2020 has been an interesting year. Some of us have worked through lockdown, some have been furloughed and stuck at home, and some have been up to other things. This last week I’ve spent time in a strange place. A high security facility, where I had my personal phone, my work phone and my work camera taken away and locked up for the whole day, and I was not allowed out of sight of a handler. Yes folks, it turns out even high security, top secret places need an occasional archaeologist. Now I can’t say that I was working at [REDACTED] because that would be against the rules and I’d get in a whole heap of trouble, but I don’t have an NDA (Non Disclosure Agreement) so I can talk about it to an extent. So, let’s pretend I was doing the archaeology for a Bond villain’s lair, because that would be cool and let’s face it, that underground volcano / Alpine lodge / space shuttle launch site probably needed planning permission and therefore an archaeology survey too!
Who knows what archaeological treasures are hidden below top secret sites! Image credit: Fair use, Wikipedia
I was actually really excited about this job. It’s not very often I get to go to high security places and put holes in the ground. There is something very strange about logging all your details and being background searched before going to a place that is both disconcerting and exciting. Anyhow I must have checked out because they allowed me a two-day window to go look at some holes in the ground.
Doing the background research on some of these places is pretty tough too. You know that it was built at some point because it exists and has done for years, because everyone knows about it but none of the OS maps show it. The place still looks like open fields well into the late 20th century (although it seems to be on current OS maps and Google Maps), so working out its modern history is tricky. There was however a potential for prehistoric activity, possible Bronze Age burials, and Iron Age occupation, in fact there’s a big Iron Age(ish) earthwork that runs right into the place. And the Romans were certainly doing things not too far away. So, I had a fairly reasonable shot at perhaps seeing some archaeology, well except for all that pesky 20th century activity that’s not on the maps. Particularly those unrecorded 20th century gas pipes and electricity cables that go to those buildings that never existed according to the maps. Then there’s the concrete footings for the invisible buildings themselves – lots of concrete.
I couldn’t take my regular site camera on to the site so I had to settle for dictating the shots to someone who was cleared to use a camera and hoping that they would show what I wanted. Although there is always the chance that the photos won’t clear the security and I may never see them. Apparently if there is anything in the background, person, part of a building, the images will be destroyed, or at the very least will never leave the site. Makes me wonder a little bit about what would happen if one my holes turned up something of national importance. Would it have to be hushed up due to the super secure nature of the site, or could it be published? It is known that a prehistoric earthwork enters the site, but what if there is a whole Bronze Age settlement there? Will my report be available in an archive or will the whole thing be redacted for secrecy? In this case I suspect my report will be the usual grey literature report so unpublished anyway.
The thing with reports is that before they get archived the client always gets to look over them, so if there is sensitive information in the report it will be redacted – covered in those secretive black lines, or I’ll be asked to do a rewrite avoiding certain words or descriptions. Ultimately, despite the sensitive and secure nature of the site it is actually just like any other watching brief and will end up in a dusty file somewhere only seeing the light of day if more work takes place in the area or someone does a PhD that happens to include a reference to this work.
On paper this probably doesn’t sound that exciting, but it was certainly an experience. I’ve never worked on another site where I’ve been allowed so little freedom, or been subject to so many security checks. Hell, I wasn’t even allowed to go to the toilet on my own! They did a sweep over my car again when I left to make sure I wasn’t smuggling stuff out. I’m interested to see how many of the site photos I get back, all in all I think it’s a good job they let me keep the pen and paper, at least I could draw sketches of the stratigraphy, I suspect I may need them.
Did I find any archaeology? No. I guess the whole volcano lair thing had dug it all out way back when those maps still showed the place as a big open field. Unfortunate really, it would have been fun to go back.
Way back in the murky past of 2002 I was in the aftermath of my exams and we had just won the fight to continue the already rare Archaeology AS (half-A level) course all the way through to a complete A level qualification. To celebrate, our archaeology lecturer had a great idea. We would build a roundhouse. A typical Iron Age (c.500BC-332BC) dwelling made of wooden posts, wattle and daub walls and a thatched roof. So, for 2 days a week we were traipsed from our college campus in Marple, to the Iron Age Hillfort just up the hill at Mellor, (a site that I also happened to be working on as a volunteer archaeologist – my first ever dig!) It was fun, it was exciting, it was subject to planning permission….
