By late spring and early summer, crops of barley, oats and wheat have become well established in arable areas. This is the time when cropmark sites begin to appear, most of which are only visible from the air.
Weather plays a very large part in the formation of cropmarks. The drier the year the better the results.
The term "cropmark" refers to the difference in crop growth which is visible - especially from the air. Such a difference can indicate buried features lying beneath the ground, many from the prehistoric period.
When earlier people built structures they dug holes and ditches in the sub-soil.
Over time these structures decayed and the holes slowly filled with soil. As a result there would be a greater depth of soil in the disturbed areas, which in a dry season will retain moisture longer than the surrounding undisturbed subsoil. The consequent stronger growth relative to the rest of the crop takes on the plan of the original buried feature, which then becomes visible from the air.
These cropmarks can appear in a variety of shapes and forms such as circles, ovals, rectangles and linear features.