- 475 - 
ised by shadows 
‘ masking of 
:h dormant-season 
is plants are 
The best time for photographing soil marks is during the winter or spring 
when there is little or no crop vegetation. The weather is important only in 
the case of very weak marks, which are best exposed on panchromatic films during 
dry periods. Where infrared imagery is specified, flights during wetter periods 
may be quite satisfactory, however. 
; old field sys- 
> define owner- 
iry by the two 
¡emnants of these 
by shadow marks. 
Although the more significant discoveries revealed by soil marks have been 
in the more arid regions, important discoveries have also occurred in humid 
areas. In certain sections of England, such as the Fen Basin and chalk regions, 
soil marks have revealed numerous remains of previous landscapes. And similar 
markings have also served to outline buried ditches enclosing ancient Roman 
fields. 
shadow marks. It 
Indian village 
.a. The site con- 
i more than a 
; broad, low 
;d for many years 
Crop or Plant Marks 
Cultivated crops and native plants (e.g., grasses), may reveal the exis 
tence of buried landscapes by variations in their color, density or height. 
Such variations, which may indicate differences in plant-root penetration, can 
result from the remains of such features as ditches, pits, or buried wall frag 
ments. Plant marks may reappear year after year, since the causal buried re 
mains may lie well below all cultivation levels. 
ind moisture of 
ihes, depressions, 
.le has been so 
¡sent surface soil, 
efinitive, even 
.1 marks may per- 
ition, and to de 
aerations of 
l beds may also 
Crop or plant marks may be classed as either positive or negative. Posi 
tive marks result where growth is stimulated by filled-in ditches, whereas 
negative marks (inhibited growth) can result from buried foundations and walls. 
Positive marks are the more common type. They are affected by the width and 
depth of the original excavation in the subsoil and are most pronounced when 
the excavation was large and deep. The minimum width required to produce a 
plant mark is probably about one meter. However, a plant or crop mark will 
seldom be as wide as the ditch or other feature beneath it. 
To produce positive plant marks, the subsoil must be well drained. Thus 
during dry periods, plants growing in deeper soil will be the only ones to 
:tor in producing 
urface soils, 
ntrasts strongly 
ed. 
flourish. Marks may not be present in areas with loose subsoils, because the 
roots of crops may extend as far down in undisturbed subsoils as in the loose 
silt of old excavations. The best subsoils for crop or plant marks are compact 
gravels, chalk, and silt. Limestone and sandy subsoils, along with loose grav 
els and clays, are generally unsuitable. Negative marks are usually indepen 
re closely asso- 
bsoils appear 
dent of the subsoil, because buried foundations, walls, and roads almost always 
have an adverse effect on the crops growing above them. 
g of a field 
ng tends to 
allow for several 
lly become less 
rm, the marks 
estroy soil marks 
Harrowing, drill- 
marks . 
In the production of crop marks, the type of plant cover is almost as 
important as the surface geology. Cereal crops are the best medium through 
which buried remains are revealed, but clover, sugar beets, and grass also give 
good results. In very dry weather, almost any type of vegetation may produce 
distinctive signs; crop marks appear gradually, with the contrast and amount of 
detail steadily increasing. During periods of wet weather, color differences 
quickly vanish, but variations in plant height and density tend to remain.