Our roundhouse was to be 1/3 size of a real one, the worry was that we may not have the time to complete construction of a full one. Logs, willow and hazel rods arrived and spades were issued. The first task was marking out, digging holes for the posts and chopping the wood. I have no idea whose idea it was to let teenage boys loose with an axe, but it seemed to work (barring one bloody finger incident). The posts were shaped to points in order to make pushing them into the ground easier and we packed dirt and stones back in around the posts to hold them in place.
To do the roof we cheated a bit, Greater Manchester is not known for its reed beds and thatch cultivation, a real roundhouse up here is likely to have had a turf roof. But we were after new skills and a professional Thatcher from Norfolk came up to show us how to make the roof. First, like with the walls we needed sturdy logs to create a cone shape. Then it was back to the hazel and willow rods to create the horizontal bars that would be the framework for the thatch. The thatch was split into sheaves and our friendly pro showed us how to sew the thatch onto the roof with a huge wooden needle, and how to bash it into shape with, what looked like a giant wooden spaghetti spoon. He got us started and we did the rest, until we got to the top where it was a bit more complicated. More willow was prodded between the thatch to create ventilation and the thatch was brought to a point. No oculus style hole in the roof for us, they have the unfortunate side effect of creating a wind funnel and dragging any hearth fire up into the thatch and burning down the roundhouse. Not that we were allowed to have a fire in there anyway, at a 1/3 size, even lighting a match in there would probably have burned it down.
Once the roof was done we got a first-hand experience of why ring ditches exist around round houses. Going up to the site after and during a rain storm showed just how effective the thatch was, at keeping the inside bone dry sure, but also at flooding the surrounding ground. So, we dug a ring ditch, no one wants rotten foundations to their roundhouse.
So, the walls were built, the roof was on the drainage ditch was dug, with spare logs laft over to create a little bridge to the doorway. There’s no archaeological evidence for these that we know of, most excavated ring ditches tend to terminate and leave a space for the entranceway, but ours kept flooding so we dug the ditch over the entrance and gave it a bridge. Then dug an offshoot into the woodland and the side of the hill when it still flooded!
Drainage sorted out it was time for the real fun stuff. Daubing the walls! Though there were some caveats, being the modern age and all, and health and safety being what it was, we were not allowed to use animal manure to make our daub, (but we could play with sharp axes and stand 7ft in the air on flimsy willow rods). So, the daub was unhistorically accurate clay and chopped up bits of thatch, which we smeared unenthusiastically on the walls and watched it crack and plop into the ditch. It was not at all sticky. What to do? DIRT FIGHT! Well, we were teenagers.
Anyhow the roundhouse got splattered as we lobbed dirt at each other and this was the solution. Lob the makeshift daub at the wattlework. In this way the clay, chopped thatch and grass mix actually worked its way in between the willow and hazel rods creating something sticky within the wall that we could build on. So, several days were spent hurling soggy wet clay at the walls. One enterprising student came up with an experiment one day. Her family cat had been to the groomers, and being a rather fluffy animal, our friend was now in possession of a full carrier bag of cat fur to add to the daub mix. It worked really well, we used the clay and cat fur mix to daub around the doorway and for years this remained the smoothest and best-looking bit of all the daubing. So, there you go, tip for the apocalypse and building your stick house, cat fur is a good bonding agent if you want a smooth finish.
Overall, we spent 6 weeks doing this, 12 days in total, which for students wholly unskilled in the art of roundhouse creation, is not bad at all. Imagine how fast these things could have gone up when being built by people who actually knew what they were doing?
To celebrate on our final day we built a fire and boiled up a stew in a replica Iron Age cooking pot, which was very welcome on a wet and miserable day! We also cooked authentic Iron Age, Linda McCartney Veggie Sausages on a fire (apparently you can’t undercook them!). We were sat enjoying our Iron Age feast complete with authentic Iron Age bread when some official looking fellow came up to ‘assess the building for council tax’!
