The factors which create crop marks at two archaeological sites, at Glenlochar, Kirkcudbrightshire and at Fisherwick, Staffordshire, are examined in some detail. Marks are best recorded at these places in cereals and show earliest over the shallowest-soils. They appear when the potential soil moisture deficit, which occurs when water transpired by a crop exceeds rainfall, is greater than the amount of water in the soil available to the plant. Marks only show in grassland when the deficit is about 40 mm more than that calculated for cereals. Marks also appear in very wet years, but are fainter than those caused by a moisture deficit, and are probably caused by waterlogging in the soil. At both sites the soils are moderately stony and shallow over terrace sands and gravels, except for soils in infilled ditches over which crop growth is better. -- AATA