Additional problems in obtaining good shadow marks are caused by shadows 
of obscuring objects, such as hills, trees, and buildings. The masking of 
minor shadow marks by vegetation may be somewhat alleviated with dormant-season 
photography, i.e., by photographing certain areas when deciduous plants are 
leafless. 
Shadow marks have proven especially valuable for revealing old field sys 
tems in England. During the Celtic era, it was the practice to define owner 
ship boundaries with ditches. A ditch was dug along the boundary by the two 
landowners and a ridge of earth was thrown up on both sides. Remnants of these 
double hedgebanks, with a ditch in between, are often revealed by shadow marks. 
Large earthworks of low relief may also be revealed from shadow marks. It 
was this technique that led to the delineation of an extensive Indian village 
site on a cultivated floodplain area at Poverty Point, Louisiana. The site con 
tains the remains of six concentric banks and ditches, each one more than a 
kilometer in diameter. Cultivation and erosion had reduced the broad, low 
banks to a height of about 1.2 m, and they remained unrecognized for many years 
because of the sheer size of the earthworks. 
Soil Marks 
Soil marks are variations in the natural color, texture, and moisture of 
the soil; these variations may result from man-construeted ditches, depressions, 
excavations, or earth fills. In many instances, the soil profile has been so 
severely disturbed that the original subsoil has become the present surface soil. 
The contrasts in photographic tones may be striking and quite definitive, even 
though such marks are rarely apparent to ground observers. Soil marks may per 
mit the archeologist to distinguish layers of past human occupation, and to de 
tect architectural patterns, ditches, canals, or other human alterations of 
previous landscapes. Natural features such as abandoned stream beds may also 
be delineated from soil marks. 
The type of subsoil present is apparently an important factor in producing 
soil marks. Light-colored subsoils, in combination with dark surface soils, 
provide excellent marks; for example, where a chalky subsoil contrasts strongly 
with a brownish surface soil, outstanding soil marks are rendered. 
In some regions of western Europe, definitive soil marks are closely asso 
ciated with the distribution of loess; conversely, limestone subsoils appear 
to be poor for forming soil marks. 
Soil marks are most easily detected after the first plowing of a field 
that has gone uncultivated for a long period of time. Weathering tends to 
emphasize soil marks in fields that have been plowed and left fallow for several 
years. Soil marks may reappear annually on plowing, and gradually become less 
distinct. Eventually, as the surface soil becomes nearly uniform, the marks 
may disappear. Tractors that plow as deep as 40 to 50 cm can destroy soil marks 
that have persisted for hundreds, or even thousands of years. Harrowing, drill 
ing, and ridging practices are particularly destructive to soil marks.