leighandgill

Archaeology in East Oxford

Archive for the tag “surveying”

Minchery Priory – Preparation

At last I’m able to get around to talking about last year’s Big Dig at Minchery Paddock – we have had to take some time off owing to day-to-day life intruding. Neither of us had imagined how complicated and time consuming selling our place in London was going to be; however we are now (fingers crossed) on the last lap so can devote a bit more time to the important things in life!

After a whole lot of work on the Team’s part, they got permission from Oxford City Council to dig in Minchery Paddock; a closed-off (in the sense of preventing vehicles in) field as shown on the map.

Location of Minchery Paddock in relation to East Oxford

Location of Minchery Paddock in relation to East Oxford

Here is a close up, showing where the paddock was in relation to the Kassam Stadium (to the right) and Oxford Science Park (to the left).

Close-up of the map above.

Close-up of the map above.

Both maps courtesy of Open Street Map –  © OpenStreetMap contributors.

The site is of interest because of the proximity of Minchery Priory – in the map you can see a building just next to the bottom right of the site; this is the “Priory and ?” pub, a Grade II* listed building,  which was rebuilt in the middle or second half of the 15th century, having been the eastern range of the cloister garth of the priory (Pantin, 1970). 

The car-park side of the Priory and ... ? pub. We never did work out what the ... ? was all about.

The car-park side of the Priory and … ? pub. We never did work out what the … ? was all about.

The name “Minchery” is derived from the Old English ‘mynecu’ or ‘minschen’, a nun. The priory (originally dedicated to St Nicholas) was founded by Robert de Sandford probably in the middle of the 12th century, was taken over by the Templars in approx. 1240 and managed by them until the order was suppressed in 1312. It was dissolved by Wolsey in 1525 after various scandals about the prioress and the nuns and passed to Cardinal (later Christ Church) College, though by 1549 it had passed into the hands of Powell family who held it until the 18th century. More information about the priory can found in an article in the VCH, and about the surrounding area in another article about Sandford, again in the VCH.

Pantin, mentioned above, has provided us with a plan of what he thought was the layout of the priory. He thought the cloister extended to the west from the existing pub, so in theory it could extend into the area which we might be digging in. However we have no really hard evidence for this, one of the reasons for digging here! The Council did think about developing the site so commissioned John Moore Heritage Services to do an evaluation of the site in 2006, which has provided us with some targets for working out where to place our trenches. Apart from this report, there have also been trenches dug when Greater Leys and the Oxford Science Park were developed. These have found prehistoric sherds and flints, evidence for Roman kilns (especially in Greater Leys), a Saxon village under the Oxford Science Park as well as evidence for medieval farm sites.

However the site did provide us with some new challenges – unlike last year at Bartlemas we did not have a friendly College to provide us with a pavilion to use for a start; we had to hire in loos, storage (especially important, we thought, after hearing some horror stories about vandalism from a nearby construction site) and a site office, and last but not least, somewhere for the poor volunteers to shelter if it tipped it down.

The other challenge was the site itself –

This gives some idea of how overgrown the site was before we had it cleared

This gives some idea of how overgrown the site was before we had it cleared – Jane, Jo and David P surveying, and no, none of them are vertically challenged!

We had to get in a commercial crew with a tractor and flail to clear the undergrowth, and to chop back branches in the south end of the site where a whole load of self-set sycamores had grown up. This was one of the reasons for us being here – the roots of these young trees could be damaging any archaeology, especially as they were growing where there might be remains of the priory. Once the clearing had been done, in came the Portacabins and a whole load of fencing as well as a load of tracking to put down so the lorries could deliver all the stuff without getting bogged down. A big thanks to Olaf for this, it was a real bit of choreography to organise everyone arriving in the correct order.

The Portacbins installed - the blue one was for storage - the cream one had the office, the mess room, and a generator - the loos were round the back.

The Portacabins installed – the blue one was for storage – the cream one had the office, the mess room, and a generator – the loos were round the back.

While all this was going on, we also were marking out where to put the trenches. As I mentioned before, we had the John Moore Heritage Services report to use as a starting point, so we planned out trenches accordingly.

Our trenches (the hatched ones) against the John Moore ones (the lines).

Our trenches (the hatched ones) against the John Moore ones (the lines).

Original diagram courtesy John Moore Heritage Services (JMH). A bit confusing of-site, as it shows a range of buildings to the north of the pub which are no longer there; it’s just a bit of a wasteland used as an overflow car park on match days and an area for a bit of gratuitous fly-tipping.

