Cropmark visible in a field
Aerial photography allows for identification of sites through cropmarks: marks on the ground caused by changes in vegetation as a result of underlying archaeology.
This is one way that archaeologists can identify sites without having to excavate. The theory is that the vegetation will grow differently over different types of archaeology. For example. If there are the remains of a wall underneath the ground, then there will be less room for the roots of the plants to grow. This may cause shorter or weaker plants than the ones surrounding them (see diagram 1).
Similarly, plants may grow thicker, taller or healthier where there are deeper soil deposits, such as pits and ditches - especially if they have had lots of rubbish dumped in them such as food and human waste which acts as a fertiliser (see diagram 2).
The differences in the growth of the plants is what makes a cropmark visible on an aerial photograph. They show up like lines, just as in the picture at the top left of the screen.
Below is an activity which is designed to mimic what happens during the development of cropmarks.
- Plastic seed tray (a fairly deep one)
- Small stones
- Cress seeds
Picture instructions are below!
Lay out all the materials required for the activity.
Carefully fill the 'pit' with soil.
Cover the base of the tray with a deep layer of stones, leaving a gap to represent a 'pit'.
Then carefully fill the rest of the tray with soil, being extra careful not to knock down the wall!
Use more stones to build a wall on the other side of the pit (this may require some glue to keep together!).
Sprinkle the seeds evenly over the tray and water. Water gently - with a spray bottle if necessary, so the seeds don't wash away!