In praise of cropmarks

Cropmarks have been making the news recently (see for example https://rcahmw.gov.uk/wales-wide-drought-reveals-further-lost-archaeological-monuments/, https://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-44812713 and https://www.nytimes.com/2018/07/13/science/newgrange-henge-ireland-drone-nyt.html ). The recent spell of hot weather has revealed a wide range of archaeological sites as cropmarks and archaeologists are getting very excited about them. As archaeological cropmarks rarely make the headlines, this publicity and excitement is very welcome.

I am happy to admit that I am one of those thrilled by these discoveries and by the potential increase in archaeological knowledge that this dry summer may bring. Now the knowledge that archaeological sites can be recorded from the air as cropmarks is not new, nor is my interest in this phenomenon. I have been involved in some way in the sub-discipline of cropmark and aerial archaeology for the last 15 years or so. My PhD thesis focused on making sense of a group of cropmark sites and I continue to research and publish about cropmark archaeology. So I have somewhat of an investment in archaeological sites recorded as cropmarks, particularly in Scotland, and have been following the publicity surrounding the new cropmark discoveries with real interest.

However, while I and others talk blithely of ‘cropmark archaeology’, this term is perhaps a little misleading; the cropmark is not the archaeology. Instead cropmarks are the way in which buried archaeology is revealed and recorded.  Buried features such as pits and ditches affect crop growth and ripening. This produces colour and height differences in the crops, which in turn can be photographed from the air and the patterns recorded on those photographs interpreted as archaeological sites. These effects are produced when there is a difference between the moisture and nutrient content of buried features and the surrounding soil, an effect that is exacerbated by dry conditions such as those we have seen over recent weeks.

How cropmarks form2

The formation of cropmarks. Image drawn by Kirsty Millican

As archaeologists we are interested in the buried sites and features that these cropmarks tell us of. So when we talk about cropmark sites, really we mean buried archaeological sites revealed to us as differential crop growth and ripening, that differential crop growth providing an insight into the riches surviving below the ground.

While cropmark archaeology is an easier shorthand for discussing sites revealed in this manner, making the distinction between recording method and the actual archaeology is, I feel, more than semantics. It is very easy to dismiss a cropmark as an ephemeral feature and inferior to sites visible above ground if one does not acknowledge the real archaeological features that create the cropmarks. Archaeological sites revealed as cropmarks are no less real than those surviving as upstanding features.

So cropmarks are the way in which the below ground features affect the way in which those crops grow. Isn’t it amazing that something someone did in a place perhaps several thousand years ago can affect the way in which modern crops grow? Just think about that for a minute – a four thousand year old pit or ditch has changed the nature of the soil so much that it has the potential to make crops grow differently today. That effect permits us a glimpse of what lies below the ground, a glimpse that we would otherwise be denied and a glimpse that without which we would know considerably less about the past. It really is a remarkable phenomenon!

In Scotland concerted aerial survey, by that I mean flying in a little plane with the main aim of photographing those crop colour and height differences, began in 1976, that other well-known dry summer. Yes, there had been some survey by Cambridge University before then, but real aerial survey really took off (pun intended) in 1976 when Gordon Maxwell of the Royal Commission on the Ancient and Historical Monuments of Scotland (RCAHMS) began a flying programme prospecting for cropmarks in Scotland. And the results were spectacular. So amazing in fact that they transformed the way in which we understand the archaeology of Scotland.

Aircraft sketch1

I’ve been lucky enough to participate in aerial survey. It is a relatively low-tech experience in a small aircraft. Before going up for the first time, I was told that it was a bit like being in an (old) mini with wings. That was a very accurate description! Image drawn by Kirsty Millican

Before 1976 it was assumed that the lowlands were simply not used much in the past; there was so little known archaeological evidence. With the advent of systematic aerial survey that was turned on its head and the lowlands were shown to be stuffed full of archaeological sites of all periods. Rather than being avoided in the past, the lowlands were heavily used throughout prehistory to the present day.

That early aerial survey recorded previously unknown site types, and cropmarks have allowed us to record sites that we would simply not know about otherwise. In particular some of those in which I take a particular interest, those built of timber. For example, without cropmark archaeology we would likely know of perhaps only the four Neolithic timber cursus monuments (early Neolithic ritual monuments) that have been excavated ahead of development. The remaining 25 or so sites that we know today would simply be unknown and the excavated examples weird anomalies we would find hard to explain. Many other site types are known only from cropmarks recorded by aerial survey or were first identified from the air. That transformative effect is not just confined to the early years of aerial survey; the recording of cropmarks still has the potential to transform the way in which we understand our past.

Cursus monuments

Plans of cursus monuments, all recorded as cropmarks on aerial photographs. Without aerial survey we wouldn’t know these sites even exist. Image drawn by Kirsty Millican

Bannockburn

Reconstruction drawing of a cursus monument, based on a timber cursus excavated at Bannockburn © Kirsty Millican.

 

So it is no wonder that archaeologists have got excited about the new discoveries and potential for more with this dry weather. While survey happens every year and sites continue to be recorded, this extended dry spell has provided a rare opportunity for cropmarks to form over sites which they have not formed over before. In years like this cropmarks will form in some areas with greater clarity, potentially giving us more detail and a richer glimpse into both known and unknown sites.

Cropmarks, and the buried archaeological sites they reveal, then, are the unsung heroes of the archaeological world. As archaeologists we are aware of the vital information they provide, but rarely do we sing their praises. In Scotland, and beyond, the vast archive of aerial photographs is testament both to the dedication and skill of aerial archaeologists over the years, and to the potential of the simple marks in crops to reveal key archaeological information.

Yet much within that archive remains untapped. Its potential is still to be fully revealed. In my small part I’m trying to release some of that potential through little pieces of research as I have the time. But there is so much more to be uncovered and study of sites recorded within that archive continues to produce new discoveries and interpretations. For example, the dating of a site excavated in Aberdeenshire led to the realisation that an unenclosed settlement recorded as cropmarks in Angus (https://canmore.org.uk/site/143379/chapelton) was not Iron Age in date as previously thought but likely dates to the later Neolithic (very rare in the archaeological record). A lowland Skara Brae, if you like. It was a remarkable realisation. This must be one of my most favourite sites recorded as cropmarks, one that is really remarkable and one that I played a part in interpreting.

Chapelton

The Neolithic settlement at Chapelton, Angus. Features mapped from the cropmarks recorded on aerial photographs. Each circle or part circle and four post arrangement represents a Neolithic round house. The fact that several appear to overlap indicates replacement over time. Image drawn by Kirsty Millican. Some the aerial images of this site can be seen on Canmore at https://canmore.org.uk/site/143379/chapelton.

How much more remains to be discovered both within the existing archives and through yearly aerial survey and recording? What more will be revealed as a result of this remarkable summer weather? How great are the riches that lie buried beneath our feet and how remarkable that the simple act of observing the growth in crops in modern fields has the potential to reveal so much.

So when this summer ends and the cropmark archaeology news stories stop, remember. Cropmark archaeology is alive and well. Long live the cropmark (and the buried archaeology it tells us about!).


If you are interested in finding out more about cropmark archaeology, in Scotland Canmore is the place to start. There is a wealth of aerial imagery available to view online and more being added all the time.

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