We decided on three trenches. Trench 1, at the north end of the site, up by the brook, was put in because JMH had found a layer of peat there – we wanted to take a continuous set of soil samples from this layer. Not only could we get environmental samples and therefore start to work out what the contemporary environment was like, but by doing some radiocarbon dating we will be able to find when the peat started to be layed down and when it stopped. Both are most probably linked to human activity changing the way water flowed in the area.

Trench 2, in the middle by the office and storage sheds, was put in next to two JMH trenches. JMH trench 3 which contained a couple of robber trenches and a possible boundary ditch and JMH trench 4, containing a well (which we planned on avoiding!), a hearth and a possible floor surface. As you can see from the plan, Trench 2 spanned the two JMH ones.

Trench 3 , in the south, spanned JMH trench 8 – they would have had trouble putting it in today as a tree had grown up in the middle of it – hence the rather odd shape of our trench. JMH found walls, aligned east-west , but we would have dug there anyway, owing to the proximity to the pub. While marking this trench out prior to the digger coming in, we came up against one of the drawbacks of using survey-grade GPS – the device does not like working near trees. It has to have line -of-sight contact with the satellites to work properly. I was finding one measurement would be OK, then it would give a ludicrous distance to the next plotted point. The marvels of modern technology!

Talking of which, the reason I haven’t mentioned geophysics is, as JMH discovered, the site had been used for doing a lot dodgy things to cars in the past, including torching them. This has resulted in a pretty even spread of bits of magnetised iron over the site, so a gradiometer just gives such a noisy result as to be virtually useless. That’s not taking into account that we discovered we had stumbled onto Mole Central – I would not have liked trying to walk in the nice and even style required by that sort of survey over a surface which seemed to have mole-hills on its mole-hills.

After we had marked out all of the trenches we let the digger loose –

The digger in trench 3 - you can see the pub in the background.

The digger in trench 3 – you can see the pub in the background.

The digger driver, Nigel, was a real asset; we had worked together before at Bartlemas and apart from having a real feel for the machine, and being a nice guy, he’s developing quite an interest in the archaeology. We rapidly came down onto (we hope) archeology in all three trenches – looking good for the start of the actual excavating!

Leigh

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Building Survey

On Friday and Saturday, there was a building survey at Bartlemas Chapel; Gill and I were booked in on the Friday session. This was to give us an introduction to the gentle art of doing a scaled drawing of the elevation of a building, in this case our old friend, Bartlemas Chapel. We met up with Jo a bit early at ArkT to pick up the gear we needed and then headed off to the Chapel. I was really gratified to see that there was virtually no trace of all our fevered activity just under a year ago.

A year (almost) after there was a large trench here, and now hardly a sign of all our hard work.

So, after the usual signing in, health and safety and introductions, Jo and Jane started to explain how we were going to go about doing the drawing. In a lot of ways it’s just like drawing a section of the side of a trench – only you’re looking up instead of down! Just like doing a section the first thing is to measure the length and depth (or height in our case) of the area which we’ll be drawing, and then work out what scale (1:10 or 1:20, say) we are going to use given the size of paper we have. A fair amount of head-scratching ensued, but there is nothing so irritating as drawing away merrily for hours, then dropping off the edge of the drawing board – a bit of time spent in preparation is very well spent. Then start by putting in the title (where we are and what we are drawing) the date, the scale and who was doing the drawing. Preparation of the paper done we then started on the wall itself.

This is where it diverges from doing a section – one can’t start hammering nails into a grade 1 listed building! The principal is the same, though; we need a datum, a reference from which all the measurements can be taken. So the first thing to do was set up the dumpy (no one can remember why it’s called that – everyone remembers being told, but as no-one remembers it must be a pretty boring explanation).

Setting up the dumpy

After it has been levelled, with a built in spirit level, we started putting in the datum line; normally we use a string (we had brought some road pins along to stretch the string between, but found we couldn’t push them into the ground) but this time we used a chalk line drawn on the stone work – it will wash off in the first rain. So using the dumpy we drew a line at the same height all along the wall which we would be drawing. Then the fun started.

Basically, the procedure is that we measure a set of prominent points (the corners of distinctive stones, for instance), draw them in, and then freehand the intervening detail. Sounds simple, eh?

Two of us measuring a point on the buttress.

Just by Jo’s head you can see the datum line continuing along the wall from where she is holding the tape so Andrew can measure the vertical distance to the point which we are going to plot. We used the measuring staff, which is leaning up against the buttress, to do the points which were too high for the steel tape.

So after we’ve got a few points measured, then the drawing starts.

The hard work – doing the actual drawing.

