A Mystery Square in the Stonehenge Landscape

 

Lidar Lens view

About 650 m north-north-east of Stonehenge, just north of the Avenue, there is a square. On the modern Environment Agency lidar it is very obvious: a ditched enclosure about 60 m across, centred at SU 1254 4278, with its sides running close to the grid. So what is it? I don't know, and that's rather the point of this post. It's a little mystery.

Who saw it first?

The first sign I know of was in 2010, when Simon Banton posted a relit image of the early lidar on his blog with a box over this spot marked "What's this then?" That post is no longer online, and I'm grateful to Simon for kindly supplying me with a copy.

Simon Banton's 2010 relit image of the early lidar, "What's this then?" Image: Simon Banton, used with thanks.

Part of something bigger

The Wiltshire & Swindon HER has a record here (MWI79347), from Michael Parker in 2024, for a feature seen on lidar: a circle about 140 m across with "an outer bank and inner ditch plus a raised area internally". I first thought that was something different, as it is round and much bigger. Having measured the lidar properly, I was wrong. The square sits in the middle of a shallow dish about 140–150 m across and 35–45 cm deep, with a low rise round its edge, clearest on the north, south and east. The hollows along the square's north and south sides are the deepest part of the dish, roughly 7–10 cm below the middle of the square and the ground just outside it. So Parker's circle and my square look like parts of the same feature, the circle seen at a broad scale and the square at a fine one.

What it isn't

I can't see it for certain on any of the early aerial photographs. That's perhaps not surprising, as it had been a cultivated, ploughed field.

Nor is it the wartime pillbox that shows on the 1943 USAAF aerial photographs (Currently offline). That stood about 230 m to the west-north-west, at SU 12320 42854.

And it doesn't show on the geophysics. The 2011 magnetometer survey covers the square fully, and there is nothing there (Darvill et al. 2013, Fig. 4B).

A 3D model, and 2001 against 2021

I've put the lidar into an interactive 3D model so you can look for yourself. You can exaggerate the height, move the sun, switch between hillshade, local relief and other views, and draw your own cross-sections. It holds two Environment Agency surveys at 1 m resolution, flown on 8 December 2001 and 4 April 2021. The square and the dish are in both, the same shape to within about a centimetre. Subtract one survey from the other and nothing has changed inside the square beyond the noise: less than 1 cm, when random patches of the same size across the field vary by about 1.5–2 cm. Whatever made it, it was there by 2001 and hasn't changed since.

So what is it?

It isn't a new discovery: Simon Banton spotted it in 2010, and Michael Parker recorded the circle in 2024. It's easy to miss because it is so shallow; it only stands out once the lidar is processed to pick out small changes in height. My first guess was something agricultural, a sheep pen or fold. Having measured it, I'm less sure. Sheepfolds on the chalk usually have a bank, often 0.4 m or more high, with a ditch outside it. This has no bank at the square, just shallow hollows inside a broad dish. Wartime or other military use, a natural hollow in the chalk, or something older all remain possible. If you know better, I'd love to hear.

Acknowledgements

Acknowledgements

Thanks to Simon Banton for a copy of his 2010 blog post.

References

• Banton, S. 2010. 'I love LIDAR and PTM', Novacaster Community blog, 4 Oct 2010 (no longer online; copy kindly supplied by Simon Banton).
• Darvill, T., Lüth, F., Rassmann, K., Fischer, A. & Winkelmann, K. 2013. Stonehenge, Wiltshire, UK: high resolution geophysical surveys in the surrounding landscape, 2011. European Journal of Archaeology 16(1), 63–93. doi:10.1179/1461957112Y.0000000025
• US 7th Photographic Group, 24 Dec 1943, sortie LOC122 frames 1022 and 1050 
• Wiltshire & Swindon HER, MWI79347.
• Environment Agency LIDAR, Open Government Licence.