72 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
become visible on the stereomodel. Thus aerial photographs sometimes pro 
vide the most reliable, as distinct from the quickest, means of determining the 
strike of the metasediments and sometimes provide geological data unobtain 
able by normal field methods (figs. 1 & 2). 
The problem of the interpretation of the dip of metasediments from aerial 
photographs is more difficult than, and not strictly analagous to, the interpre 
tation of strike, because: 1. indisputable dip slopes are infrequent; and 2. meta 
sediments are characteristically tightly folded and steeply dipping, so that the 
variation of their strike-trace with the topography, which is the most common 
criterion for the determination of their dip, provides only the weakest evidence 
for the direction and amount of dip. 
The analogy, with the interpretation of strike breaks down because, whereas 
it is theoretically possible for lineaments resulting from bedding to be depicted 
on the photographs simultaneously with those resulting from foliation, even if 
the foliation planes should be dominant, it is not equally possible for dipping 
planes resulting from bedding to be depicted simultaneously with those result 
ing from foliation. If the foliation planes are dominant, and if their dip differs 
from that of the bedding, it is unlikely that true bedding dip slopes will develop. 
These are rather theoretical considerations; but it can be suggested that the 
most reliable criteria for the recognition of dip in metasediments on aerial 
photographs are: 1. the traces produced by clearly defined lithological bound 
aries as they cross topographical features, and 2. apparent dip slopes asso 
ciated with indisputable bedding structures. 
Because aerial photographs frequently supply the clearest and most reliable 
indication of the bedding direction in metasediments, they also frequently 
supply the clearest and most reliable evidence of folds. It was stated above 
that the determination of the strike of the metasediments from field work might 
be an arduous task. If complete fold structures in the metasediments had to be 
elucidated by the multiple determinations of strike in the field, it could indeed 
be a mammoth task, and many important structures would inevitably be over 
looked altogether. 
Because aerial photographs sometimes provide the most reliable evidence 
of the strike of metasediments they also sometimes provide the most reliable 
evidence of faults (fig. 3). In soil covered areas of regional metamorphism, 
many faults are now mapped and their apparent displacement sometimes meas 
ured on aerial photographs, which would be entirely unnoticed by the field 
geologist working without the photographs. 
Banding in the permeation gneisses (fig. 4) is recorded on aerial photographs 
even when the gneisses are covered by a thick layer of residual soil (fig. 5). If 
a single photograph of a soil covered area of permeation gneiss is inspected with 
the naked eye, a “flowing” appearance will frequently be noticed which is 
typical of such areas, and which results from the presence of a very large number 
of individually insignificant lineaments. It is noteworthy that the appearance 
of “flowing” shows with much greater clarity on a single photograph of a large 
area than it does on a magnified stereoscopic model of a smaller one. This is