This, of course, is the whole point of the exercise. We have had a laser scan done of the building, and have obviously taken a shed-load of photos, but there is no substitute for drawing – the human eye is capable of much better discrimination than any photo. It’s not so much a matter of  “accuracy”, but the ability of a combination of really looking at a subject, then using the drawing to bring out the relevant details.

So after a lot of hard work the end result looks something like this – this is one I prepared earlier ( to coin a phrase) – it’s the end, and the start of the other side, of the buttress that Jo & Andrew are measuring in the photo above.

The end result – the second group started quite a bit later than the first one.

Another group carried on on Saturday, and when Gill & I popped in on Saturday afternoon to have a chat, Jane said we might well carry on Sunday, as quite a few of us are going to be at the Chapel as part of Oxford’s “Open Doors” event. Christopher & Sarah Franks are opening the Chapel so the Project is going to lend a hand (they got a bit overwhelmed last year by the unexpected number of visitors) and do a bit of explaining about what we found during the dig last year. Hopefully there will be a bit more of the chapel to show in a later blog.

Leigh

Topographic survey by tractor, etc

Olaf has done had his first stab at doing the topo survey of South Park (see Geophysics at South Park). After having worked out the best way to attach our GPS unit to the tractor –

The GPS unit strapped to the front of the tractor with gaffer tape

Then Olaf pressed the button and off the tractor went, gathering info as it went. The grass was pretty long, as you can tell from the picture Jo sent me –

The tractor in full flight – you can just see (hopefully) the aerial of the GPS device poking up at the front of the bonnet (?, perhaps engine cover) of the tractor.

Gang mowing in action, in the service of archaeology! Olaf had set the GPS to collect data every 1 second, which gave the following set of collection points –

Each green circle represents a sample point.

Which when he had crunched the numbers gave us the following –

The results of the survey.

I’m not quite sure what the drop-outs represent – I’ll have to have a chat with Olaf when he returns from his travels. I do know he plans to have another go when they next mow the park, as they will do it at right angles to the last time, so we’ll get a different set of data-points, which will increase the resolution of the final image.

I had written quite a lot more, but when I tried to save it, I was asked if I was sure I meant to do that (?), and when I said yes – you’ve guessed it – everything I had just typed in disappeared. Thank you, WordPress.

Main point was I’ve finished adding the fields to the Cowley Enclosure map –

All the cultivated areas added to the background details.

I’m now letting my eyes return to normal!

We’ll report on yesterday’s talk on Oxford’s Medieval Farming Landscape and other tings in the next Blog – I will have calmed down by then.

Leigh

Various Updates

Nothing specific going on, just continuing with this and that, though some interesting stuff has come to light.

First off, Olaf has sent me the following images following on from our morning in South Park.

Olaf’s professionally presented results showing the sample points from when we did the initial test walk. Imagery © Bing Maps/Microsoft Corporation and its data suppliers – this applies to the next 2 images as well.

Pretty slick, eh. Pity it doesn’t reproduce so well at this sort of resolution, though the background image does show up the ridges and furrows rather nicely. I’ve extracted the relevant bit and blown it up a bit – what you are looking at is the processed result of the scan overlayed with a red circle showing each point at which a sample was taken.

The same image, enlarged, so it’s hopefully a bit clearer.

We all did a few back and forth scans, then Olaf did the wander around (this was before opening time, honest!). The next shot is of the processed results, again blown up a bit from the centre of the image Olaf sent me.

The processed results of our wandering around.

The spotty bits are where we bounced a bit while carrying the GPS, but you can clearly see the ridge and furrow – the slightly strange bits at the top right and bottom left are where the software didn’t have enough data to work on and was trying to be creative.

So that was the “proof of concept” trial, and it seems to have worked pretty well, so Olaf went off this morning and met up with Andy to strap the GPS on to the tractor and do the real data collection run. I’ve heard it went OK, and Jo sent me this photo –

The GPS unit strapped to the front of the tractor.

Olaf went back to the gaffer tape, I see – I’m looking forward to the final result, I’m just hoping the tractor didn’t bounce around too much. With no suspension and the bumpiness of the terrain it could be a problem, but we can only wait and see what comes out in the processing.

We’ve also found a lot more about the Enclosure map, or maps. Graeme came across this map http://www.icowley.info/words/15,and started to look into it a bit more. I contacted the web site in question, but they just pointed me at the Oxfordshire History Centre. Graeme eventually found this new map on microfilm at the OHC, which has opened up some new avenues which he and Christopher are looking into. With the help of Carl Boardman, History Services Manager at the OHC, we realised that when the Enclosure award was worked out, the commissioners had their own working copies of the maps, which ended up in London, and are now at The National Archive. They also deposited one copy with the diocese, which is the one at the OHC, and another copy with the parish – this one often got lost. As I said, new avenues to look down.

