400 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
Such small features as the post-holes of his buildings, or the ditches round the 
fields he tilled, provide most valuable and informative evidence about the life 
of prehistoric communities. Hitherto, the recovery of such details has been 
largely due to chance, as when some stray find revealed the position of an an 
cient settlement. It is, however, a fact that once the land-surface has been 
disturbed by whatever agency, the effect of that disturbance marks it practically 
for ever. Even if ditches, pits, trenches and holes are filled up and obliterated, 
their filling never attains the same compactness as undisturbed subsoil, and 
will ever after tend to affect the density or colour of vegetation growing over 
them. Thus, differences in vegetation, best seen in bird’s-eye view, reproduce 
the form or plan of features long lost to sight. Much here depends upon the soil, 
climate and type of growth. The best results are observed in arable land, since 
certain long-rooted cereals are most sensitive to such differences of soil. The 
resulting “crop-markings”, as they are called, reveal to an air observer, often 
in great detail, buried remains of which no trace appears on the surface. 
The principle is simply stated: its study and application are more compli 
cated. Every site has its own problems and requires careful and repeated ob 
servation if it is to be photographed to best advantage. Features still existing 
in low relief require sidelighting with a low sun. Crop-markings must be 
watched until the stage of growth is reached revealing the greatest detpil. In 
normal agricultural conditions a suitable crop may be sown in a given field 
only once every five or six years. Yet the fact remains that for a few months 
each summer, before harvest-time, the history and prehistory of Britain are 
displayed in her growing crops. The long succession of human cultures that 
have swept over these islands, the variety of British soils and the wide extent of 
arable land combine to make available to an air observer an exceptionally 
detailed and valuable body of material for the study of man’s social develop 
ment.