthening works.

A detailed description of each context recorded within the excavation and shown in brackets in the text is provided in the appendix. A site archive consisting of all primary written records, plans, sections and photographs has been prepared and catalogued and lodged with the North Yorkshire County Record Office.

Catterick Bridge spans the River Swale some 2km to the north-west of Catterick village in North Yorkshire (SE 227993) (Fig. 1). The bridge is presently crossed by the A6136, but originally carried the Great North Road across the Swale prior to the construction of the existing A1 dual carriageway to the west.

A number of phases of building, alterations and repair to Catterick Bridge can be identified from documentary sources (Cardwell and Simpson 1995). The first recorded bridge across the Swale at this point was built between 1421 and 1425 by William de Burgh and seven other nobles of the area.

St Anne's chapel was a late 15th century addition and its location to the east of the southern end of the bridge is recorded from later documentary and illustrative sources.

The bridge was substantially rebuilt and repaired between 1562 and 1590. An agreement was made in 1562 between Roger Burgh of Brough and two other nobles of the area with two freemasons to build up and re-edify the southern pillar which had become decayed for £55. Further repairs were made to the bridge in the following years but even so it was recorded as being in a ruinous condition in 1674 (Slack 1986, 1993).

In 1792 the bridge was widened by 13 feet on the downstream side by John Carr of York. The chapel, by then ruinous, was demolished in the process. The bridge itself had been in poor condition as shown by an engraving of 1780 by Grenville (Fig. 2) and it was necessary to reface the north and south arches on the upstream side.

When Catterick Camp was established in 1914 the railway line to the camp ran across Catterick Bridge along the western carriageway of the existing bridge until 1922, when the steel girder railway bridge was built further upstream (Ludlam 1993).

Excavation of a 20m length of the western carriageway was carried out over the southernmost pier of the medieval bridge. The resulting trench was 4.8m wide and entailed the removal of an area covering 100sq m including a pedestrian retreat sited over the bridge pier. Surface layers were broken up and lifted using a heavy toothed bucket after the perimeter edge had been cut out. At this point the telecommunication services and lighting ducts were exposed, as were the tops of the spandrel walls. The services required regular support along their exposed length. This was achieved by tying them to planks placed at right angles across the width of the trench. As excavation proceeded it became clear that the cavities within the bridge pier and between the spandrel walls were much smaller than anticipated. As a consequence of these restrictions a JCB with a ditching bucket was brought in to complete the operation. All loose infilling materials were removed as were any thin or weakly mortared details. The final cleaning out of the bridge structure was done by hand, joints were brushed clean and the whole area dusted with an airline.

Where structures or features of archaeological interest were exposed, these were cleaned and recorded with drawn sections, plans and photographs. A series of profiles across the excavation were drawn and levelled with reference to the bench mark to the north of the bridge at a height of 64.09m OD.

The persistent snow showers which fell during the last two days of the excavation prevented a full photographic record being made.

The material infilling of the bridge pier was removed exposing its internal structure which was constructed of mortar bonded river cobbles. At the lowest and central point in the pier, the floor or saddle was 2.7m below the modern road surface (Fig. 4). The circular nature of the structure was accentuated by its battered sides rising at an angle of 45 degrees to the base of the parapet wall within the pedestrian retreat (section B, Fig. 5). Spandrel wall 06 continued into the bridge pier on the downward side as a low cobbled wall 0.9m high. Above this wall the layers of material that had filled the pier were present in the section.

To the north and south of the pier the decking over the arches of the bridge curved up towards their respective crowns and were surfaced with flat rounded cobbles (19) at each apex. The interior faces of the pier spandrel walls rose from this decking before merging into the sloping inner core of the bridge pier. The southern end of the western spandrel wall (05) was 3.0m in length and had a maximum height of 0.6m. At the point nearest the pier a large re-used(?) sandstone block 0.7m in length was incorporated into the wall of roughly coursed cobbles. These cobbles were 0.1 - 0.4m in diameter and common to both sets of spandrel walls. Parallel to wall 05 the eastern spandrel wall (06) was of a similar length and separated by a gap of 1.2m. This wall contained a number of dressed sandstone blocks and continued around and into the bridge pier for a further 3.0m. It rose almost vertically for 0.75m at its deepest point within the spandrel before sloping eastwards where it supported masonry (07). The spandrel walls of the bridge arch to the north (section A, Fig. 5) showed a greater gap between them of nearly 2.0m and a length of approximately 5.0m was exposed. Though superficially they appeared to be of the same make-up as walls 05 and 06 their construction was supplemented by at least three separate horizontal layers of mortar separating bands of cobbles in a red sandy matrix.

A line of parapet stones (07 and 16) was visible along the easternmost side of the excavation with a gap over the pier. These stones were displaced and broken only at the extreme ends of the trench over the apexes of the arches. A further course of stones was visible beneath these over the spandrel wall north of the bridge but it was not possible to ascertain whether these were also chamfered. The outer rings of arch stones (18) set on end were visible where the parapet stones had been removed at the point where trial trench 2 had been positioned at the north-east corner of the strengthening works.

