FEATURED, Landscape

Crop Marks, Natures’ History Trail

What lies beneath the crop in a field can affect the rate of growth? Hint: we’re not talking soil and fertiliser here.

The answer is that previous activities which may have led to the creation of ancient settlements, buildings, ditches, pits and other features all leave their mark in the subsoil too.

They affect the depth of soil and consequently, impact what is growing above them.

At various points during the growing season, the plants can be affected in terms of colour and vigour. This may not be visible from ground level but can easily be spotted by a drone or aircraft from above.


Crop Marks – It’s all Perfectly Logical

Archaeological features buried deep in the ground almost inevitably affect the growth of crops planted above them.

This works in two ways. Some features provide a greater depth of soil, so the crop has deeper roots and enhanced growth, others reduce soil depth, like a buried wall or compacted Roman floor or road.

Oakley Down Barrow Cemetery
Oakley Down Barrow Cemetery on the Wiltshire/Dorset border.

Depending on the type of plant, the differences can be observed at ground level in the field and from above and may form a distinct pattern or shape which gives a clue about historical remains.

The marks can be confusing as often different settlements overlap so the site may reflect layers of past human activity rather than date to one specific period.

Read More: The Forgotten Roman Roads

How does the Science Work for Crop Marks?

Different crops vary in what they can reveal but the overarching key to crop marks is the amount of moisture content in the soil regardless of what is growing in it.

Less moisture generally exaggerates the effects of both greater and lesser soil depth.

The lack of moisture may be caused by reduced rainfall either before or during the growing season and depends to some degree upon the behaviour of that particular crop.

Crop mark
Sketched diagram of a negative crop mark above a wall and a positive crop mark above a ditch

This usually means that in dry periods, it’s easier to spot natural and archaeological features.

Visibility is also dictated by the type of soil and subsoil with crop marks more common on gravel and chalky soils compared to clay, which is a poor draining soil and better at keeping hold of water.

Also, cereals tend to produce better crop marks than grass.

The Heatwave of 2022

During the heatwave of 2022, the UK experienced record temperatures but what was more significant for aerial archaeology, is that it remained warm and dry for an extended period well into September.

Read More: Ancient Trackways: Walking in the Footsteps of Neolithic People

The lack of rainfall produced hidden treasure for aerial archaeologists from Historic England as well as amateur drone operators.

New discoveries included Iron Age settlements, Neolithic ceremonial monuments and a Roman farm. Even known sites which are already mapped, revealed secrets that hadn’t been visible previously.

The last bonanza year for Historic England was in 2011 when they discovered around 1,500 new sites using aerial mapping, mostly in East Anglia.

Oakley Down Barrow Cemetery
Where two worlds collide, a rare Bronze Age disc barrow that has had the bottom sliced off by a Roman Road. This is the Old Sarum to Badbury Rings Roman road cutting through the Oakley Down Barrow Cemetery on the Cranborne Chase.

Because the heatwave was so widespread and not just confined to one region of the UK.

There were new discoveries almost everywhere including a prehistoric settlement with an unusual layout at Lansallos on Cornwall, one of many new finds in this county, and Iron Age square barrows which are burial sites at Pocklington in Yorkshire.

Read More: Menhirs Date From the Neolithic, But What are They?

Even on a local level, the heatwave revealed geographical features to landowners and farmers which were previously unseen and ancient gardens and buildings at stately homes.

crop mark
Really good example of how crop marks work.

The Advent of Drones

Drones have revolutionised aerial archaeology and mapping. Farmers have been using drone technology for a while now to inspect their crops but for reasons of yield rather than because of any historical interest.

Drones have also become a popular hobby among many just for leisure but have some varied uses like looking for lost dogs, taking interesting images at a wedding or even identifying fire pockets to help the Fire Brigade to battle field fires during the heatwave.

Abandoned medieval village.
Crop marks are all that is left of a medieval village. You can see a very defined street in this shot.

Drones make regular aerial images much more accessible and frequent so mapping the seasonal changes in a particular field or area irrespective of freak weather events, is now more than just a possibility. 

Crop Circle Season and Controversy

Overnight sensations, crop circles have been appearing worldwide for a while now.

These are not differences in the plants caused by ancient activity some metres below ground, but patterns created in the standing crop where seemingly, the plant has been trampled down or flattened.

From ground level, the marks and shapes are meaningless but a bird’s eye view often reveals a complex and exact design.

Crop circle viewed from Maiden Castle an Iron Age hillfort
Crop circle viewed from Maiden Castle an Iron Age hillfort

Crop circles are defined by the fact that they seem to appear overnight and unseen by an observers.

Although they have manifested across the globe, the southwest of England appears to be the crop circle capital of the world with the county of Wiltshire at its epicenter. Stonehenge and Avebury are crop circle hotspots so this just adds to the frisson.