And talking about maps, the digitising of the Cowley Enclosure map is coming on – I’ve done the rivers and streams, the existing roads (as of 1853) and the roads which were to created after the enclosure, and I’ve made a start on the houses with their surrounding property. Here is a quick glimpse of how far I’ve got.

The whole map.

And a close up –

Temple Cowley

Just as I was about to publish this post, Olaf phoned me up, alerting me to a couple of emails he had sent me about the results of today’s topo survey – they look great – and I’ll do a report about that tomorrow.

Leigh

Geophysics at South Park

I’ve just got back from a meeting with Olaf & David Pinches at South Park,

South Park, in relation to the rest of East Oxford

where Olaf had arranged to meet up with Chris & Andy from the City Council’s Parks and Leisure Department. Chris (whose background is in archaeology!) is being really helpful towards the project, and has agreed to try a novel form of surveying – we are going to strap the GPS to the side of a tractor so that we can gather a whole load of info while Andy mows the park.

The GPS has a setting whereby you can get it to take readings at specified time intervals, say once every second, so in theory, we could get a complete survey of the park done in the time it takes to mow it. It is going to need a bit of fiddling with the attaching of the GPS to the tractor – the two main problems are going to be vibration, as a tractor has no suspension. The only thing that stops (barely, at that, from experience) the driver getting shaken to death is a sprung seat. The other one is the actual way of attaching the GPS – both Chris & Andy suggested cable ties rather than gaffer tape, which had been our first idea. The main point is avoiding a horribly expensive sound as the GPS drops off and promptly gets mowed, with extreme prejudice!

Olaf is going to meet up again with them when Andy brings the tractor and gang-mower along to have a go at sorting out the practicalities. After they had gone off to carry on working (this is about the busiest time of year for them, everything growing like mad and with the school hols, the parks being used like mad) we had a go at using the GPS in this automatic data collection mode. First we started off by doing an area near where we were (the entrance on Headington Road), all taking turns to do a bit, then we set off up the hill to so some more serious work.

David’s area of interest is (rather imprecisely) Oxford during the Civil War, which ties in nicely with doing a survey of South Park, as there are, hopefully, the remains of the earth-works the Parliamentarians set up to bombard the city which was the Royalist capital. You can see why – you get a marvellous view of the “dreaming spires” up there. After a little confab –

David and Olaf working out which area to walk to get a representative sample.

Dave set off at a brisk pace, walking up and down the hill, to get a mini survey covering a section of the ramparts. In the above photo you can also just about pick out the medieval ridge-and-furrow field system – the yellow Plantain is flowering along the tops of the ridges which you can see stretching diagonally across the photo. Cycling across the park must almost make you sea-sick, the ridge-and-furrows are so well preserved.

David treading boldly!

The purpose of all this hard work was to get some representative samples of the sort that the tractor might gather, so that Olaf can play around with it a bit, to see how it comes out. He is also going to look into if there is any LIDAR data for the park – but it’s a bit random as the main amount of data collection has been by the Environmental Agency for compiling flood-risk maps. Whether areas off to the side get looked at depends on flight paths, which rivers, and streams are their responsibility, and all sorts of other factors, so it’s a bit of a lottery, but worth looking at as the data is very high quality. You can manipulate it in all sorts of ways to bring out detail which might not be noticed under ordinary viewing.

We finished up and I gave David a lift back to Rewley House with the GPS (it packs into a rather unwieldy large red flight case) while Olaf went off to a meeting with Jane.

I’m carrying on preparing for a Test Pit which Gill and I are running, just off Cowley Road, tomorrow and Friday, which I think is going to be interesting – regardless of what finds turn up, the location is in the area of Cowley Marsh. We have been thinking about what precisely “Marsh” might mean. When we did a test pit in the Elder Stubbs allotments we came down onto sandstone after about 0.40 metres which doesn’t seem all that marshy. But do we know it is pretty damp around Bartlemas, so more information is going to be useful. As an aside, I was talking to Christopher Franks, who lives in the Farmhouse opposite Bartlemas Chapel the other day and he says that the trench around the Chapel, which was our “excuse” for last year’s dig there, is a great success. During the downpours earlier on this year, the chapel stayed dry throughout!

I’ll be back with what happens at the test pit this weekend and what Olaf comes up with from the survey.