The details of the infilling layers within the bridge structure could not be accurately described because of their extraction by machine. However, deductions could be made from the occasional sampling that was possible and the results of previous trial holes dug within the area of the excavation (trial trenches 2 and 8) (Fig. 3).

Trial trench 8 centred over the bridge pier to investigate its structural integrity and loading capability was excavated to a depth of 2.5m. The earliest layer encountered consisted of a pink silty sand which was partially excavated to a depth of 1.5m. Above this was seen a layer of yellow-green silty sand 0.6m thick containing occasional fragments of sandstone masonry and which extended up to the base of the modern road make-up. These layers then represent the general fill of the pier base and almost certainly of the southern bridge spandrel too.

The east section wall against Carr's pier base showed a similar sequence of layers but these were complicated to the north by a mortar layer (12) interposed between the layers of sand. This mortar layer was 0.5m thick against the lower course of masonry 16 and seemed to spread from within Carr's bridge pier around the northern lip of the earlier bridge pier within the trench. The mortar thinned out and separated into three layers 0.1m thick across the area between the bridge spandrels and became incorporated into the construction of wall 10.

Trial trenches 2 and 5 were sited over the crown of the northern bridge arch and were contained in part within the excavated trench. Here the structure of the arch was exposed at a depth of 0.45m below the modern road revealing the tops of large squared vertically set courses with tightly mortared joints. Also recorded were the blocks of masonry representing the 15th century chamfered offset stones of the bridge parapet only 0.30m below the modern road surface. Against these was found a small area of cobbled road surface (14) measuring 1.0m x 0.8m and composed of flat rounded cobbles 0.15m in diameter set in loose mortar..

The aim of the 'pre-strengthening works' was to remove all material seen as non-structural prior to refilling the bridge pier and spandrels with layers of mass concrete. This was undertaken with some speed and completed almost totally by machine. Observations made during machining and an inspection afterwards confirmed that the lower fill of trial trench 8 was the primary deposit across most of the excavation except for the crown of the northern arch. This layer of broken angular pink sandstone in a sandy matrix is suggestive of waste from stone-working and could therefore have been readily available at times when the bridge was under repair. This layer was found at the base of trial trench 1 which was sited 8.0m further south over the bridge abutment. This was more than 0.6m deep and its use as a levelling layer is postulated. Sherds of 16th century Cistercian ware were associated with it suggesting the date of its deposition. This layer was also recorded in trial trench 7 which was placed over Carr's southern bridge abutment. Here it lay directly over the mortar bonded structure of the bridge and against the partly dismantled masonry at a depth of 0.9m below the modern road surface.

If this layer is solely a 16th century event, when documentary evidence states that substantial work was carried out on the southern half of the bridge, then it should be contained within the earlier structure and not be part of Carr's 18th century work. Carr could have re- used this pink sandstone infilling from the earlier bridge to cover his structure as seen in trial trench 7, or he could have used his own debris from working similar material. Comparisons of the fills of this excavated bridge pier with those of its 18th century counterpart in trial trench 9 showed that they differed substantially. Also the pink sandstone layer was not present in either of the 18th century bridge piers sampled in trial trenches 9 and 11. This in turn would appear to demonstrate that the lower layers within the medieval bridge, at least in the case of this sand deposit, remained undisturbed by the 18th century activity.

The northern arch spandrels show a different method of construction from those of the southern arch. Boulders within a matrix of this pink sandy material form an integral part of the walls which would date their construction as being earlier than Carr's improvements, but later than the construction of the southernmost arch. The upstream spandrel of this southern arch is very wide and contains Carr's additional widening. This extra arch ring is clearly visible from below (Fig. 7) but the top of the spandrel wall was obscured by services preventing any constructional details from being recorded.

That most of the internal fill of the spandrel wall was preserved during Carr's activities is surprising but possible given the fact that the original downstream 'face' of the earlier bridge shown by the line of coursed parapet stones (07 and 16) survives. Considering also the ruinous state of this bridge pier as evidenced in contemporary etchings (Fig. 2), the only keying-in to link the old and new bridge structures was through the piers themselves, the arches remaining free standing.

A slight deviation in the line of the eastern side of the earlier bridge structure could be seen in plan. If this line is projected as far as trial trench 7 it must coincide with the north-south aligned wall (77) whose east facing outside face showed signs of weathering. This in turn has the remains of the porch of St Anne's chapel butted to it and both are absorbed within the structure of Carr's bridge.

Only a small area of road surface (14) survived which had a direct relationship to the earlier bridge parapet base stones against which it was preserved. It could be dated to the 15th or 16th century period of bridge construction.

The extensive nature of the bridge strengthening works exposed a considerable area which enabled the full interior structure of the pier and spandrels to be recorded. Useful information was obtained which confirmed the documentary detail of the bridge repair and development.