DART: Excavation and probe installation at Harnhill, Cirencester and Diddington, St Neots in April and June 2011-06-18 Keith Wilkinson 18 June 2011 Introduction Excavation and probe installation was carried out on three of the four DART ‘sites’, namely Quarry Field and Cherry Copse, Harnhill and the pasture field on gravel at Diddington between 11 April and 8 June 2011. This fieldwork followed on from previous magnetometer and borehole surveys of all four DART sample sites undertaken in January to March 2011. The excavation and probe installation of the final remaining site at the clay field, Diddington will take place in the 18-29 July 2011 interval. Preliminaries to excavation and probe installation Magnetometer surveys were conducted by Rob Fry and David Stott at three sites in Harnhill on 24-27 January 2011 and four locations at Diddington on 7-10 March 2011. On the basis of the results of these surveys, presumed archaeological features were selected on the four sites for further investigation by borehole drilling. Borehole surveys were subsequently carried out by Keith Wilkinson and Nick Watson (an ARCA consultancy technician) at Harnhill on 3 March and at Diddington on 9-10 March. These latter studies suggested that the ditches selected for study at Harnhill (both in Quarry Field and Cherry Copse) were of c 0.6m depth, while that on the clay field at Diddington was c 1m deep. However, the ditch identified for study on the pasture/gravel field at Diddington proved to have fills that were inseparable from the background soil profile, while the feature also appeared as a notable depression in the present field surface. Consequently the decision was made to target a second ditch on the Diddington pasture field and a further borehole profile was drilled by Keith Wilkinson, Nick Watson and Natsuhito Fuji (an undergraduate student at the University of Winchester) on 16 March 2011. This latter demonstrated that the newly selected ditch was c 1m deep and therefore suitable for probe installation. During the latter visit to Diddington the locations of all boreholes were accurately surveyed using a Leica System 1200 dGPS. Trench locations were planned on the basis of the geophysics and borehole results. Trenches on the two clay sites (i.e. Quarry Field, Harnhill and the clay field at Diddington) are ‘L’-shaped, the base of the L cutting tangentially across the ditch and the longest axis passing along the centre of the ditch (e.g. Figure 1). Trenches on the well-drained sites (Cherry Copse, Harnhill and the pasture field at Diddington) are straight 8m long slots that cut tangentially across the targeted archaeological feature (e.g. Figure 2). Unfortunately Tom Overbury (Royal Agricultural College [RAC]) informed DART in early April 2011 that English Heritage had contacted him to indicate that they had scheduled the Cherry Copse site in February 2011. Subsequent discussions between Ant Beck and the English Heritage Bristol office led to the decision to move the DART sample trench in Cherry Copse away from the scheduled area – and therefore outside the area previously surveyed by magnetometry and boreholing. As a result Keith Wilkinson and Natsuhito Fuji surveyed in three new geophysics grids in Cherry Copse outside the newly scheduled area on 8 April 2011 and using a Leica System 1200 dGPS. The opportunity afforded by this visit to Harnhill was also used to survey more accurately the boreholes drilled in March and to survey in the Quarry Field excavation trench. A magnetometer survey was conducted by Rob Fry and David Stott of the three new Cherry Copse grids on 10 April 2011 and as a result of that work a new trench location was determined.

Figure 1: Quarry Field - Trench and borehole locations plotted against processed magnetometry data. Excavation and probe installation Harnhill Excavation and probe installation at Harnhill took place between 11 and 16 April 2011. Rob Fry, David Stott, Natsuhito Fujo and Keith Wilkinson manually removed topsoil from both the Quarry Field and Cherry Copse trenches on the 11-12 April and then excavated the fills of ditches that were revealed beneath. The latter procedure was carried out using standard archaeological procedures and on a context by context basis. The excavation at Cherry Copse demonstrated that the selected ditch was 1m deep, cut through weathered limestone of the Cornbrash Formation and was infilled by a poorly sorted mixture of red brown silt/clay and limestone boulders and pebbles. Finds from the ditch all dated to the last 100-150 years and included glass fragments and a tin can. It can therefore be concluded that the ditch is a modern feature. In contrast to the relatively simple stratigraphy at Cherry Copse, the excavations at Quarry Field revealed a more complex story. The sampled ditch was cut through clay strata of the Forest Marble Formation to a depth of 1m, while the infilling sediments largely comprised reworked clay, albeit modified by soil forming processes near

