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A Dry Summer Shows Buildings Nobody Has Seen

About 13 minutes

The summer of 2018 was fiercely dry across much of Europe. Fields went brown. Rivers dropped.

And in aerial photographs taken that year, buildings began to appear in the crops — the outlines of enclosures, farmsteads and field systems that nobody had ever recorded. Not one of them was dug up. Not one was even touched. They simply became visible for a few weeks, and then the rain came back and they were gone again.

The plants are the instrument

Nothing is showing through the soil. The ground is flat and the crop is unbroken. What changes is how well the plants are growing, and the difference lines up exactly with what is buried underneath.

Wheat sends its roots down. How the crop does depends on what the roots find.

A ditch and a wall do opposite things

Two kinds of buried thing, two opposite results.

A filled-in ditch is a trench somebody dug long ago and that gradually silted up. The material inside it is soft, deep and dark. Roots go a long way down and there is more water held there. So plants above a buried ditch grow taller, stay greener, and ripen later than their neighbours. From the air, the whole line of the ditch shows as a dark green stripe.

A buried wall or road does the reverse. Stone or rammed gravel just under the surface leaves the soil above it thin and quick to dry out. Roots hit the hard layer and stop. Those plants stay short and turn yellow first, so the wall shows as a pale line.

One buried feature makes the crop stronger, the other makes it weaker, and either way the crop copies the shape.

A crop growing taller over a buried ditch A field seen from the side. A firm level line runs the whole width: this is the surface of the ground, and it is flat, because whatever is buried was filled in and levelled long ago. Standing on that line is a row of evenly spaced stalks, each a straight stem with a small solid head on top. Most are short and all the same height. Six of them, in one run near the middle, stand more than twice as tall. Under that run, drawn below the surface with a dashed outline and lightly shaded, is a wide flat-bottomed trench cut into the ground and long since filled with soft soil. The soft filling holds more water and lets roots reach deeper, so the stalks above it grow taller. At the surface itself, nothing marks the trench at all.

The ground is level and the ditch is completely buried. The only thing changing is the weather.

In a wet year every plant has all the water it wants, so the deep soft soil over the ditch is no advantage at all and the crop is even. Turn the dryness up and the plants without a deep root run start to struggle first. The ones over the ditch keep going.

The buried ditch has not changed. What changed is whether it makes any difference — and a mark you cannot see in a wet year is not hidden, it is genuinely not there to see.

You have to get above it

Stand in that field in a drought and you will most likely notice nothing at all. The difference between one stalk and the next is small, and you are looking along the crop rather than down on it.

Go up a few hundred metres and the same small difference, repeated across hundreds of plants, becomes a line. Lines become corners. Corners become rooms and enclosures and boundaries. From above, the field is drawing a plan.

Aerial archaeology got going in the 1920s, when people who had learned to read photographs taken from aircraft started pointing cameras at ordinary countryside. The technique has not changed since. The camera goes up, the summer does the work.

A crop growing taller over a buried ditch A field seen from the side. A firm level line runs the whole width: this is the surface of the ground, and it is flat, because whatever is buried was filled in and levelled long ago. Standing on that line is a row of evenly spaced stalks, each a straight stem with a small solid head on top. Most are short and all the same height. Six of them, in one run near the middle, stand more than twice as tall. Under that run, drawn below the surface with a dashed outline and lightly shaded, is a wide flat-bottomed trench cut into the ground and long since filled with soft soil. The soft filling holds more water and lets roots reach deeper, so the stalks above it grow taller. At the surface itself, nothing marks the trench at all.
A section through the field. The surface is flat and gives nothing away. The advantage is entirely below ground, and it reaches the surface only as a difference in how tall the crop grows.

What this method does not give you

It is worth being blunt about the limit, because it is a large one.

A crop mark gives you a shape, and nothing else. It shows a rectangular enclosure with a gap on one side. It cannot tell you whether that enclosure is three hundred years old or three thousand, or who dug it, or what it was for.

Shapes are a real clue — certain layouts belong to certain periods, and a good eye can make a sensible guess. A guess is what it stays. To turn it into a date you need something the photograph can never contain: a coin in the ditch fill, which would put a floor under it, or a timber from a post-hole, which could give the year the tree came down.

Why do crop marks show up best in a dry summer?

Finding is not the same as knowing

This is a method for finding, and it is extraordinarily good at it. A single dry summer can add more sites to the record than years of digging, at no cost to the ground at all, because nothing is disturbed.

But finding and knowing are different jobs. The photograph says something is here, and this is its shape. Everything else — when, who, why — has to come from somewhere else entirely.

That is not a weakness in the method. It is what a method is: a tool that answers one question honestly and stays quiet about the rest.