WORKING GROUP 2 
ALLUM 
71 
than the foliation does not imply that one can frequently find on a ridge an 
outcrop in which the relic bedding is clearly visible with its strike parallel to 
the ridge, and with the foliation making an angle to them both; on the con 
trary, the foliation is usually clearly visible and apparently approximately 
parallel to the ridge, whereas the relic bedding is usually difficult to discern in 
a single outcrop. Thus the field geologist frequently wrongly suspects: 1. that 
the ridges producing the photographic lineaments are caused by foliation 
rather than bedding, and 2. that in metasedimentary areas aerial photographs 
indicate foliation rahter than bedding. 
The evidence that a ridge in fact represents the relic bedding is that, if the 
ridge in one place is found to be a clearly recognisable lithological horizon, 
and if the ridge be traced on the photographs through all its contortions, then 
when it is again inspected in the field, perhaps several miles from the site of the 
first inspection, it is still found to consist of the same clearly recognisable litholo 
gical horizon. The fact that a particular ridge can be traced for miles, perhaps 
through numerous contortions, indicates that it consists of a rock which is more 
difficult to erode than the rocks immediately adjacent to it. This difference in 
resistance to erosion usually results from a difference in the rocks’ mineral 
constituents, which in turn, usually results from the fact that they were origin 
ally different sedimentary beds (fig. 2). 
Of course, closely spaced jointing and other structures can produce domina 
ting photographic lineaments which may greatly obscure the relic bedding 
lineaments, but it is usually possible to recognise the dominating lineaments 
for what they are, and thus look for, and recognise, the relic bedding lineaments 
making an angle to them. When there are several directions of lineaments, 
those resulting from relic bedding may be relatively obscure, but it is very 
rare for the relic bedding to be completely unrecorded. 
In soil covered areas of metasediments, outcrops are often small and sparsely 
distributed, relic bedding planes are difficult to recognize in the field, and the 
determination of the strike can be an arduous task (fig. 1). The geologist, 
working without aerial photographs in country of this type, is compelled to 
use the ridges as a general guide to the strike direction of the metasediments, 
so using the same criterion as the photogeologist; but his observations are more 
laborious and less conclusive. 
One of the reasons why the observations of the field geologist may sometimes 
be less dependable than those of the photogeologist is that the field geologist 
lacks the advantage of vertical exaggeration when he observes topographic 
forms. He is compelled to estimate the crests of the ridges, which may be in 
distinct, and then to judge their direction. He sees only a short length of the 
ridge at a time, and must either extrapolate on his map or walk the length of 
the ridge. Aerial photographs however are almost invariably taken in such a 
way as to produce vertical exaggeration in the resulting photographic stereo 
model; as a result of this exaggeration, low, rounded, subdued and indistinct 
ridges appear relatively sharp on the stereomodel and can be followed with 
precision, and very subdued ridges, which are imperceptible on the ground,