The First Geophysical Survey to help establish Batford's History: March 2015
Traces of early settlement, needing urgent further survey
Re-evaluation, March 2018
Whilst the interpretation of Alexander Thomas’ geophysics results have been queried in some quarters, there can be little doubt his initial theory – the idea that Batford Field had the potential to be an archaeological site – has been vindicated beyond measure. The public interest generated by his surveys undertaken in 2015 and 2016 has only added to the site’s communal value, which further strengthens the case for the preservation of this important site.
The extensive pre-planning archaeological evaluation in late 2017, carried out by Archaeology South East in response to Hertfordshire County Council’s planning application for a new secondary school (named after Rothamsted’s Katherine Warington), revealed features indicating human activity from the Neolithic to the Industrial periods. These included flint scatters and evidence of ridge and furrow ploughing. However, any remains of the buildings forming Bonny Boys Farm (c. 18th Century), opposite Batford Mill, alluded the survey as the majority of these buildings were likely lost through road-widening earthworks in the early 20th Century.
The discovery of a Middle Iron Age enclosure and an Early Medieval burial ground was most unexpected. A late 7th Century AD, late Christian Conversion period, burial ground is a rare find, particularly in Hertfordshire. This burial ground represents the first archaeological evidence of activity in the Batford / Harpenden landscape during the Early Medieval period. These features make the site locally, and nationally, important. The site has the potential to help us to understand the development of our town.
Archaeology is the study of the human past through its material culture, its animals and its environment. The discipline achieves its aims through a number of different scientific and non-scientific methods. The field on the corner of the Lower Luton Road (B653) and Common Lane was identified as an archaeological site through map regression analysis using GIS (Geographical Information Systems) and field name analysis using the Tithe Award Map apportionment. It was surveyed using a Geoscan RM15 resistance meter and a Geoscan FM256 Fluxgate Gradiometer (magnetometry) during the weekend of Saturday 14th and Sunday 15th March 2015.
The aims and objectives of the survey were three-fold:
- To record any surviving remains of the buildings recorded in the historic maps of the area
- To investigate whether any other features of archaeological interest survived
- To test the conclusions of the desk based assessment produced for Hertfordshire County Council.
This article aims to summarise how the surveys were conducted, the results of the surveys and their interpretation. We should never take heritage for granted. The results from this survey are significant due to the fact that archaeological remains are a finite resource and also because of the limited archaeological fieldwork conducted around Harpenden. Nevertheless, the data from Harpenden demonstrates that Batford is an ancient settlement that has its roots in prehistory and the results from this survey challenge yet further our understanding of our local area.
Methods of “Discovery” and Survey
It is unusual for archaeologists to a “discover” a site. Sites are brought to the attention of archaeologists via members of the public, such as metal detectorists, or when a site is threatened with development. Archaeologists rely on an engaged and informed public. I was made aware of the urgent need to investigate the site on the Lower Luton Road/Common Lane because of the potential school development, though I had always intended doing this at some point. The first things I looked at were the cartographic and field name evidence. This meant doing map regression analysis using GIS and field name analysis.
Map regression analysis requires an archaeologist to acquire all the modern and historic maps of the area that include the site being investigated. The aim of the exercise is to chart how a site has changed over time. Ten to 15 years ago map regression analysis would have required many blown up maps and overlays of acetate, today it can be done through a computer and GIS. GIS is type of software that allows users to overlay vector and raster maps, and create distribution plots and the maps generated are to scale. The use of GIS has become the norm in academic and professional archaeology. Field name or place name analysis focuses on the origin of names and this can give archaeologists hints about what was there before, particularly if one is studying the Roman, early-medieval or medieval periods. What the initial analysis in Batford showed was that there was a series of buildings present in the field, possibly medieval or post-medieval in date, and that they had been demolished some time between the Tithe Award Map in 1840 and the Ordnance Survey First Edition in 1879. These buildings appeared to be associated with Batford Mill, recorded in the Domesday Book under the entry for Wheathampstead. The name of the field, according to the Tithe Award Map, was “Bonny Boys Farm Homestead and orchard”. This description has significance, following field name analysis, as “homestead” derives from the Old English, hamstede, meaning settlement and ‘Bonny Boys’ is also intriguing. The field is also close to other fields named “ham” which indicates a place of settlement.
It is might be surprising to some, particularly viewers of television’s Time Team that modern archaeological practice tends to err away from excavation. This is because excavation is a destructive method and means that an archaeological site can only be dug once. It is also very time consuming, very expensive and demands many resources. Therefore archaeologists often use other cheaper non-destructive methods to survey a site. One such method is geophysical survey. Geophysical survey is considered to be both an intrusive and non-intrusive means of survey: resistivity is intrusive; magnetometry and ground penetrating radar are non-intrusive. Resistivity measures and records the changeable electrical resistance, using a resistance meter, of the soil in a set area up to about one metre in depth. The reason why it is considered intrusive is because it requires metal probes to be inserted into the ground at set intervals. Magnetometry uses a gradiometer to measure and record the changing levels of magnetism within the soil in a set area up to about one metre in depth. Ground penetrating radar, or GPR, images the soil layers by producing cross sections using radar pulses. For the survey on the site Lower Luton Road/ Common Lane resistivity and magnetometry were used. Predominately, resistivity can pick up the remains of buildings and rubble whist magnetometry can detect the presence of ditches, pits and post-holes.