Some of us always intended to sleep over in the house, especially during excavation season but I don’t think anyone ever successfully managed it, the place was pretty draughty, though it was always dry and was certainly a favourite place for the dig team to skive off on wet excavation days.
The roundhouse is still there to this day, although the college no longer re-daub the walls and tidy it up like they used to. About 10 years ago or so I heard that the drip gully had been filled in due to health and safety concerns. The roof was collapsing a little the last time that I saw it but that must be several years ago now … perhaps I should head back for one last look before it falls in completely.
It was an unforgettable time and fuelled my passion for archaeology as well as allowing me to see what happens when you trust teenagers with sharp tools. – For anyone who has read my books, let me tell you, a lot of Jerry is based on several characters building that roundhouse. It was certainly some of the most fun I’ve had on site and I loved explaining how we did it to visitors on Mellor open days. I would love the chance to do something similar again does anyone know if channel 4 would be up for a new experimental archaeology show? Though I’m still not sure about being trusted with an axe!
In my blog travels through commercial archaeology so far, we have: undertaken a Desk-Based Assessment, marched through the field undertaking a Geophysical survey and opened up a few Archaeological Trial Trenches as part of an evaluation. And, what did we find? Well for the purposes of today’s blog post, not a lot.
If our evaluation turned up mostly blank trenches, but did have a few scattered dated features, then we do still have an archaeological interest in the site. It is at this point that we will undertake an archaeological watching brief. In this case we observe any groundworks pertaining to the new development. The development itself having been assessed as likely to cause minimal damage (if that) to the limited archaeological remains on the site. Alternately we may be in an area of known heritage, say a scheduled monument or stately home with a considerable amount of suspected archaeological remains. In these cases, full-scale excavation is undesirable and an archaeological watching brief is preferred as it is the least destructive means of obtaining information about the monument or property, allowing for the preservation of remains outside of the development footprint without disturbance (in-situ).
The watching brief then is the final part (with the exception of the report) of archaeological work that we will undertake on a site with limited archaeological potential. We are generally expecting very little (except for the scheduled monument scenario outlined above), and record everything we see for posterity, so that it can be referenced should any other work take place in the area. In general we see a lot less than we would on an evaluation and considerably less than on a targeted excavation or an open area. Like the evaluation, what we are gaining is a snapshot of the site, but unlike the evaluation, what we see on a watching brief is being removed as we watch, our watching brief record will be the only record that the feature ever existed. Unless as on my current site we have something like a wall or culvert that the new pipe can be routed beneath.
It is not a fool proof method, and the archaeological observations can be sporadic, meaning that several people may have done a site visit, which can also cause nightmares for the one who ends up writing the report. But it is important to note that even finding nothing at all can be important for future research in the area, to quote that famous phrase – absence of evidence is not necessarily evidence of absence, and noting that nothing of archaeological interest was found in one field whilst three roundhouses were found in another can bring us closer to understanding the wider landscape of an area.
In my experience the watching brief is the least liked task of the field archaeologist (site dependent) as we actually have very little chance to really analyse what we are looking at. It is important to note that the watching brief is actually a very versatile form of site investigation and can range from an afternoon spent observing the excavation of footings for a new conservatory, to several weeks of sporadic trips observing the creation of a new road, to the several weeks of stripping small areas (the footprint) of the buildings of a small development, however this latter example borders on targeted excavation and open area which I will discuss next time.
In general, an archaeological watching brief is about observation. The archaeologist is not running the site (unlike an evaluation or excavation), we are just there to observe the groundworks involved with the construction. This can make the process incredibly frustrating for both parties – the developer wants to get on with the job and the archaeologist wants to record everything as accurately as possible. Occasionally there can be friction. This is not always true: some groundworkers and developers are actually great to work with. For example, the current team that I am working with, under the challenging conditions of Covid-19 and social distancing, are actually some of the best lads I have worked with on a site. We manage to give each other space whilst maintaining our observations and when we come across yet another post-medieval brick drain or foundation wall, they pause the machine and jump in with shovels to clear all the debris around the wall so that I can then hop in with my gear whilst they have a break, and do my final cleaning, photos, section drawings, scaled plan and measured tie-ins from established structures or grid points and record my contexts sheets. This can take anything from 10 minutes to the 3.5 hours I spent recording a monstrous intersection of several walls on Wednesday morning. These guys have been great and a real pleasure to work with, and I wish I could yell their company name from the rooftops, but I’m bound by Client and site confidentiality until the job is all done.