Leigh

Geophysics Week

A while ago (apologies for the gap in posts, I’ve been getting my head around a new GIS package, ESRI ArcGIS, and working on the Cowley Enclosure map – more about that in a later blog) Olaf organised a geophysics week. We knew it was in the pipeline, but he has had to do a lot of work behind the scenes to get all the necessary permissions for access; some sites were public recreations grounds and one was a school, so a lot of negotiation was in order.

I missed the first few days as my car had given up the ghost (Oh Woe, Oh decreased bank account)

The first site to be surveyed

Olaf started off by surveying the public recreation ground next to the sports field at Rose Hill Primary School that I talked about in an earlier blog (09 May – Geophysics Weekend) – this was done while I was busy. I met up with everyone after lunch on Wednesday at Donnington Sports Ground.

Donnington Sports Ground (to the west) & Larkrise sports ground (to the East of Iffley road

I found a couple of things had changed since the Geophysics Weekend, both for the better.

First Olaf had sorted out the GPS; it turns out that you have to explicitly log on to the OS network. Survey grade GPS works by being basically the same as your off-the-shelf satnav, but it has a built-in mobile which dials up a network of stations maintained by the Ordnance Survey which give the unit a whole load of extra data to refine the information coming down from the satellites. Why anyone would spend about £12,000 and then not want to log on, which then drops the accuracy back to a £150 satnav, is beyond me – but that’s the way it goes. This was why we were getting such a lot of problems with the GPS on the previous weekend.

The second change was that we have upgraded the gradiometer to the two tube version.

Two tube gradiometer.

This has two advantages – first, the obvious one, is that you collect the information twice as fast, or to put it another way, you walk half the distance to collect the same amount of info. The other is that being quite a bit heavier, it comes with a harness, which makes it a lot easier to use.

So while the gradiometers were settling down (the first thing that happens when we arrive on site is that the gradiometers are assembled and left to come up to ambient temperature) we went and marked up the grid of squares to be surveyed – with the GPS now working properly this is pretty quick; we marked the vertices of the squares with an aerosol as the sports ground is a public space, we didn’t want bamboo sticks sprinkled around (Health & Safety).

We had two gradiometers with us, the upgraded Project one, and another one which we had had lent to us by the Continuing Education department at Rewley House. This meant we were able to run two teams (it takes three people in a team – one to do the actual survey and two others to move the trapeze) so we could really get going! Well, in theory, but as this was a training exercise, not all of us were zipping along at the same speed as Olaf, even when Olaf, David Griffiths and Jane were doing the tricky bits – it’s bad enough doing the grid squares, but doing the odd, non-square bits round the edge need a bit more confidence. You have to have the basics down pat before starting on the bells-and-whistles.

We ran into another problem then, also to do with it being a public open space – a load of guys turned up to use the football pitch. We will have to come back really early to finish off the remaining grid points, though I’m not sure what sort of results we are going to get around the goal posts as the gradiometer is thrown out by large lumps of iron (Olaf said it could have an effect as much as 10m away).

Next day we went off to the Larkrise Sports Ground – this was a different kettle of fish as it was part of (confusingly) Gregory the Great school so we had to sign in and get issued with ID tags – in fact, as we thought we might have some of the sixth-form students coming along, we had to have one person with a CRB (Criminal Records Bureau) check present; though as it turned out, they had something else to do so never turned up.

Olaf left Tim, Steve and I to do the GPS part of the survey – setting up the grid squares – so we each did about 6 grid points each. You really start to get the hang of it with a bit of repetition! I had to shoot off at that point (I was booked in on the next Animal Bones workshop) but Jane arrived just as I left to take over and make up the numbers, so they were going to have no problems with finishing off.

Slightly depressingly, one of Olaf’s next-door neighbours turned up and was telling us that he had planted the hedge and assorted trees around the edge of the field – he said the digging was awful;it looked like the level had been made up with some modern “soil”, so we weren’t at all confident of finding anything. This was a bit of a blow as we knew that this was part of the medieval field system, and I had found a couple of marker stones shown on the Cowley Enclosure map, but with a load of modern stuff laid on top of the older surface, any archaeology was most probably going to be masked.

Saturday turned out to be the last day; Olaf had planned on doing Sunday as well, but it looked like we were going to finish off so he felt like cancelling Sunday and having a bit of a (well deserved) lie-in. I managed to set up the GPS and check if we could set up another grid point – we couldn’t; trees caused me to lose the signal about 6 metres away from the grid point – but it was good experience doing it from scratch on my own.

