Cropmarks are a type of archaeological evidence, by which sub-surface archaeological, natural and recent features may be visible from the air. They can reveal buried archaeological sites not visible from the ground.
Different heights of crop growth result from a wall and ditch below the ground
Crops in a field can grow differently depending on what lies below them. Water will naturally collect in a ditch buried underneath crops. This will encourage the crops above it to grow tall and strong. On the other hand, a wall underneath the surface will dry out the soil and lead to shorter crop growth immediately above it.
These different rates of growth will naturally follow any features buried below ground. Although the differences in growth may appear small close up, from the air these are more visible. When the sun is low to the horizon, shadows cast by the taller crops can also become visible.