Other times watching briefs can be a real pain, as the archaeologist is not in charge of the site we cannot realistically halt the operation, however we are supposed to be given enough time to make our records and observations. Minutes can often be enough to make basic records, but often full recording is challenging, as the machine crunches on relentlessly and we are left playing catch up and recording features that may once have been ditches or pits in the section of the trench. Hopefully we will have managed to see the shape of the feature on the ground and thus determine whether it was a circular pit or a linear ditch. Although this can be deceptive – I recall vividly recording something that I and everyone else on site swore was a ditch in an evaluation, only to find when we returned and stripped the whole site that it was a very, very big pit and not the potential early medieval boundary to Bury St. Edmunds.
And, there may be another problem, (now this could just be me, but I suspect not), sometimes we simply are not listened to. On one of the worst sites I ever had the pleasure of doing a watching brief on, I found myself having to phone my (male) manager to have him explain that yes I was qualified to do this job, yes all those qualification cards in my wallet (CSCS, Cat and Genny, Overhead cables, manual handling, vehicle marshalling, my company ID card and several others) were really mine and yes I had earned them despite the desperately unfortunate handicap of being a woman! I had to phone my manager several times that day and indeed that week to have him repeat to the site foreman word for word, what I had said I was doing/ needed to do and why. This guy was a nightmare, and (being stubborn) I refused to swap the site with one of my male counterparts because by this point I was annoyed – I’d also lodged a formal complaint and wanted to see how it panned out, so this nightmare continued for about 2 weeks, whilst I recorded the, actually fairly substantial amount of pre-historic activity in the area of a new small two-house development. (I did actually get a very grudging apology – that was in no way sincere at the end of the job, but I also found out later that he also complained that a woman (me) had written the site report too). Fortunately, there seem to be less and less of these “gentlemen” around, but unfortunately construction is still rather male dominated – come on Ladies I want to see more female plant operators and site forewomen! Usually, once a difficult Forman realises that I do actually know what I’m doing and that I’m not trying to hold them up, but am willing to work with them to keep the site moving (so far as is reasonable), they usually relent and let me get on unhindered.
Anecdotes aside, the archaeological watching brief is a short observational analysis of a site. It is not something we would usually do in an area where lots of archaeological remains were expected – unless it is a significant site and the activity is minimal, such as a new pipeline for example. As a result, the ground interventions that we are looking at are generally half a metre wide and a metre or so deep (depending on the pipe, footing etc), so we only have a very small window to see what is going on. I once did a watching brief on the footings for a new garden centre, this involved 100 1m x1m squared excavated to a 1.5m depth and immediately filled with concrete. Fortunately, there was actually nothing to see, though I did manage to photograph every hole from all 4 angles just in case something showed up later. I’ve never found a major discovery during a watching brief and the chance of making one is very low because of the speed we have to go, but it is still possible and I would love to hear stories from other archaeologist who have made interesting discoveries during a watching brief!
Next time I’ll be looking at my favourite type of site, the targeted or open area excavation; a strip, map and sample – this is where all the real fun happens.
Following on from my previous blog on geophysics and the preceding one on Desk-Based Assessments, this time I’m discussing one of the ways we can take the archaeological process forward following those processes. In this case: Archaeological Trial Trenching. It is important to note here that trenching is intended to give archaeologists as much of a view of the site as possible whilst causing the least damage, and that what we see is only a window into what lies below the ground.