the surface. However, two ceramic drainage pipes dating from within the last 150 years were found at the base of the ditch. On visiting the excavation on 12 April Tony Norris (RAC) said that he did not know about the pipes, but gave us permission to cut through them. While the excavations were proceeding Laura Pring and Dan Boddice assembled three TDR boxes and two weather stations at the RAC farm facility in Harnhill village, while Natsuhito Fuji carried out a dGPS survey of a 15 x 15m area around the Quarry Field and 10 x 10m around the Cherry Copse trenches to produce models of surface topography.

Figure 2: Cherry Copse - Trench and borehole locations plotted against processed magnetometry data. On 13 April a tracked mechanical excavator was used to excavate out first the Quarry Field and then the Cherry Copse trenches to 1m depth. On the Quarry Field site this procedure resulted in the discovery of a concrete-lined drain cutting at right angles through the dual ceramic pipe drains in the body of the ‘L’. The former was capped by concrete paving, which

when removed, revealed that the drain was still active. A spread of gravel was found to the immediate south-west of the concrete drain that had also been placed in a cut through the ceramic pipes. The fact that the drain was still active and the truncation caused to the stratigraphy by the insertion of this feature and the gravel led to the decision to discontinue work in the body of the ‘L’ and focus all attention of the north-east part of the trench (Figure 1). Both long sections of the northern arm of the ‘L’ were drawn at a scale of 1:10 by David Stott and then photographed, and then the infilling ditch and non-ditch sediments were sampled in duplicate series (one each for the University of Birmingham [for geotechnical purposes] and University of Winchester [geoarchaeology]) of monolith tins (6 monolith tins were taken in total) (Figure 3). The monolith tins were photographed in situ and then extracted. Laura Pring, Dan Boddice, Nicole Metje, David Chapman and Giulio Curioni then commenced the TDR and weather station installation. Sixteen TDR and temperature probes were placed in the ditch sediments and sixteen outside. The dGPS was meanwhile used to collect positional data for the reference nails used in the section drawings, the tops and bases of the monoliths and the corners of select experimental growing areas used by the RAC in their crop trials1.

Figure 3: Monolith samples in situ in the ditch fills and soil profile at Quarry Field, Harnhill with the mechanical excavator in the background. Note the open top of the concrete drain in the longer section of trench (the drain’s lid is lying on the edge of the spoil heap to the left of the trench). Once the mechanical excavator had excavated a depression in which the TDR boxes and weather station would sit at Quarry Field, it was moved to Cherry Copse to excavate the transect trench to 1m depth (Figure 4). The cleaned faces were photographed and then David Stott and Rob Fry drew the exposed long sections at a scale of 1:10. A duplicate

1 Unfortunately the wrong reference information was input into the base station, meaning that the

dGPS data collected at Quarry Field on the morning of 13 April will need to be re-processed.

series of monolith samples were taken from the Cherry Copse section on 14 April by David Stott and Rob Fry (aided by the mechanical excavator) and a total of four tins collected. The mechanical excavator was then used to excavate a depression to house one TDR box and a weather station, after which Laura Pring, Dan Boddice and Giulio Curioni installed eight TDR and temperature probes in the sediment sequence infilling the ditch and eight outside2. Laura Pring, Dan Boddice and Giulio Curioni spent the period from the afternoon of 14 April to 15 April infilling (with the aid of the mechanical excavator) and compacting the excavation trenches. Several valuable lessons were learnt as a result of work at Harnhill, namely: 1. We had not allowed enough time for probe installation and backfilling. 2. The mechanical excavator had originally be hired for one day, but in practice three days of machine time were required. 3. Carrying out two excavations/installations simultaneously/in quick succession and within a single week led to some comprises in recording and was found to be overly stressful for those conducting the fieldwork.

