- 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.