I did a couple of grids and then I had to do one of these partial grids. Instead of walking the full 30 metres, one stops short at, say, 28 metres, then starts again at 28 metres to go. Sounds simple, but you do have to be fairly confident of walking at a steady pace to do the return walk and get to the end marker at the correct time.

I’ve been chatting to Olaf about getting the results to show here – we’re having problems emailing it (we don’t know which end is limiting the file size) – so I’m going to call it a day now and do another blog when we have managed get the files across, and when I’ve worked out how to display them!

Prehistoric Landscapes

On Thursday, 17th May, Olaf Bayer, our new Project Officer, gave a talk entitled “Prehistoric Landscapes” at the ArkT Centre in Church Cowley.  Sorry about the lack of photos, but as he was using the projector, the light levels were a bit low, and I didn’t want to get into all the copyright problems involved with asking if I could use some of his PowerPoint slides. He was introduced by Dr David Griffiths, who also mentioned that the Archeox project had been shortlisted (last three) for the Best Community Archaeology Project for this years British Archaeology Awards – though we won’t know the final results until early July, so stick around!

Olaf started off with some definitions: –

Mesolithic       10,000 BC to 4,000 BC

Neolithic           4,000 BC to   2,000 BC

Bronze Age       2,000 BC to      500 BC

Then he went on to sketch out some of the salient points to remember about the periods; it was a transitional time, with people slowly changing from being hunter-gatherers to being farmers – from a mobile life-style to a much more stable one. Though he emphasised that this was not sudden change; it was a slow process, with hunter-gatherers starting to manipulate their landscape and gradually starting to extend the periods when they stayed in one place until eventually they ended up permanently occupying one site.

He then described the characteristic technologies of the periods; in the Mesolithic microliths (small, sharp flints which were mounted side by side in hafts to produce tools) predominated, then in the Neolithic ground stone tools (handaxes, etc) appeared, to be followed by copper – the Chalcolithic – and then bronze – the Bronze age, by which time the use of stone tools had been mainly abandoned. Along side the changes in technology, there were allied changes in the monuments that these people have left in the landscape,  from burial sites, to tombs, to enclosures. Then to the idea of “Landscape” itself – not just a view, nor just a map, or even a set of resources – more a meaningfully inhabited space. In all human behaviour, there are always choices to be made – this would have been as true in Mesolithic times as it is today.

After laying this groundwork, Olaf went on to describe his post-grad work in the lower Exe valley, Devon where the Culm to the East and the Creedy, to the West joined the Exe. This is a broad valley, defined by areas of hard geology to North and South, which has been extensively farmed since Neolithic times, so the soil has been churned up by ploughing since time immemorial; so one of the main lines of investigation Olaf followed was a collection of stone tools (over 16,000!) amassed by extensive field-walking by John Uglow (1921 – 2007) and Thurston Shaw (later of Cambridge University). Rather than measure each and every item in the collection and apply statistical analysis, for some strange reason Olaf decided to select a sample which could be dated with a degree of certainty (this was done by use the “debetage” – the chippings left over from stone working – which gives different chippings if one is producing long stone blades, say, as opposed to small flaked blades) of about 1-2% of the whole collection, about 250 items. These were then mapped so as to show their distribution in time as well as place – two of the main conclusions Olaf drew were A): that there were other reasons apart from just resources that determined where people were gathering, and also, rather interestingly, B): that there seemed to be no difference in finds density between “ritual” sites and other sites (in more recent periods one can tell a church, say, from a house, by the difference in the assemblage of finds).

Olaf then talked about the various methods he had used to investigate the current landscape in order to tease out what remains from past – hard enough at the best of times, but this area had, as mentioned above, been under the plough for centuries. He used aerial photography (to see crop-marks and the like)  and LIDAR, a method using airborne lasers to obtain very accurate elevation data. LIDAR is useful not only for its inherent accuracy, but also for the way in which the data can be manipulated to bring out details which would otherwise be missed. Most rivers and their surrounds have been LIDAR mapped for information about flooding, but the data that has been obtained this way is an invaluable archaeological resource. These views were then used to pin-point areas for further investigation – geophysics leading to excavation.

We were shown slides of a couple of trenches Olaf had put across a the ditch defining a large enclosure – the trouble he found (apart from the weather) was that the soil was such that there was very little discernible difference in soil colour between the normal ploughed soil and the ditch infill. He had decided to put in a large “sondage” – in this case a mini-trench within the main trench going from one side to the other – to obtain a section across the ditch. Even this was difficult to unpick as worm- and root-action had blurred any sharp distinctions; the main definition between the soil and the infill was the angle of the large stones which had fallen into the ditch and lain at an angle on the side of the ditch! The dating evidence was pretty uncertain too – Olaf had some radiocarbon dating done, but the samples were so small that they could have been moved through the soil by worms or even gravity, so that Olaf reckoned he could not really draw any firm conclusions from the dates he got, even though they were pretty accurate in themselves – it could well have been Bronze age, but could also have been post-medieval . As we are finding out for our selves – trying to reconstruct a story from the evidence in the ground can be incredibly difficult!