Figure 4: Cherry Copse – the excavated ditch, also showing as a crop mark (darker green stripe) in the background.

2 Keith Wilkinson, Natsuhito Fuji, Nicole Metje and David Chapman had had to leave Harnhill on the

evening of 13 April.

Figure 5: Pasture field, Diddington - Trench and borehole locations plotted against processed magnetometry data. Diddington Excavation and probe installation in the pasture field on gravel took place at Diddington from 6-8 June 2011, the field team comprising Laura Pring, Dan Boddice, Rob Fry, David Stott, Ant Beck and for 7-8 June, Keith Wilkinson. The trench location had been laid out by Keith Wilkinson using the dGPS on 13 May 2011, during which visit a topographic survey at 0.5m resolution was also carried out of a 10 x 10m area around the trench (Figure 5). A conventional, wheeled JCB mechanical excavator was initially used to remove the topsoil (context 1) along the length of the trench and a compact silt/clay mixed with rounded and sub-rounded flint pebbles was revealed (context 2). This deposit is likely to be the result of mixing (by the plough) of floodplain alluvium and gravel from the underlying terrace. Context 2 was also removed by the JCB to reveal a ditch in the centre of the trench. The ditch was then manually excavated on a context by context basis (contexts 3-5) by David Stott and Rob Fry, with finds from each context being separately bagged. The latter comprise ceramic

sherds of up to boulder size and of probable prehistoric age3, and animal bone. Meanwhile the JCB was used to excavate a depression into which Laura Pring and Dan Boddice placed the TDR box and weather station. Finally the JCB excavated a 6m long section of the trench down to 1.2m depth exposing Pleistocene gravels (contexts 6 and 7) in locations outside the ditch. Excavation of the pit for the TDR box and weather station had partially truncated one of the long sections of the trench, but the remaining part together with the other long section was photographed and then drawn by Keith Wilkinson at a scale of 1:10 on 7 June. Two series of monolith tins were then taken through the ditch sediments on the south side of the trench (a total of four tins) and through the non-ditch stratigraphy on the north side (two tins). The JCB had been off-hired for 7 June. TDR and temperature probes were installed by Laura Pring and Dan Boddice on 8 June. Eight probes of each kind were placed in the ditch fills and eight on the outside. Once the probes had been emplaced they were marked on the section drawing and surveyed in with the dGPS. The dGPS was also used to survey in the reference nails used in the section drawings, the position of two 0.5m-long steel pipes inserted in the trench base on 6 June (for calibrating future GPR surveys) and the ‘real’ corners of the trench (the trench had been planned with a width of 1m, but the bucket of the JCB that was used to excavate the trench was 1.2m wide). The trench was then backfilled both manually and with the aid of a mini-excavator (that had been hired instead of the JCB - at the suggestion of Lattenbury Service’s driver [Mark]). An attempt was made by Keith Wilkinson and Rob Fry to take core samples from outside the ditch, but while the core sampler penetrated the topsoil, compacted gravel/silt/clay and the gravel of the underlying terrace, it proved very difficult to extract. Further attempts to obtain core samples were thus abandoned. However, 12 locations for future boreholes were surveyed with the dGPS and marked with red surveyor’s pegs along the course of the sampled ditch. Quinton Carroll (Cambridgeshire’s County Archaeologist visited the site on 8 June to view the works and the finds that had been recovered. He requested that DART produce a report on the results of the fieldwork for the county’s HER. The fieldwork at Diddington progressed more smoothly than at Harnhill, largely because of the greater time that was available. As a result recording was of a better quality and both probe installation and backfilling proved to be less stressful activities. Acknowledgements All those involved in the fieldwork between January and June 2011 would like to express their particular thanks to Tom Overbury and Tony Norris at the RAC and Vicky Dickens and Richard of Thornhill Estates, Diddington. We would also like to thank the machine drivers from Bison Hire (Harnhill) and Lattenbury Services (Diddington) for the care they displayed in excavating our trenches.

3 The pottery has been taken to Winchester and a report on these artefacts will be commissioned from

an appropriate specialist once excavations at the clay field, Diddington have been completed.