There is a distinct advantage therefore in evaluation trenching following a geophysical survey as we can use the survey results to identify potential archaeological hotspots, or individual features and so strategically target our trenches to assess the dates and nature of the expected archaeology, and test the accuracy of the geophysical survey. If there is no geophysical survey available, trenches are usually planned to be excavated either in a herringbone pattern across the site or alternating east-west / north-south to cover a specified percentage of the overall site area.
Trenches can be any size and shape. Usually we mean something 1.80m wide by 10-50m in length, but a trench can be as small as a 1m x 1m hole (often referred to as a test pit). There is also no set number of trenches for an archaeological evaluation. The number and size of the trenches required is determined on a site by site basis. Smaller sites requiring between 1 and 10 trenches and larger ones anything from 30 -300 or more.
There is no set depth that we excavate trenches to either. We look for the archaeological horizons, so the uppermost layer of strata in which we encounter archaeological features, this also on a site by site basis; the archaeological horizon may be the upper level of the natural geological strata or it may be a demolition layer into which pits have been cut. Rural sites often produce shallow trenches usually 0.50m or less. (although deeper stratigraphic sequences may be seen at the base of hills or in heavily worked areas. By contrast urban sites produce trenches that are frequently over 1m in depth and can be several meters deep due to the use and reuse of the site over time.
The aim of the archaeological evaluation trenching is to assess the archaeological potential and the expected significance of any archaeological deposits or features encountered. A large number of archeologically blank trenches indicates a low potential for previous activity to be present on the site. Likewise, a substantial amount of modern landscaping which has resulted in heavy disturbance of the underlying natural geology of the site will indicate a low potential for the survival of archaeological materials. However, trenches which produce archaeology can be indicative of moderate to high potential for previous land use and settlement in which case further archaeological works are likely to be requested before any proposed development of the land will be allowed to proceed.
So, how is the evaluation conducted?
Usually there will be at least two archaeologists present on an evaluation job. A field supervisor to monitor the machine and field archaeologists to undertake recording and excavation of the features. Archaeologists on site will work to a Written Scheme of Investigation (WSI) which details the intended development, known historical background of the area including any previous archaeological works, the expected geology (as noted in the British Geological Survey), height above Ordnance Datum, and if we are lucky, the service plans for the site detailing where any underground cables and/or service runs may be (these are frequently inaccurate see my previous services blog post!). We also have a plan of the intended trench locations. These days we usually load this data into a GPS unit and set out and record our trenches onto the machine. However, it is possible to both set out and record using tape measures and triangulation, although as use of the GPS becomes more frequent, this skill is being lost.
The first thing the supervisor should do on arriving on site (besides greeting the client and if on an active construction site undertaking an induction) is to ensure that the site is safe to work on and that welfare is available (this may be arriving later or may be local facilities). The supervisor should also walk the site with the trench plan to look for hazards such as overhead cables, fences, trees or other potential issues that may require the repositioning of a trench or may cause a hazard to the staff working on the site.
Trenches can be laid out using a plan mapped onto a GPS or can be measured in from landmarks on a map. To ensure that the correct percentage of the site is covered by the evaluation trenching it is a good idea to extend each trench by 1 or 2 meters as some inevitably end up smaller. It’s also a good idea to dig a test pit in at least one trench to test that what looks like the right natural layer is really the layer we are looking for.
Trenches are stripped by machine with a bladed bucket, affectionately referred to as the big yellow trowel, and are monitored by an archaeologist at all times. It is often useful to extend the trench by a few meters when laying out as they have a tendency to shrink once access ramps are in place, and it is also useful to excavate a test pit in at least one of the trenches to confirm that what looks like the undisturbed natural geology is actually the natural geology. Redeposited natural layers can be confusing and geology like brickearth can be hard to spot to the untrained eye.
What we look for during the excavation of the trench are the changes in geological build up, where the topsoil changes to subsoil, where the subsoil changes to undisturbed geological layers – we call it natural geology however a more correct term would be the undisturbed upper layer of the natural strata. At least this is the case on rural sites, as outlined above urban trenches can be several meters deep with huge built up layers. (The difference between the two types of site is worth a blog post on its own!). In either case what we are looking for is evidence of previous human activity.
