Lumps, bumps, humps and how to date dirt!

Tom Wood
Tom Wood

I’m spending a lot time of thinking about archaeology at the moment.

Apart from watching Time Team and marvelling and the various fashions of Tony Robinson and how Phil’s shorts conceal less than they should, I have never had more than a passing interest in the subject, but I do love spotting a bit of land use history- an old hedge bank or granite gate post here, a coppice stool there.

Now I am in the position of helping to organise a large volunteer exercise on the sites I look after known as ‘ground truthing’ and am trying to pretend I know what’s going on.

Back in 2013/14 when we took on more of the Teign Valley, we organised to fly a LiDAR survey of the entire area which I cover. What is that, I here you ask? LiDAR stands for ‘light detection and ranging’, the knowledgeable amongst you will be thinking “Ah ha- RADAR stands for radio detection and ranging I remember that much”.

Essentially LiDAR works the same way as RADAR, but using a Laser*, it also is used over much shorter distances and is usually used vertically rather than horizontally (like when RADAR is used on a ship or aircraft).

Essentially an aeroplane flies over the ground shining a spread of laser beams at the ground. When they hit something they reflect back and the instrumentation can work out how far the object it bounced back from is. Basically what this does is give you an amazing 3D map of the landscape.

You also get returns from things other than just the ground; trees and hedges for example, and you can see those as a separate layer or remove them from the image if you just want to see the final return (the ground).

Using the images an archaeologist can identify potential sites of
interest which can’t be seen easily from the ground, circular ditches or mounds etc (remember how Time Team loved a circular ditch or series of small walls!). However these could of course be the result of more recent activity and nothing to do with archaeology (things like ring feeder locations show up in fields looking like small iron age huts) – that’s where ground truthing comes in.

Effectively someone has to visit every feature visible on the LiDAR that might actually be something and decide what it might be, or at least whether it’s of historical interest or not.

I am working with a dozen or so volunteers now who are tirelessly out every week tramping through the woods with ranging poles and tapes, identifying, marking, measuring, photographing anything interesting, or noting what a feature is if its modern. Features we are finding range from the remains of small buildings (they found a cooking pot in one) to banks, ditches and charcoal burning platforms (complete with charcoal).

The take home message is just how much stuff there is out there when you start looking, particularly on Dartmoor. I was speaking to a lady who was helping the Dartmoor National Park Authority analyse their LiDAR data, she was using small grid squares overlain on the LiDAR (I forget the exact size but not very big) and had yet to find a grid square containing nothing of interest when I spoke to her.

Its just amazing the effect humans have had on the UK landscape when you really start to look and understand. Almost nothing can be said to be truly untouched, and some wilder feeling areas were clearly hives of industry at one time.

I’m now thinking about dating, not pottery or coins (a la our
television friends) but soil, yes you can date soil – who knew! Specifically when looking at a monument (such as the Deer Park wall I am interested in) you can remove soil from beneath the base course of stones and date when the soil was last exposed to light, in other words when the foundations for the wall were laid. The technique we are considering using is known as Optically Stimulated Luminescence (OSL).

OSL works because all sediments and soils contain trace amounts of radioactive isotopes. These slowly decay over time and the radiation they produce is absorbed by minerals in the sediments such as quartz and potassium feldspar. The radiation causes a charge to remain within the grains in structurally unstable ‘electron traps’. The trapped charge accumulates over time but is reset (bleached) by exposure to sunlight/ UV. Hence the clock is reset after the minerals are once again covered and this accumulated charge since last exposure can be measured. Using techniques like this you could date walls, hedge banks etc, assuming you had time and assistance or funding and couldn’t find out an easier way (like a written record of someone having built something- which involves less spade work).

Anyway, I’m finding all this fascinating, it’s a largely new area for me and its really exciting to see all these new features emerge out of the landscape. If you live in the Dartmoor area and would like to be involved please get in touch through the paper.

*Laser stands for- Light Amplification by Stimulated Emission of Radiation- if you wanted to know.

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