Page "Cropmark" Paragraph 3

Crop marks appear due to the principle of differential growth.

One of the factors controlling the growth of vegetation is the condition of the soil.

A buried stone wall for example will affect crop growth above it, as its presence channels water away from its area and occupies the space of the more fertile soil.

Conversely, a buried ditch, with a fill containing more organic matter than the natural earth, provides much more conducive conditions and water will naturally collect there, nourishing the plants growing above.