If there is a high level of archaeology we may not investigate every single feature that we see in a trench. Archaeology is a destructive process and if the evaluation indicates a high presence of archaeological features, their excavation may be best left until a later date when the whole site is stripped and the features can be seen properly. We record everything that we see and locate it with the GPS unit so that when we, or another unit returns to the site, we can locate our evaluation trenches and confirm how what we recorded in the evaluation fits into the site as a whole.
We record everything we do in the evaluation as once we have removed something we cannot put it back. The trenches are tied in to the OS national grid so that there is a record of exactly where everything was found. Each trench and feature are planned either by hand at a scale of 1:20 or, more frequently with the GPS unit. Spot heights are taken, at the top and base of the trench and each feature. Context numbers are assigned for everything that was excavated, photographs are taken, section drawings are created and a thorough record is made before the trenches are backfilled.
A report detailing what was found on the site, including any potential dates from analysis of the finds is produced and submitted to the local planning authority. It is they who will decide what the next step is for the site. If the results of the evaluation match the geophyisical survey, or there was a lot of archaeological deposits and features recorded, then it is likely that further investigation of the site will be required. If there was very little material then perhaps a watching brief will be conducted, observing the groundworks associated with the development, and recording anything that may be observed.
The evaluation therefore gives us the first (second if a geophysical survey was undertaken) look at the site via a series of small windows. It is not a fool proof method. I have heard tales of sites that found a modest amount and on further excavation showed that each trench had been excavated between the burial plots of a vast cemetery. I have done a site myself where the evaluation suggested that some medieval activity had occurred but little of major significance, when we went back to do the excavation we found a medieval brew house. One of our trenches had gone through the doorway and missed out the walls - we had only recorded a shallow gully where the threshold should have been! Our mistake was clear when we returned but during the evaluation we could only work with what we could see.
And it goes the other way too. A few years ago, I dug a site where the evaluation indicated there was high potential for a Roman temple. We stripped the whole area, found an Iron Age settlement and a lot of high-status Roman tile from a bathhouse, but no temple and no sign of structural remains or robber trenches, just a lot of dispersed tile and pottery rubble suggesting a villa was likely nearby. Evaluation trenching is merely a window into what lies below, it cannot tell us everything about a site and so a site’s journey from DBA to development is unlikely to end at this stage. In my next blog I’ll look at the approach we take if we found little in the evaluation stage. A watching brief.
In my last blog we looked at Desk-Based Assessment, the usual first link in the process of commercial archaeology preceding a development. There are several ways to continue forward from the DBA, the easiest (for the developer) is when the assessment strongly indicates either nothing archaeological is likely or that the modern disturbance will have already annihilated anything of archaeological interest, in this case no further work may be required. However, this is rare.
More often the next step will be one of three things; an archaeological watching brief (if nothing much is expected), a trial trench evaluation (if there is reason to suspect archaeological remains), or geophysical survey (if ground conditions are suitable and a significant amount of archaeology is expected). In this blog post we’ll look at geophysical survey as the results of this survey often lead to further archaeological involvement of the other types, which I’ll get to in future posts.
Geophysics then. Let us start at the very beginning, what is it? In short it provides a way of seeing below the ground without digging it up. A kind of X-ray for the earth. Anyone reading who is old enough to remember Time Team, or has watched the re-runs on various channels will recall the geophysics team plodding up and down fields with their gizmos and producing what looked like a heavily pixelated graphic that may or may not show a wall or something. The technology has come a long way since then and a number of commercial archaeology units now have their own equipment, though specialist archaeological geophysics companies also exist as geophysics and archaeology are separate disciplines.
There are several different types of geophysical survey that can be used for archaeological purposes, Resistivity, Magnetometery, Ground penetrating radar (GPR), Lidar, and sonar. Sonar is only used for marine archaeology which is something completely different and involves all of its own specialisms and techniques. GPR is probably the best known type of geophysical survey although is not necessarily widely used for archaeological purposes as there are limited conditions in which it can be used effectively.
