A geophysical survey of approximately 2.6 ha was conducted over the henge and barrow sites known as Arbor Low and Gib Hill, Derbyshire (National Monument Numbers: 11503 and 11501) at the request of the regional Inspector of Ancient Monuments. Though Arbor Low has been subject to some intrusive investigation (Gray 1903), little work has been done to place the two sites into a wider setting.

The aim of the survey was to investigate the geophysical response over the two sites with a view to providing more information on the archaeology, both within and surrounding the henge. In addition it was hoped that the survey would inform future management programs.

The site (SK 158 633) lies on well drained silty soils of the Malham 2 association (Soil Survey of England and Wales 1983) developed over Monsal Dale Limestone (Institute of Geological Sciences 1978). At the time of the survey the field was under grass and used as a recreation area and pasture.

Magnetometer survey was chosen for the rapidity of coverage it would give, as well as its ability to detect pits and other features that might be expected within a henge (eg Clark 1996, 93; David, 1998). The survey was conducted over all the numbered grid squares* (Figure 1) using the standard method outlined in note 2 of Annex 1. To highlight areas of interest better within the henge, a higher resolution survey was carried out over grid-squares 3-5 and 8-10, with a traverse interval of 0.5m.

Due to the erection of a fence around the mound of Gib hill, it was not considered feasible to carry out a magnetometer survey within this small area owing to the adverse effects of the fence on the magnetic response.

Plots of the data-set are presented as both an X-Y traceplot and a linear greyscale, at a scale of 1:1000 on Plan A. The only corrections made to the measured values displayed in the plots were to zero-mean each instrument traverse to remove heading errors and to ‘despike' the data through the application of a 2m by 2m thresholding median filter (Scollar et al 1990) to reduce the detrimental effects produced by surface iron objects. In addition the lower and upper values have been trimmed for presentation as a traceplot.

*Four grid-squares (1,2,11 and 12) were re-surveyed in September 2000 due to corruption of the original data.

An earth resistance survey was conducted to provide complementary information to the magnetometer survey, particularly with respect to the henge interior and the linear earthwork extending from the south of the henge. Additional work was also carried out inside the fenced area at Gib Hill. The resistivity survey was conducted over all the hashed grid-squares (Figure 1). Measurements were collected with a Geoscan RM15 resistance meter, PA1 mobile probe array in the Twin-Electrode configuration. Readings were collected using the standard method outlined in note 1 of Annex 1, but with a sample interval of 0.5m x 0.5m for grid-squares 3-5 and 8-10. The remainder of the area was surveyed at 1.0m x 1.0m intervals. Plots of the high resolution data-set over the henge monument are presented as both an X-Y traceplot and a linear greyscale, at a scale of 1:500 in Plan B. This data has also been interpolated to combine it with the lower resolution grid-squares, and is presented as both an X-Y traceplot and a linear greyscale, at a scale of 1:500 in Plan C. The data-set from the work around Gib Hill is similarly presented in Plan D.

A graphical summary of the significant anomalies discussed in the following text is provided on Plan E1.

The overall magnetic response is unusual, with a background of very local variation against which a more widespread texture of rectilinear alignments is apparent. Much of the background activity seems likely to be a response to local geology and soil conditions; the linear anomalies are more probably of anthropogenic origin.

The more intense magnetic responses are probably modern in origin. The magnetic disturbances at [1] correlate with the presence of a gateway between two fields. The magnetic noise at [2] most probably relates to disturbance of the adjacent barrow (Gray 1903, 466). The disturbance at [3] may be due to excavations over the narrow dyke running from the south-west of the henge (ibid 482). The disturbed areas at [4] and [5] are due to ferrous fencing at the field boundaries. The disruption to the magnetic signal at [6] coincides with a bank terminal and is likely to be due to the remains of a steel channel used to support an MPBW information sign (K Smith pers comm). The disturbed magnetic responses at [7] could be associated with fence posts, or a gate, associated with a previous management regime on the site. However, there is no record of any such structure being present within the last 30 years (K Smith pers comm).

Outside the henge, various linear anomalies criss-cross the site. Running north-south in the east of the survey area are two sets of positive linear anomalies [8]. Across the surveyed area, broadly parallel to [8], are several other linear anomalies [9]-[12], and approximately orthogonal to these are another series [13]-[20]. These linear anomalies are difficult to interpret, particularly as there is some variation in the character of both positive and negative responses. They may represent ditches, or perhaps the accumulation of soil against former field boundaries; but it is also conceivable that they correspond with as yet un-recorded joint systems in the underlying Carboniferous limestone.

Amongst the linear anomalies are more discrete positive magnetic readings. A large (~ 8 x 5m) raised response is visible at [21], which may be related to the positive alignment at [22]. South of [21] is a smaller positive magnetic anomaly [23] that possibly relates to a pit. Directly north is a larger but more subdued response [24], selected in this interpretation owing to its correspondence with the location of a barrow (see Figure 1) excavated by Thomas Bateman in 1844 (Barnatt 1996, site 8.9). The latter four anomalies are all recorded within an unexplained area of uncharacteristically subdued magnetic response [25]; the latter seems to partly over-lie the double negative linear anomaly at [13].