Several weeks before the survey I contacted the farmer of the field, Mr. Phil Holt, to ask permission to survey the field. He very kindly gave me permission and so I assembled a team from the Department of Archaeology and Anthropology at the University of Bristol to survey the site. I also had to complete a few forms for health and safety purposes!
The team arrived on site on the Saturday morning ready for the off. Before any surveying could begin, we needed to set out a series of grids using a number of measuring tapes, as all archaeological surveys’ or excavations’ results need to be tied into the National Grid. Six 30 by 30 metre grids were set out starting from the south-western edge of the field on the corner of the Lower Luton Road and Common Lane with three along the bottom and three along the top. 30 by 30 metre grids are standard practice for surveys undertaken by Historic England (formerly English Heritage).
The Resistivity Survey
The first survey to be conducted was the resistivity survey with only the bottom three grids to be surveyed. The team divided the grids in to lanes, with each lane set two metres a part starting at one metre along the X-axis. Grids are divided into lanes to help keep to the sample interval, which was a one metre traverse along the Y-axis. The field was surveyed using a zig-zag traverse mode. The resistivity meter is the product of the 1980s: it is white box with a series of buttons, a two line LED display and a memory disk to record the data points. The white box has a series of plug sockets that enable it to be connected to the different probes and a computer when the data is ready to be downloaded and processed. The white box sits on a black frame with two probes at the bottom spaced half a metre apart. The white box is connected to these probes as well as a remote probe – every time a reading is taken and recorded an electrical circuit is created between the two sets of probes (see figure – top right). The resistivity survey was successfully completed by the end of the first day.
The Magnetometry Survey
The second day was devoted to completing the magnetometry survey. Magnetometry is used in professional archaeology to complete rapid surveys because once the equipment is set up the survey can be completed very quickly. As the equipment used is very sensitive the surveyors cannot wear any metal during the survey as this would produce anomalous results. Before the equipment can used it must calibrated to the magnetic field of the site which means finding a “quiet place” where there is low magnetic variation. Using a compass, the equipment must be pointed to each of the four compass points (north, south, east and west) as well as up and down ensuring the different magnetic readings between the six points are within a certain range – normally within one nanotesla (nT) of each other. The difference will invariably be greater and so it is necessary to use the different inbuilt dials to adjust the settlings so that the readings are within the 1nT range. For this survey all six grids were to be surveyed. The X-axis sample was the same as the resistivity survey, whilst the Y-axis traverse sample was set to 0.5 metres. The zig-zag traverse mode was kept the same too.
In order to complete a magnetic survey successfully it is also necessary to know the operator’s walking speed and the length of their pace. A gradiometer automatically takes readings during each traverse at regular timed intervals, though this setting can be adjusted before the survey starts. Nevertheless, it is imperative that each traverse takes roughly the same time to complete and the equipment produces beeps to help keep the pace.
Downloading and Processing the Results
After every survey the results had to be downloaded from the equipment to a computer and processed using specialist software. The resistivity results were processed in a program called TerraSurveyor, whilst the magnetometry results were processed in an application called GeoPlot.
The resistivity results was processed thus:
- Clip: 2.00SD.
- Despike: Threshold: 3×3.
- Deslope: Horizontal using Polynomial.
- Normalize: Complete survey.
- Periphery Match: All grids in the survey.
- Interpolate: X and Y doubled.
The magnetometry results were processed thus:
- Despike: X=1, Y=1, Threshold=3.
- High Pass Filter: X=10, Y=10, Wt=U.
- Zero Mean Traverse: Grids=All, LMS=On.
- Low Pass Filter: X=1, Y=1, Wt=G.
- Interpolate: Y Expand – SinX/Y x 2.
- Interpolate: X Expand – SinX/Y x 2.
- Interpolate: Y Expand – Linear x 2.
- Interpolate: X Expand – Linear x 2.
Post-report and results analysis
It was a couple of weeks later that new evidence came to light which may shed light on Harpenden’s prehistoric past. A large pinch of salt is needed here as further work is required to test this hypothesis. I was made aware of the existence of at least one a tunnel discovered in the 1950s very close to the field surveyed and there is evidence to suggest that it could be part of a network. During the survey we noticed that the field surveyed was strewn with small pieces of flint and chalk and in the eastern section of the surveyed field there are some very large earthworks which are circular in shape and concave in profile. These features were clearly man made and this was confirmed once again after further consultation with prehistory experts. The features cannot be watering holes as the field is right next to the ford and the River Lea and when I asked Mr Holt, the farmer, about them he had no idea of their origin but suggested the cattle were spooked in the area near the holes. Cattle may react to a number of things, but one is the vibration caused by unstable ground. The circular features are not marked on any of the historical maps which may suggest that they are extremely old; similar shaped features have been identified as mine entrances elsewhere in the county and these may date to prehistory. Much, much more work needs to be done to establish this hypothesis, but if our hunch is correct then this is an extremely exciting prospect.
Whatever the date or function of these geophysical and earthwork features, it is clear that there is important archaeology in this field in Batford and this needs to be investigated further. We should never take our heritage for granted: archaeological remains are a finite resource. These archaeological discoveries are particularly important given the limited archaeological fieldwork conducted around Harpenden. The results from this survey challenge yet further our understanding of this historic, and may be prehistoric, town.