Crop marks are 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.