Comparable in size to [21] is another large and isolated anomaly, [26], south of the henge. This is likely either to be a large pit, or perhaps more probably, an area of burnt deposit – of unknown age.

In the southern area of the survey grid, just distinguishable against the background magnetic variations are a number of apparently discrete positive magnetic anomalies, for instance at [27] and [29]. To the north of [27] is a positive curvi-linear anomaly [28].

There are various anomalies associated with the henge ditch and bank, but very little patterning is apparent within the henge. Parallel to the outer edge of the bank is the positive linear anomaly [30], possibly an artefact of medieval ploughing up to the henge bank (K Smith pers comm). The area of raised magnetic response at [31], and possibly anomalies [15] and [16], most probably relate to the linear earth work and previous excavations of it (ibid). The henge ditch has been identified at each of its terminals at [32]. The small areas of disturbance [33] in the centre of the henge could relate to excavation activity (such as of the skeleton, ibid 480), or modern detritus; they are unlikely to have prehistoric origins. The positive linear anomaly [34] running between the disturbances at [7] is most likely due to a trackway into the henge.

A graphical summary of the significant anomalies discussed in the following text is provided on Plan E2.

The resistivity survey is generally dominated by high resistance readings, mainly due to the shallow depth of soil. Bedrock is very near the surface at [R1] and [R2] but less so in the central area [R3] around the Cove. In the central area itself, within [R3], lower values suggest either a greater overburden of soil here, greater compaction, or an association with former backfilling.

Due to the cutting of the bedrock, and silting up of soil, the ditch terminals are just evident as areas of low resistance at [R4] and [R5]. However, more obvious than this is the linear low resistance anomaly [R6] which corresponds to the linear earth work (Figure 1).

At Gib Hill the resistivity variations are amorphous and, without the benefit of wider survey coverage, cannot be shown to form part of any recognisable broader pattern. The high resistance anomalies indicated in Plan E3, if not of archaeological origin (eg residual cairn material) may just as well reflect natural variations in the underlying soil depth (cf. Barnett 1996, 20, fig 1.8 I), as at the henge. Unrecorded interventions at the site may also have been a contributory factor.

Although the magnetic response over Arbor Low was varied, disappointingly little extra information seems to have been gleaned by the magnetometer survey. Known features such as the henge ditch and linear earthwork were identified, as well as evidence of former fencing. However, no new features, such as timber post settings have been distinguished. The remainder of the survey area was characterised by a linear patterning of uncertain origin and of unknown association with the henge. Other magnetic anomalies have been detected in the vicinity, including possible pits, but - again - a definitive pattern or interpretation cannot be made.

The resistivity survey of the henge interior has mainly recorded the proximity of the bedrock to the ground surface. The one archaeological feature identified in this survey is that of a low resistance linear anomaly, probably deriving from the silting up of the ditch associated with the visible earthwork extending southwards from the henge.

The resistivity survey at Gib Hill complemented the findings of previous earthwork survey, but recorded little additional information.

In general the geophysical response at Arbor Low and Gib Hill has been disappointing and this must in large part be attributable to unfavourable soil and geological conditions. Whilst some of the grosser earthwork features have been detectable, there is little geophysical evidence for subsidiary associated features. However, the absence of appropriate anomalies, or patterns of anomalies, cannot be taken to imply that significant components of this ceremonial area are not present. Further and more extensive geophysical survey might elucidate larger patterns or linkages within the area but so far the indications from the recent surveys are not very encouraging.

British Geological Survey, 1978, Buxton, England and Wales, Sheet 111, Drift Geology, 1:50,000.

Barnatt, J 1996 ‘Barrows in the Peak District: a Corpus' in Barnatt, J and Collis, J Barrows in the Peak District. Recent Research. J R Collis Publications.

Clark, A 1996, Seeing Beneath the Soil; Prospecting Methods in Archaeology. Batsford.

Gray, H St G 1903 ‘On the excavations at Arbor Low, 1901-2.’ Archaeologia 58 part 2. 461-98.

Scollar, I Tabbagh, A Hesse, A and Herzog, I (eds.), 1990, Archaeological Prospecting and Remote Sensing. Cambridge.

Soil Survey of England and Wales, 1983, Soils of England and Wales, Sheet 3, Midland and Western England.

Figure 1 Location plan of survey grid squares over base OS map (1:2500).

Figure 2 Linear greyscale plot of magnetometer data over base OS map (1:2500).

Figure 3 Linear greyscale plots of resistivity data over base OS map (1:2500).

Plan A Traceplot and linear greyscale of raw magnetometer data (1:1000).

Plan B Traceplot and linear greyscale of resistivity data at 0.5m mobile probe spacing (1:500).

Plan C Traceplot and linear greyscale of resistivity data at 1.0m mobile probe spacing (1:500).

Plan D Traceplot and linear greyscale of Gib Hill resistivity data at 1.0m mobile probe spacing (1:500).

Plan E Graphical summary of significant geophysical anomalies (1:1500).