The dominating grain size in dunes is medium to fine sand (0,6—
0,06 mm).
"Wounds" in the surficial deposits very often can give valuable in-
formation.
Slip "wounds" occur in frictional material. They have rather steep
walls, the steepness depending on the angle of repose.
The "wounds" characteristic for cohesive material are ravines and
slide scars, The ravines can very often be distinguished as steep and
deep often ramified valleys in the vicinity of lake and river shores.
The slide scars can be of different appearance. On the air photos they
very often are reproduced as semicircle or bottle-shaped hollows in
which the backwalls form very steep scarps.
There are often intimate relations between ‘vegetation and the grain
sizes. Some examples of such relations already have been given. It
may also be mentioned that coarse grained deposits (= 0,2 mm) as
well sediment as till very often are covered with grey lichens which
causes a very light tone on the air photos. When dark grey sections
have been found in such light areas they have without an exception
appeared to have a soil, dominated by a grain size not coarser than fine
sand (< 0,2 mm), in most cases finer (fig. 4 and 5).
The importance of the scale of photography when distinguishing geo-
logical elements is obvious. Comparisons which have been made in areas
photographed on different scales, have shown that, as a rule, it is not
possible to obtain a satisfactory interpretation result on photos of a
scale less than 1:20 000. For normal interpretation the scale 1:15 000
has turned out to be the best: When one is concerned with interpreta-
tion for engineering and similar purposes it is often necessary to make
use of photos of scales larger than 1:10 000. Owing to the costs air
photos only in exceptional cases can be produced especially for the pur-
poses of photo interpretation. With the resolving power the camera
objectives have to day the difficulty to obtain suitable scales for the
purposes of photointerpretation can however in many cases be evaded
by using enlargements from negatives with scales of up to 1:40 000.
High glossy photo papers give considerably higher resolving than
ordinary photo paper This is of special interest when identifying for
instance, boulders in till.
Shadows and screens on the air photos restrict to a high degree the
possibilities of photo interpretation which is especially the case in well
wooded terrain. Experiments performed at the Photogrammetric De-
partment, Stockholms Institute of Technology, have shown that when
observing stereoscopically wide angle lens photos over a sunny "nor-
mal forest” it was possible to see the ground to an extent of only 1,2 %
of the total area of the stereo-model. If there were no shadows the
corresponding value was 12 96. Normal angle lens photos gave a little
more, namely 1,4 respectively 17 96. These figures speak for themselves
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