We mentioned in a previous blog that our drone “Dronita” had picked up some spectacular crop marks, but what creates these marks, and what can we actually determine from them?
When we mention crop marks most people’s first response is “crop circles? Wow!” But there are no mysterious alien forces at work here! A crop circle is a design pressed into the crop (by whichever process you choose to believe) but a crop mark is an area of land where subsurface archaeology has prevented or limited the growth of the crop. Much less exciting (or more, if you’re an archaeologist)! As a result, when a field is in full crop it can be easy to spot lines or structures as there are sections where nothing has grown.
But how exactly does the underlying archaeology prevent the crop growing? Obviously we’re not talking about small pieces of pottery here, but much larger structures such as buildings or roads. The bricks and stones below ground prevent roots from growing to their full ability and as a result the crop grows less well and appears more stunted than those surrounding it which can grow without any obstructions.
Additionally, the presence of stones takes water away from that area and into other sections of the soil, making them more fertile and in these sections the plants will grow more robustly than the others.
Depending on the crop, the marks can be more or less obvious. Thankfully, M.C. the crop growing in many of the fields we surveyed in 2017, was quite short and completely unable to grow above the subsurface archaeology, which made the crop marks incredibly apparent, which was great for us!
More difficult to see from the drone is a “positive crop mark” which is essentially the opposite of what we’ve been talking about above. With a positive crop mark, the area below the plant is likely a depression deeper than the rest of the field. Often, these ditches or holes are filled with much more organic and fertile soil, thus resulting in the plants growing stronger than others in the field.
You can see in this rough diagram (drawn by yours truly) how all this works.
And that is everything you need to know about crop marks! Any questions? You can always ask on our Facebook, Twitter, or Instagram . We’ll be answering some FAQs in a later blog, so if there’s something you’ve always wanted to know about archaeology, Italy, or our field school in general, get in touch! And if you’ve been enjoying our recent updates, don’t forget you can follow our blog by email and get notifications every time we post, so you’ll never miss an update!
Libarna Urban Landscapes project 