He then went on to refer to a dig he has been involved with at Damerham, West Hampshire, which is a combination of university placements and community archaeology – as we were getting near the coffee break, he showed us a couple of slides and pointed us at the website.

After the break he continued by describing how he was planning to go about trying to obtain information about the prehistoric landscape in East Oxford. There is plenty of evidence of prehistoric activity – “crop-marks” in university Park, evidence from various excavations (a henge, circular enclosures) in the centre, so we know the area has been occupied for a long time – so where to start?

Initially, a look at the collections of stone tools, etc held in the various museums and collections in Oxford (The Pitt-Rivers, Ashmolean, County records etc) and looking at aerial photos of the area (aerial photography was almost invented around here). As I mentioned in an earlier blog (the recent one about geophysics) Olaf wants to do gradiometer surveys of as much of the open spaces as we can get our hands (or wellied feet) on and when the time is right (one needs to do it after the crops have been harvested and the area ploughed, and just after rain, to damp down the dust), we could do some field walking to the south of our area – south of Greater Leys and Littlemore. Apart from that, we need to carry on with the test pits, and inquiring about finds which people may have turned up in their gardens and allotments – not much to do then.

A fascinating talk, and an intimation of lots of interesting work to come – so much so that some of us (all of the professionals and a few of the volunteers) repaired to the pub (the Rusty Bicycle) to carry on the intellectual discussion – the highlight of which seemed to be how to wind up thirsty , but broke, theology students!

Geophysics Weekend

We have just spent the weekend – well the days, at least! – attending a geophysical survey weekend at Iffley’s Rose Hill Primary School. Olaf Bayer, our project officer, has negotiated permission to do a survey of the playing field at the back of the school, and decided to use the opportunity to familiarize more people with the kit involved.

Location of Rose Hill Primary School

He intends to start a programme of doing surveys on as many of the open spaces as he can get access to, and this weekend was a start on building up a team to carry out the work. We turned up on Saturday morning, being really thankful for the fact that it wasn’t raining (after all, we do have a hosepipe ban), though up on the top of the hill, with allotments to one side (North West) and a recreation ground to the East, the wind cut right through us – and in the morning it was bitter.

The kit all laid out, gradiometer out of its box, signing-in sheet ready for action, with our wellies, lunch etc.

Olaf started off, after the obligatory signing-in and confirmation of having read the Health & Safety info (easy to take the mickey about, but not so funny when things go wrong), by explaining about the preparation needed before we started doing any geophysics and the equipment we would we using. Initially we got the piece of kit – the gradiometer – out of its box to settle down to ambient temperature, and then he started to talk about the need for laying out a grid. This is laid out in squares of 30 metres a side over the area to be surveyed – the size of the squares being a compromise between large (less faffing about surveying but bigger gaps fitting the squares into an irregular shape) and smaller (more surveying, more turning around and toing-and-froing during the survey but fewer gaps around the edges of the survey). We needed to lay it out accurately in the sense of accurate squares (obviously) but also in terms of position as we want to extend the survey into the recreation ground to the East of the Primary School – it’s separated by a tall, painted steel fence – and we need to be able to marry up the future survey with the one we’re doing this weekend.

The area we could survey in and a rough idea of where the grid was laid out

The idea was to use the projects survey-grade GPS (it’s like a sat-nav but works to centimetre accuracy, instead of the 3-5 metre accuracy of your normal off-the -shelf device – you pay a fortune for the precision, though) to fix the points where the squares meet. Not all of them, just along two lines at right angles to form a cross, and then use tapes and ranging rods to fix the other points from that base-line. The GPS has a neat facility where you can input the co-ordinates in the comfort of your own home or office, and then it’s just a matter of selecting the right co-ordinates and up pops a screen like this:-

Screen on the GPS device

The two arrows on the right of the screen show how far – in metres –  that you have to move the device to get to the exact position that you had earlier keyed in, get the numbers down to the sort of precision you are interested in and there you are; change to a different Point ID and do the next position. The trouble was the numbers kept on jumping around, even when we held the device totally still! At first we thought it was because we were right next to a big sycamore – survey-grade GPS relies on a direct line-of-sight link to the various satellites – its major draw back; but when we had done four positions in a line, we noticed that they were definitely not in a line! Much head-scratching and re-checking ensued. Olaf has used this sort of kit a lot, and has never come across this sort of problem, but he said he has never used this particular make and model before. I think it’s down to the perennial problem with modern technology, the more functionality you pack into one device, the more details you have to get spot-on or you get problems down the line; and I think we have all had experience with struggling to wrest information from manuals. So we decided to cut our losses and go back to the old way of doing it – tapes, right-angled triangles and using an optical square and Olaf would sort out the device at a later date.

