Today we’re talking about crop marks! And no, I don’t mean the ones that people think aliens made.
We’re talking about the visible markers that can be seen from above through different forms of aerial reconnaissance. When I say aerial reconnaissance, that can be anything from aerial photography from a plane or a drone, climbing up on a big cliff of mountain to survey a landscape, or even using satellites or Google Earth to get a good look at the surface of the earth.
Crop marks are the shapes and outlines of archaeological features and structures created by the varying growth heights of the crop being grown over the area. A lot of archaeological sites have been found on farmland and other open fields with this method because crop growth is directly influenced by the conditions of the soil underneath.
Let’s say we have a small Roman farm settlement in England. The people who lived there built a nice house, maybe a couple walls, and a ditch for drainage or something, there.
Eventually, that settlement will become abandoned. The ditch gets filled in naturally, a lot of the building stones get taken and repurposed by other people in the area, and eventually whatever remains of the site becomes buried and there are only a few surface indicators that something used to be there.
A few hundred years later and then for centuries afterwards, people plough the land for farming. All this repeated ploughing removes all surface remains, but those buried features are still there, and they have an effect on the crops that are grown overtop of them.
And based on the type of growth, we can determine what sort of archaeological feature is underneath! Crazy right? This is so cool and a lot more interesting than aliens, let me tell you.
Features under the soil will either have a positive, or negative effect on how tall crops grow and this is dependent on the soil depth and the availability of nutrients and moisture.
Ditches, pits and other negative features that are dug in to the subsoil are deeper than the subsoil itself, so they contain a lot more organic material and moisture than the natural surroundings. This means the crops overtop of this area will have access to an environment that is better for growth and will therefore grow taller than the other crops on normal subsoil.
On the other hand, crops that are planted above a wall or other sort of structure will have the opposite effect because it takes up space within the subsoil and can channel water away from the area. If the structure is made out of brick, it’ll just absorb the water for itself and draw all those nutrients away from the soil. This means the crops over these areas won’t be as tall in comparison to the crops planted in the regular subsoil or those planted over a buried ditch.
These changes in height might not be that noticeable when you’re standing in the field, but once you’re elevated, entire outlines and structures can be seen from the air by looking for changes in tone or colour, or from shadows cast from the sun when it’s low on the horizon. It’s like a big game of aerial Eye Spy.
Of course not all crops will be as effective at showing these crops marks as others, but cereal crops like wheat and barley, and root vegetables like potatoes and peas are very good for revealing what’s hidden underneath.
Spotting crop marks isn’t always an easy task though. Because crop marks are… well dependent on crops, they can only be seen seasonally, and might not be visible at all except during droughts or very wet years. Droughts are useful for seeing crop marks in areas that have vegetation that is harder to show the differences in heights such as grass.
Another, more science-y way we can see crop marks is through thermal imaging. The differences in water loss in the ground will show up as different temperatures, which will create an image of thermal crop marks, which can potentially be seen regardless of the level of crop growth.
All that being said, having a solid knowledge of past and recent use of the area you’re looking in is very important as well, so that way you have an idea of what you are looking for and what structures you might find. It’s also important to remember that some features might not even show up at all and you’ll need to use another method for finding that archaeological site.