158 CONSTRUCTION SOURCES OF GRAVEL, DISCUSSION 
Discussion 
Mr L. F. Cunris: Some of the points raised 
by Professor Mintzer in his paper are of interest 
to the soil surveyor. Firstly, I think I must say 
as a former soil surveyor that I do not consider 
that the “walking it out” or field survey method 
is by any means obsolete as yet. Particularly, I 
was interested to know the comments he made 
concerning the relationship which exists between 
tonal features and drainage conditions. On the 
whole, I agree with his statement that light tones 
are often indicative of free drainage, and also 
that dark tones are often indicative of poor 
drainage. Nevertheless, I think it is pretty im- 
portant to accept the fact that this is not always 
true where there is vegetation cover. I could 
give examples of this country, for example the 
wet moorlands of the south west of England, 
commonly carry a cover of purple moor grass. 
When this is photographed from the air in the 
spring, it gives rise to extensive areas of very 
light tone, and yet these are on the whole very 
wet areas. 
In this area too where drainage is better, 
there tends to be a mixed assemblage of grasses 
which generally gives rise to medium tones. 
Therefore, in this case, the reverse of the general 
statement which Professor Mintzer has made is 
true. 
On the whole, I have found in my experience 
that even this soft tone rather than depth of 
tone is a better guide to drainage conditions. On 
the whole, I have found that even tones are 
associated with free draining areas, whereas 
poorly drained areas commonly show mottling 
or unevenness of some kind. 
Professor Mintzer in his paper has noted that 
the granular terrain with which he was con- 
cerned showed alternating light and dark spots. 
I should like to ask him whether he found 
features on the ground which explained this 
mottling on the photographs. 
There are two other points which I should 
like to raise. Firstly, I note from his paper that 
the scale of photographs used in the United 
States was 1 : 20,000, and 1 : 9,600. I should 
like to ask him about the scale in Germany and 
in India. 
Later in his paper, Dr Mintzer has said that 
soil types were delineated along the proposed 
highway centre line. There are two queries 
which I should like to raise in this connection. 
Firstly, was this delineation done from the aerial 
photographs? Secondly, is he using the words 
"soil types" in a technical sense as used by soil 
surveyors, that is to say, were these delineations 
of areas showing similar profile characteristics 
and similar surface textures? 
Prof O. W. MINTZER: I am glad you have 
brought up this point about mottling being 
somewhat different in your experience. This 
bears out very well that in interpretation of 
photographs for engineering purposes or for any 
other purpose, for that matter, it is important 
that the experience of the engineer or inter- 
preter, whoever he may be, is with the area 
which he has in mind to study. Your experience 
is somewhat different from mine, as you pointed 
out, even though you have mottling you may 
not always have gravel. 
The next thing was about scale. With regard 
to India, the scale was about 1 : 30,000; with 
Germany it was about the same, 1 : 30,000. So 
they are very small scale photographs, yet they 
are very good for interpretative purposes of this 
general nature. 
In regard to your question about soil types, 
the soil type that I speak of there I am sure 
should have been clarified. In engineering prac- 
tice we have what is known as a Highway Re- 
search Board classification. This classifies the 
soil in terms of whether it is granular, silty, 
sandy or clay. In effect, it is similar to the petro- 
graphical classification used by soil scientists, 
yet the soil type is never definitely absolutely 
known until a field exploration has been done to 
couple it with the airphoto study. Therefore, it 
is a combined use of airphotos and field which 
establishes the type. 
Mr TA LIANG: I should like to comment on 
this tone question. This is something which we 
looked into when we were trying to find out what 
we could about sand, gravel, clay or whatever 
the soil may be. It is true that in mottled tones 
you could have sand and gravel or you could 
have clay, but there is one additional thing to 
look into: whether the model you have is clear 
between the dark and the light, whether you 
have a clear boundary. I think that in the major- 
ity of cases if itis a well-drained soil you will find 
the mottling where your dark changes to light 
has a clear-cut boundary, whereas in your clay 
soil your change of tone is rather gradual. This 
one might think of when you look at the beach, 
if you have a sandy beach you generally find 
that the wet area and the dry area have a rather 
soft demarcation, whereas if you go into a 
muddy beach you find that the changing of tone 
from the dark to the light is rather gradual. 
  
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