SURVEY RESULTS

2005 / 58 Queenborough Castle,
Isle of Sheppey, Kent

1.	Survey Area

1.1	The castle site was investigated using a combination of resistance survey (Geoscan RM15 resistance meter), Ground Penetrating Radar (Pulse EKKO 1000 GPR unit with a 225MHz frequency antenna) and magnetic survey (Bartington Grad 601-2 fluxgate gradiometer). Figure 1 shows the location of the survey areas at a scale of 1:1000. 

1.2	The survey grid was set out by Dr Henry Chapman and tied in to the Ordnance Survey grid using a Trimble real time differential GPS system. 

2.	Display

2.1	Figure 1 shows the limits of the resistance and gradiometer survey areas and the location of the GPR survey areas together with the direction of the traverses. 

2.2	   The resistance and gradiometer results are displayed as greyscale images in Figures 2 and 4, while Figures 3 and 5 are summary interpretation of the survey results at a scale of 1:625. Figure 6 shows a summary of the GPR results and interpretation diagrams at various scales for ease of display. Figure 7 shows a drawing of Queenborough Castle circa 1640 and a plan. Both show the general morphology of the structure although the scale may not be accurate (Project Design 2005).

2.3	The display formats are discussed in the Technical Information section, at the end of the text. Letters and numbers in parentheses in the text of the report refer to magnetic/resistance and GPR anomalies, respectively, which have been highlighted in the relevant data plots and interpretation diagrams.

3.	General Considerations - Complicating Factors

3.1	Ground conditions were moderate to good for data collection; most of the area had a relatively short grass cover and was free of obstructions. However, the centre of the survey area contained two wells which had been capped and covered by a raised brick/concrete rectangular platform (15m x 12m). 

3.2	Generally the quality of the resistance data was good, allowing identification and interpretation of the suspected archaeological features, however, the ground conditions were extremely dry which did hinder probe contact. 

3.3	For Health and Safety reasons a metal, 2m high, perimeter fence was erected around the survey area. The fence restricted the use of the gradiometer and calibration of the instrument was also affected. 

3.4		The ground conditions were not suited to GPR. The soil and underlying London Clay severely reduced the depth of penetration. While depths have been indicated on the GPR diagrams, these have to be viewed with caution. The conversion from time to depth depends on the velocity of the electromagnetic signal through the ground. Given the nature of the site, this is likely to vary markedly over relatively small distances and, as a result, any depth conversion is only an approximation. An average velocity of 0.08m/ns has been used for the time to depth conversions following velocity analysis using graphical methods involving the fitting of curves to point source reflections.

3.5	Where there is a strong electromagnetic contrast, the GPR signal can be inter-reflected or reverberated, producing a delay in the reflection of the signal. This is termed 'ringing'. This happens, to some extent, with all reflections and results in a greater apparent depth than actually exists. As a result, it is often not possible to detect the base of features; only the tops of buried features/deposits are detected with certainty (Annan 1996).

4.	Results of Survey

Gradiometer Survey

4.1	The data are dominated by the linear highly magnetic anomaly (A) which runs northwest-southeast through the centre of the gradiometer survey area. This is presumed to be a large pipe possible originating from the now obsolete wells that were used to supply naval vessels until the 19th Century.  A second linear anomaly (B) runs parallel to (A) but gives a far weaker response and may be a drain or similar feature. There is a strong magnetic spike at point (C); this anomaly is believed to be unrelated to the castle, as are the anomalies at (D) which coincide with the location of a WWII air-raid shelter. The area of magnetic disturbance at (E) may be due to the disturbance associated with the concrete platform.

Resistance Survey

4.2	The resistance survey proved to be the most successful of the three geophysical techniques used at this site.  

4.3	The linear high resistance anomaly (1) coincides with the weak magnetic response (B) and represents a modern pipeline, while the rectilinear responses (2) provide a clear plan of the air-raid shelter, magnetic anomaly (D).  

4.4	The core of the site, surrounding the concrete platform, comprises a broad block of high resistance readings that presumably relates to the main castle structure. The curving band (3) probably represents the walls of the inner Keep, though whether the inner or outer walls is uncertain. Other responses are thought to be associated with demolition rubble and landscaping, though the picture is far from clear. The readings at (4) are interpreted as the inner entrance into the castle (see Figure 7).  

