A cropmark is a mark visible in crops caused by underlying archaeological features. The presence of things like walls, ditches and pits will affect the moisture level and chemistry of the overlying soil. This in turn may affect the growth of the vegetation above, which may be visible. Typically features such as pits and ditches lead to denser, higher growth whilst underlying walls lead to a shorter, thin vegetation growth. These marks, especially when viewed from the air, may be used to identify and plot the extent of the archaeological features. Typically, cropmarks become more pronounced when there is less than average rainfall. The type of vegetation overlying the archaeology also affects the appearance of cropmarks, some plants are more tolerant than others and less likely to vary in growth due to soil conditions. Cropmarks can be caused by features dating to many archaeological periods and can rarely be dated without further investigation. An example of a monument found as a cropmark are the possible Civil War earthworks south of Akeman Street or the Roman villa at Yewden.