WORKING GROUP 3 
WEBSTER 
147 
to look for annotated air photographs of an area analogous to that in which one 
is interested one still requires a means by which it can be located. At this stage 
it is worth considering the genetic aspect. 
Christian [1957] has pointed out that a land system, or pattern, owes its 
existence to a broadly uniform genesis; the result of a particular climate or 
climatic sequence acting on a particular type of lithology in a given stage of 
development. In fact one may expect to find similar patterns wherever these 
factors, Davis’ process, structure and stage Davis [1899] are the same. In soil 
survey work it is usual to have preliminary data of this sort and the location of 
an analogous pattern could be aided if patterns and the air photographs 
illustrating them were indexed under these headings also. Unfortunately one 
cannot assume that a given combination of climate, rock type and stage of 
development, or major relief, will produce only one pattern, or that one’s data 
are sufficiently precise to locate it. Perhaps the best that one can hope for is 
that reference to the index will suggest a few patterns, two or three, which are 
likely to be relevant in the study of a new area. But by a comparative study of 
the new area with the annotated photographs and descriptions of each of the 
possible analogues the selection of the correct analogue should be relatively 
easy. 
Conclusion 
It is believed that implicitly physiographic units of the type described above 
are already widely used in interpretation. But by giving them explicit recogni 
tion it would be possible to index and annotate air photographs to stress anal 
ogues and thus facilitate air photo interpretation in areas of difficult access. 
Acknowledgements 
The work on which this paper is based has been carried out under contract 
from the Military Engineering Experimental Establishment. Thanks are also 
due to F. Mosely from whose original fig. 3 is drawn. 
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