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REPORT OF COMMISSION VII 45
or near the surface of the ground, or where excavations must go deeply enough
to penetrate into covered bedrock, the engineering geologist applies the methods
of structural geology, stratigraphy, and petrology. Where the engineering
operations are entirely limited to unconsolidated deposits, however, the princi-
ples of geomorphology, sedimentation, Pleistocene geology, and soil science are
applied. In the latter connection, it may be noted that the term so is used with
different meanings by the engineer and by the geologist and soil scientist. To
the engineer, ‘‘soil’’ includes all unconsolidated materials below the ground
surface, even where these reach a thickness of hundreds of feet. It is in this
sense that the term is used in the science of soil mechanics, which is devoted to
the laboratory analysis of the physical properties of unconsolidated deposits in
relation to engineering purposes. To the geologist and soil scientist," however,
the term *'soil" means the uppermost layer of unconsolidated material which
supports the growth of vegetation, together with immediately subjacent mate-
rials having a distinctive vertical zonation reflecting the action of specific chem-
ical and biological processes. In this sense, soil ranges from a few inches to a
maximum of perhaps a few tens of feet in thickness. It is underlain by a substra-
tum or parent material which also may be unconsolidated, but which may be of
considerable diversity in geologic characteristics and age. Such unconsolidated
materials as underlie the soil, in this strict sense, belong to the province of the
geomorphologist or Pleistocene geologist. In this paper, the term ''soil'" is used
in the sense of the geologist and soil scientist.
In agricultural engineering, concerned with the prevention of soil erosion and
with other conservation measures, together with planning for most effective
utilization of the land, the relevant fields of earth science are geomorphology,
sedimentation, Pleistocene geology, and soil science.
HisrORICAL DEVELOPMENT
With the above background in mind, the utilization of air photography in
earth science may be considered, first in general and then in particular. The
beginnings date back to World War I. During that war, air photos were used in a
very limited way for purposes of military geologic mapping in enemy territory
( Brooks, 1920). Shortly afterward, the advantages of the aerial view in studying
various geologic features were noted by Willis (1921) and by Johnson (1921).
In 1922, Leedealtatsomelength with the value of air photos in the study of the
earth's surface features. During the remaining years of the decade, substantial
progress was made in the use of air photography for general mapping, particu-
larly in Canada, and this, although somewhat indirectly, wasa contribution of air
photography to the stock in trade of the earth scientist. Photo-interpretation,
per se, progressed more slowly and haltingly, and, owing to the high costs of
photography, was limited to commercial projects. There grew a realization,
however, that the aerial photograph possessed certain unique advantages
for the study of earth features, providing a perspective and a degee of detail
difficult or impossible to attain in other ways (Bourne, 1928; Walker, 1929;
Eliel, 1929).
In the early thirties there began a period of development and expansion which
continued until World War II. Equipment and techniques were improved stead-
ily, and applications to both basic and applied science grew apace. The paper
by Logan in 1932, and the comprehensive summary by Gill in the same year are
indicative of progress at that time. An event of particular importance in the
* Soil science, or pedology, was developed and is now practiced almost entirely by workers in
the field of agriculture.