It was the first time any of us had come across an optical square; a little hand-held optical device for locating you on a straight line between two points, and then working out a right angle from that base-line (you can get some more details about the principles involved here). But first we had to establish straight lines which we did by sighting along the ranging rods and using tapes to work out the 30 metre grid points. We then used a combination of the optical square and good old Pythagorus (3-4-5 triangles, and a new one Olaf told us – if the two sides are 10 each, the hypotenuse is 14.14) to lay out the complete grid. If it looks like there are a lot of gaps from the diagram above, it was because there was a lot of rough ground around the edge of the playing ground proper, as well as a small pavilion and some vegetable beds planted out (I don’t think we would have been flavour of the month for stomping over them in our wellies).

After a break for lunch and, more importantly, something warm to drink, we started on actual surveying. First we had to divest ourselves of anything ferrous; the gradiometer is incredibly sensitive. It works by detecting tiny variations in magnetism so anything magnetic near it throws out its data; so no zips, eyelets, metal buttons, watches, mobiles, steel-toecaps, etc.. It basically means wellies, track-suit bottoms and tee-shirts – though Olaf had some waterproofs he has customized by cutting out all the zips and other metal work and then sewing in Velcro. The sensitivity is why we had to take it out of its case to settle down and why it has to be calibrated – the tube which you can see in the photo has a magnetometer at either end.

Calibrating the gradiometer

By subtracting the reading from the one at the bottom of the tube from the reading from the top one, you “edit out” the effect of the Earth’s magnetic field and are left with any magnetism caused but stuff buried in the ground; archaeology hopefully but just as probably services, old nails or any old iron. To do the calibration Olaf had to wander around a lot to find a relatively “clean” area – a place with as little magnetic noise as possible; you can set the device so it beeps when the magnetic field changes so you aim for as quiet an area as possible. Then you go through a routine of facing the points of the compass and turning the device upside-down (basically you are following the instructions  on the screen that you can just see being peered at above). Once it is calibrated we were almost ready to get stuck in – just one more thing before we started; we all had to checked to make sure no stray ferrous items had crept in by accident!

Queuing up to be checked for unknown metal bits

Then we could start. The basic procedure is that one has to walk up and down in lines in the 30 metre square, a metre apart at a time until the whole square has been covered. To make this easier, we use a “trapeze”; two poles with holes a metre apart through which are threaded cords 30 metres long – you can see where the name trapeze comes from – and the mid-point of each pole is marked with tape. Then, on opposite sides of the square we stretch measuring tapes which have marks along their lengths at every other metre. You lay the trapeze down lined up with marks on the tape, the operator walks up one cord then back down the other one, the trapeze is moved up to the next mark, operator walks up the cord and so-on. The cords have marks every metre and the gradiometer beeps every metre so that when you are the operator, you aim to get the beep happening when the tube is directly over one of the marks; you set the device with a speed you are comfortable with for that to occur – fast for Olaf and embarrassingly slowly for the rest of us.

Gill taking readings – you can see the trapeze (the blue cords) with the spacing pole at my feet in the distance. Thanks to Olaf for the photo.

You would think that just walking up and down a laid-down cord at a steady pace would dead easy, wouldn’t you? Just goes to prove that you have to try something before you can pass judgement. It really does take a bit of getting used to – quite a bit of deleted and repeated lines, and later on in the day Olaf stepping in finish in a reasonable time! We did the southern most three squares (North is at the top in both maps, sorry Jane, as usual I’ve forgotten to put in the North arrow) on Saturday and finished off the rest on Sunday. At the end of the day on Sunday, Olaf got out his laptop and did an initial pass through the data we had collected – a fairly obvious services pipe at the North end of the field and some rather symmetrical oddities dotted around – but at first glance, no henges! Early days, though, and a valuable learning exercise for all involved – a big thank-you to Olaf for being so patient with us, especially as he is more used to the speed at which the pressures of commercial archaeology force one to. I know Gill and I are both looking forward to putting our experience to use in the future.

Leigh

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