4.5	Surrounding the core of the site is a broad band of low resistance (5) that is thought to represent the courtyard between the Keep and the outer defences. These defences, which comprise a moat ditch and wall, are visible in the data at (6). The high resistance reflects the collapse rubble from the wall and the relatively dry top of the broad ditch feature. At (7) the readings are particularly high and on excavation it was discovered that large quantities of stone from the castle had been pushed into the moat at this point. It is uncertain if the high readings at (8) relate to the castle or whether they are more recent in origin.

GPR

4.6	The GPR survey did not show any evidence of the suspected castle remains.  Figure 6 shows two radargrams from the site. Radargram AR1 was collected prior to a trench excavation. The excavation confirmed the pipe identified in the radargram and the depth of the feature. A few anomalies identified within the GPR traces related to modern features, pipes, which were later confirmed by excavation.  

4.7	Radar data were also collected over part of the Keep. Radargram, CR1 Figure 6, shows the limited penetration depth and the lack of response of a wall later confirmed by excavation. The clayey soil is presumed to be highly conductive which reduces the penetration depth of the radar waves and produces the strong parallel reflections, seen in all the radargrams. The nature of the soil effectively masked the subsurface features.

5.	Conclusions

5.1	Results from the resistance survey proved successful providing a basic footprint of Queenborough Castle. Although no clear plan was obtained this simply reflects the archaeological situation at the site. The castle has been heavily robbed, the ground consolidated and landscaped. It is perhaps surprising that so much has survived intact. It proved very difficult to resolve the matter of the scale / size of the castle due to conflicting interpretations of the historic documents and maps / plans; this did not help with the interpretation of the geophysical data.

5.2	The gradiometer and radar surveys added little to the detailed picture apart from confirming the courses of service pipes. 

Project Co-ordinator:	I Wilkins 
Project Assistants:	J Gater, C Stephens & E Wood

Date of Survey:		17th August and 19th August 2005
Date of Report:		20th  December 2005

References:

Annan, A.P., 1996 	Ground penetrating radar (workshop notes). Sensors & Software Inc., Canada.

Gillam-Smith, N. 2005	Proposed archaeological evaluation at Castle Green, Queenborough, Isle of Sheppey. Project Design, unpublished.	

SITE SUMMARY SHEET #

2005 / 58 Queenborough Castle
Isle of Sheppey, Kent.

NGR: TQ 913 712

Location, topography and geology 

The town of Queenborough is located on the west side of the Isle of Sheppey and on the north side of the River Swale, immediately south of its confluence with the River Medway. The oldest part of the town lies on a coastal plateau less than 5m AOD. The site of Queenborough Castle sits at the eastern end of the High Street. The underlying geology is London clay surrounded by alluvial deposits which once formed extensive marshes of low lying. The soils are unclassified. 

Archaeology

Queenborough Castle was completed in 1367 and survived for three hundred years before being systematically demolished. There are no visible remains of the castle today; the site is a public area that is grassed over. The eastern area of the site was cut into by a railway line in 1860; Queenborough Elementary School was built on a portion of the western area in 1864 and in the 19th century a well house was constructed in the centre of the site, on top of the original medieval well. This pump house that sat over the well is now marked by a concrete platform. 

Aims of Survey

The aims of the survey were to establish the size and morphology of the castle as part of a wider investigation carried out by Time Team for Channel 4. 

# Background information taken from Gillan-Smith 2005 

Summary of Results 

Magnetic, resistance and ground penetrating radar (GPR) surveys were carried out in an attempt to locate the foundations and walls of the castle buildings. Of these three techniques the resistance survey provided the most informative results; the footprint of the castle was identified together with evidence for the key elements of the complex. Although a clear plan was not obtained, this result reflected the reality of a site that had been systematically demolished and subsequently landscaped.

List of Figures

Figure 1	Location of Survey Areas						1:1000
Figure 2	Gradiometer Greyscale	 					1:625
Figure 3	Gradiometer Interpretation						1:625
Figure 4	Resistance Greyscale							1:625
Figure 5	Resistance Interpretation						1:625
Figure 6	Selected GPR Radargrams						nts
Figure 7	Drawing and plan of Queenborough Castle				nts
Figure 8	Archive Gradiometer Data as XY Trace				1:625