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Because the sand dries out rapidly you have a 
clear-cut wet or dry area. 
I think we should emphasise the fact that you 
should not depend entirely on one of these 
things, such as tones, because you have very 
light tones in desert areas, and where you have 
an alkali soil you have very light tones also. 
Similarly, we have found that the areas which 
have looked as if they are well drained, and are 
actually well drained, are not necessarily sand 
and gravel such as a lot of soil areas in tropical 
countries. You would see that it generally pre- 
sents a well drained pattern and it could if it 
were sand and gravel if the area were in the 
temperate countries. 
Therefore, it comes back to the point that 
Mr Curtis and Professor Mintzer mentioned, 
that experience is important. I want to em- 
phasise one more thing, that we always want to 
separate what is directly recognisable in the 
photographs and what is inference. In other 
words, you say that you recognise a certain 
feature on the photographs, but the sand and 
gravel is only there from inference. You actually 
do not see such sand and gravel on the photo- 
graphs, but you infer it from certain recognis- 
able features on the photographs. 
Prof O. W. MINTZER: I think those com- 
ments are very explanatory in themselves. I 
could not take issue with Mr Ta Liang. I am 
certainly glad to hear the emphasis is on ex- 
perience, because that is the important element. 
There is one other feature. There are several 
steps to this. There is a certain amount of 
literature, a certain amount of areal concept, 
that is engendered in the mind of the interpreter. 
There is the experience that comes in together 
with his photographic studies coupled with field 
work, and this has been referred to in other 
places as convergent evidence, a way of search- 
ing for what you wish to find. So it is done alone, 
just from the model patterning. 
I wonder if anybody else has another com- 
ment. 
Mr VERSTAPPEN: I should like to continue 
a little on what Professor Ta Liang has said 
about features directly seen on air photographs, 
and other things that are only obtained after a 
deduction and further consideration. One of the 
most important things which can directly be 
seen on air photographs are the landforms, as 
have already been emphasised by Professor 
Mintzer: landforms, erosional features, and so 
on. Therefore, if we map those landforms accu- 
rately from the air photos we are absolutely 
  
    
  
    
    
  
    
  
   
    
  
    
    
    
   
   
   
   
  
    
    
    
    
   
    
   
    
    
    
    
   
   
   
     
    
   
   
   
    
   
    
    
   
    
    
   
    
    
  
    
   
      
    
   
    
    
CONSTRUCTION SOURCES OF GRAVEL, DISCUSSION 159 
safe. If then we want to draw some conclusions 
as regards engineering uses and other physical 
characteristics of those landforms, the thing we 
ought to know is the genesis of the landform, 
how it originated, because the very genesis tells 
us something about the physical characteristics. 
For instance, if you want to have sand, it is 
important to know whether it is river sand or 
whether it is wind-blown sand, especially for 
inherent purposes. So we can distinguish be- 
tween the landform mapping which is very 
factual, and which can be done without much 
difficulty and without many deductions being 
made, and the genesis of the landform, how it 
originated. That is the most difficult thing, I 
think, in our way of interpreting. 
Professeur F. RUELLAN: La petite communi- 
cation que je voudrais faire se relie directement 
à ce que vient de dire Monsieur Verstappen. 
Elle a trait à la carte géomorphologique dressée 
d'aprés des photographies aériennes en étudiant 
ses buts et ses usages. Il s'agit de traduire d'une 
manière statique et objective les formes obser- 
vées stéréoscopiquement par des signes conven- 
tionnels en réduisant l’interprétation à ce qui 
est évident. Exemples: le tracé du drainage; les 
lignes des faîtes et les formes des faîtes; les 
aplanissements limités à ce qui est visible sans 
généralisation; les surfaces horizontales; les 
versants plans inclinés; les formes des versants 
droits convexes ou concaves; les ruptures ou 
changements de pentes supérieures ou infé- 
rieures des versants; les surfaces ondulées; les 
surfaces rugueuses; les alignements de géoclasse 
sans préjuger de leur nature s’il n’y a pas de 
preuves formelles; les formes d’alluvionnement 
et d’érosion; les dépôts de pentes; les éboulis; 
les cônes de déjection; les cônes alluviaux; les 
terraces; les levées; les moraines; les falaises; 
les cordons de galets; les plages; les bancs etc. 
Joindre à cette carte des mesures des dénivelle- 
ments et des mesures des pentes obtenues avec 
des appareils choisis suivant le degré de préci- 
sion que l’on désire, et après exécution de profil. 
Quand il y a des plongements évidents de couche 
on les figure et on les mesure. 
Telle est la carte géomorphologique telle que 
nous la comprenons. Les usages: une carte de ce 
genre est une base, d'abord pour l'étude des 
processus, c'est à dire pour la géomorphologie 
dynamique. 2^ — pour l'interprétation géomor- 
phologique, pétrographique, y compris la re- 
cherche des matériaux; pour l'interprétation 
tectonique; pour l'interprétation stratigraphique; 
pour l'étude des sols, de la végétation et des 
cultures; pour le tracé des voies de communi-