Lidar, (optical remote sensing using laser pulses) is slowly becoming more common as more parts of the country undergo Lidar survey, however it remains largely prohibitively expensive for commercial companies to conduct Lidar surveys of site. Should a Lidar survey of the area in question be already available then the Lidar data tends to be utilised as part of the Desk-Based Assessment.
For archaeological purposes resistivity and magnetometer surveys are favoured. Resistivity surveys were certainly common when I was on student excavations in the early 2000’s, although the trend these days is for magnetometer use. However, both are worth looking at as they work in different ways and each is better suited to a particular type of archaeological feature.
Resistivity surveys have been used on sites for several decades and were introduced as archaeological tools to the general public by the aforementioned Time Team (I’d certainly never heard of it before watching John Gater and his team trudge the fields). The survey equipment tends to look like a metal frame wrapped in bits of white plastic pipe, with two or four spikes along the base, attached to a battery pack, (or at least that’s what the last one I used on a student dig back in the early 2000’s looked like – I suspect it was a homemade one). The resistivity survey works on the principle that damp ground conducts electricity faster as there is less resistance to the current. A resistivity meter attached to the electrodes (spikes) measures the varying degrees of resistance during the survey. So, ditches, where the soil retains moisture, will allow a current to pass through relatively easily, whereas something solid like a wall, will slow it down. It works really well for ditches and pits in well-draining geology like chalk and gravel. Downside is, this is really slow, and the kit is really heavy.
Which is why magnetometer surveys have become increasingly common in the commercial sector, where time is a huge factor as developments tend to move quickly (or at least they want all the archaeology done yesterday). Though be warned the magnetometer comes with its own problems. Point 1, users can not be wearing anything metal. So goodbye steel toe caps, farewell zippers, lose the underwire bra, earrings, wedding rings and any other metal you may have, or if you’re anything like me and are surrounded by your own personal EMP just stay well away from them. I’m serious, I can’t touch the things, the magnetic reaction is like dropping a demon in holy water or putting a magnet next to a floppy disk (wow that makes me sound old!) and the whole survey comes out blank. Lucky me.
There are a few different types of magnetometer but for archaeological purposes we usually use a gradiometer with two sensors (as opposed to a single sensor), because archaeology tends to be close to the surface in areas where we would undertake the survey, and the two sensors provides better resolution at shallow depths.
So what does it actually do that makes it better for commercial archaeology than the resistivity survey? For starters it can be used in a variety of conditions, although like most equipment (and archaeologists for that matter) it tends not to like the rain too much. The survey can rapidly cover a large area, as there is no need to stick probes in the ground. The magnetometer detects metals, specifically reacting strongly to steel and Iron, but will also detect burnt or fired materials such as brick, scorched earth (hearths), pottery kilns and certain types of rock which are highly magnetic, however it can also be used to detect smaller things like decayed organics or disturbed soils if magnetic rock formations are not present – hence its usefulness to archaeology.
So why do we undertake geophysical survey? Once the survey has been done and the data processed, a geophysical survey can give us a good idea of what is lurking under the ground, which helps inform the next step. If we can see what looks like several ring ditches and a load of other potential features, a geophysical survey gives us targets to dig – thus informing the location of evaluation trenches (see my next blog post, subscribe and be sure not to miss it!). If the geophysical survey detects little, then we may go forward with a more limited watching brief on the area (more on those later too).
A desk-based assessment was our first stop to obtain a history of the area from maps and records, a follow up geophysical survey is telling us whether or not there are potential archaeological features under the ground, not only that but the geophysical survey gives us an indication of what type of features we are looking at, ring ditches are a good indicator of prehistoric settlement, square shaped enclosures may suggest Romans, long regular lines are likely to be medieval ridge and furrow. Critically the survey allows us to give the developer an estimate on how long we might need on site and the type of excavation that we will be required to do, which informs the cost of the project, both for us and for them. Geophysics also prepares the site supervisor for getting boots on the ground and pointing the big yellow trowel in the right direction. Having an idea of what you’re looking for and a rough plan of where you might find certain features is very useful.
S. M. Porter
Professional archaeologist and author, S. M. Porter loves history, adventure and digging in the mud. Her career is in ruins - just where she wanted it to be.