Read More: Pingos, an Ice Age Relic Hidden

This phenomenon appeared relatively recently in the 1970s and took off in the 1980s when both the frequency of occurrence and the complexity of the designs increased.

The astute have observed that creating artwork in the fields is already something of a local specialty as there are eight chalk horses in Wiltshire alone.

In 1996, a crop circle appeared opposite Stonehenge depicting a mathematical fractal called a Julia set.

Theories Abound

A similar design appeared in 2001 on Milk Hill and was one of the largest recorded, stretching 900ft.

Activity has diminished in the 21st century with around 30 formations being reported each year and yes, you’ve guessed it, over three-quarters of these are in Wiltshire.

Maiden Castle Iron Age hillfort in the back ground and wheat neatly laid in the centre of the crop circle.
Maiden Castle Iron Age hillfort in the back ground and wheat neatly laid in the centre of the crop circle.

Theories abound as to their origin; well, we Brits do love an unsolved mystery! The crop is trampled down so this could be achieved without machinery and relatively silently but creating such complex and accurate designs to scale is no mean feat.

Speculation is endless about their origin with some suggesting ley lines, supposed mystical seams of spiritual energy that intersect at certain points, including Stonehenge and Avebury.

Read More: The Mighty Medieval Tithe Barns

Another theory is the presence of aliens trying to prophesize doom, supported by the appearance of a crop mark in the shape of a coronavirus in May 2020.

Other commentators suggest that mathematical designs are commonplace in nature, witness, the fractal branching of snowflakes, which could account for human intervention tapping into some kind of innate consciousness, so these are manmade, but the designs are not pre-planned.


If crop circles and designs are manmade, then they are an incredible feat.

Complex geometric designs appear literally overnight, created in the dark and on uneven terrain. If you walk through any field of crops, you’ll understand. And there’s more.

There are reports of car engines suddenly switching off in the vicinity of crop circles and watches stopping plus, strange orbs of light, with stationery unidentified objects hanging in the air and then flying off at great speed, although these types of sightings are not just associated with crop circles.

crop circle at Maiden Castle
Very complex designs are seen in crop circles, and they seem to become more complex year on year.

Stephen Hawking suggested that flattened crops were caused by cyclonic wind, which is easy to witness in any field of standing plants as there will always be odd areas which have apparently been flattened.

But this still doesn’t explain the complex and perfectly formed geometric designs which have been seen in the 20th and 21st centuries.

Rather like the Loch Ness Monster, the subject of crop circles is an area ripe for hoaxers.

Two self-confessed hoaxers, Doug Bower and Dave Chorley were responsible for hundreds of circles and in 1991, they demonstrated how they did it using ropes to measure the design and planks of wood to flatten the crop.

However, local farmers were skeptical that they were responsible for the more complex designs, especially the very large ones some of which went over the brow of a hill and which would be almost impossible for two people to produce.

Read More: Colours From the Countryside, Dyes Derived from Plants

Led Zepplin

It’s not long before people try to capitalise financially on the crop circle phenomena.

In 1990, a very famous circle appeared which was called the Eastfield Pictogram and this went viral, pictures were beamed all over the world and even used as an art cover design by the famous rock group, Led Zeppelin.

The farmer was quick to spot a profit when he saw so many people turning up to look at it.

He charged a £1 a person for access to the field and even had keyrings and T-shirts made. The farm accounts revealed that this was his most profitable quarter of an acre ever.

Celtic field systems can be seen as crop marks
Celtic field systems can be seen as crop marks

The advent of drones, social media and YouTube has brought crop circles to the masses although the frequency of them has declined in recent years. Many farmers view them as trouble and cut them out if they appear in their fields.

Their likely derivation, whatever you believe, is a world away from the more respectable and historical analysis of crop marks, the origin of which is understood and is just ancient history coming to light in layers of soil.

Before you get Excited…

With crop circles keeping amateur sleuths entertained, crop marks are of enduring interest to archaeologists, and drones mean that aerial archaeology is not just confined to large organisations like Historic England.

However, there are some potential pitfalls before you think you’ve made the next great Iron Age discovery.

crop marks
Kite aerial photo of crop marks at Nesley, near Tetbury, Gloucestershire. Cereal crop left, beans right. The relative intensity in the crops was reversed in the near infra-red.

Soil depth changes can occur for lots of reasons many of which are basic and rather dull, sometimes manmade intervention and nothing to do with ancient settlements.

A change in land drains or recently removed field boundaries will all leave crop marks as can weed killing protocols which create linear or circular variations over large areas.

Nature also plays a hand with simple changes due to fissures in extreme heat or frost cracks in prolonged cold weather. The moral is to always take a step back and maybe talk to the local farmer about recent land management or get your find checked out by an experienced aerial archaeologist.