DETECTION TECHNIQUES 
When the archeologist relies on ground reconnaissance for the detection 
of archeological sites, he is limited to sites which are (1) small enough to 
comprehend on the ground from visible remains, (2) accessible within practical 
and economical limits, (3) still visible in spite of modern-day cultivation and 
construction, and (4) recognizable, even though the erosional effects of nature 
may have been operating over a long period of time. 
Fortunately, aerial discovery techniques are not as severely limited by 
the foregoing conditions. Evidence of past landscapes which are too large to 
be comprehended from the ground, or which may have been incorporated into the 
present landscape and thus gone unrecognized, are often detectable on some 
form of aerial imagery. And the advantage of a greater range and vertical 
viewpoint, as depicted on an aerial view, helps in understanding the pattern 
of things that are seen but not understood on the ground. 
For example, subtle suggestions of buried landscapes are sometimes revealed 
on conventional aerial photographs by shadow patterns, variations in soil color 
ation, or differences in the height, density, or color of the plants that grow 
above the buried features. And some remote sensors operate outside the visible 
portion of the electromagnetic spectrum, thus providing the archeologist with 
"another eye into the past." For example, thermal infrared scanners react to 
emitted, rather than reflected energy. These images record variations in heat 
waves emitted by the earth's surface. The structure of rock and soil beneath 
the surface affects the emissivity of these heat waves, and may be revealed on 
the infrared image. 
Shadow Marks 
Shadow marks are site indicators produced by the sun's rays falling ob 
liquely on minor terrain configurations or irregularities. Such surface irregu 
larities may have been caused by soil accumulations on the ground, by mounds 
of earth that resulted from older structures, or by buried remains of archeo 
logical value. Old earthworks, unnoticed banks and ditches, and other charac 
teristics of previous landscapes may sometimes be discernible from shadow marks 
on aerial photographs—even though such features are virtually invisible to a 
ground observer. 
Shadow marks dénote variations in surface relief through contrasting tones 
of shadows, normal photographic tones, and highlighted areas. Since an oblique 
sun angle is ordinarily required to produce good shadow marks, photographic 
flights should be planned for early mornings or late afternoons, and preferably 
during the winter months in regions of higher latitude. 
The direction as well as the altitude of the sun at the time of photography 
are important for producing diagnostic shadow marks. When the sun's rays are 
parallel to a bank or ditch, the tone on the photograph may be perfectly uniform 
and non-revealing. For the best contrast between light and shadow, the sun's 
rays should form an angle as close to 90 degrees as possible with suspected 
linear relief features. In planning exploratory flights, it may therefore be 
necessary to photograph an area from two or more flight-line orientations.