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CHAPTER 1
THE SCOPE OF GEOLOGY
1. The study of earth materials goes back to the first use of metals, but the
interpretative study of the minerals and an understanding of the true origin of
rocks dates effectively from the beginning of the nineteenth century. The study of
geology has subsequently become enlarged by the growth of numerous subdisciplines, some like geophysics being now as well known as geology itself. The
term earth science (or geoscience) has been coined to embrace once again the
whole field.
2. The various sub-disciplines which are defined below are concerned with:
a. Basic description of earth minerals.
b. The application of other sciences to earth materials.
c. Methods of obtaining information for historical synthesis of earth
evolution.
d. Practical applications to human demands and problems.
3. Definitions:
(1) Mineralogy is essentially descriptive of the naturally occurring com­
pounds and chemical elements that in various combinations constitute
the rocks of the earth; the investigation and classification of these
minerals is mainly chemical and crystallographic.
(2) Petrology is first descriptive (petrography) of rocks that are naturally
occurring associations of minerals. Secondly from the nature and
distribution of rocks, the sequence of their mode of formation is inferred
(pedogenesis).
(3) Geochemistry is the application of chemical methods and theory to
explaining the pattern of distribution of minerals and rocks, and it
makes a major contribution to the prediction of occurrence.
(4) Geomorphology is descriptive of the present exposed surfaces of the
rocks of the crust of the earth, and seeks to interpret these surfaces in
terms of natural processes (chiefly erosion) which lead or have led to
their formation. The term 'Physical Geology' has also been used in this
sense.
(5) Pedology (Soil Science) is concerned with those parts of the present
earth surface which have become weathered or otherwise modified
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Chapter 1
in situ by solar energy and by the effects of organisms to form a soil
which is of primary importance to man in agriculture.
(6) Sedimentology comprises the description and interpretation of the
products of erosion (deposits) and of large assemblages of organisms
which result in accumulations of their mineral skeletons. The study is
extended to include processes of sedimentation, the interpretation of
past deposits of this kind and of the natural changes they have sub­
sequently undergone (diagenesis).
(7) Stratigraphy (Historical Geology) includes first the description of
rocks locally in their observed sequence of formation. Secondly a com­
posite reference scale of layered rocks is selected to represent a con­
tinuous period of time. Thirdly the synthetic activity is the
time-correlation of any other set of rocks with the reference scale of
rocks by comparison of evolutionary (or occasionally unique) events
inferred from observations on the two sets of rocks.
(8) Paleobiology (Palaeontology) is concerned with recording details of
all traces of past plant and animal life. Usually an attempt is made to
interpret the mode of life of the organisms (mostly extinct); this is
however only a step towards establishing the evolutionary history, for
its use in stratigraphy, of the group of organisms concerned. P a l e o ­
biology is currently the principal evolutionary basis of stratigraphic
correlation.
(9) Tectonic geology is the study of the gross arrangement of major rock
bodies in the crust of the earth, and the elucidation of the origin and
development of the vertical and horizontal movements that have led to
this arrangement.
(10) Structural geology is the study of the effects of forces causing flow
folding and fracture structures to develop in previously consolidated
rocks. Interpretation from the observed fractures and folds and their
patterns, of the forces and of their directions and constraints, is the main
basis of tectonic reconstructions.
(11) Metamorphic geology is the study of the effect of natural heat and/or
pressure on the mineral content of previously formed rocks in terms of
the chemical and physical stability of individual minerals under these
conditions, but short of complete melting.
(12) Vulcanology is the study of high-temperature melted rock
phenomena resulting from internal earth processes, either confined
beneath the observable crustal surface, or sometimes unconfined at the
surface in the form of volcanoes.
(13) Geophysics is the study of all the gross physical properties of the
earth and its parts, and is particularly associated with the detection of
the nature and shape of unseen subsurface rock bodies by measurement
of such properties and property contrasts. Small scale applied geo­
physics is now a major aid in geological reconnaissance.
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Chapter 1
( 1 4 ) Seismology is a branch of geophysics concerned with recording of
earthquakes and the interpretation of these records. Increasingly impor­
tant is the art of prediction of earthquakes with the eventual possibility
of ameliorating their surface effects.
( 1 5 ) Hydrogeology is the study of the natural (and artificial) distribution
of water in rocks, and its relationship to those rocks. In as much as the
atmosphere is a continuation of the hydrosphere, and is in physical and
chemical balance with it, there is a close connection with meteorology.
(16) Glacial geology is the study of the direct effects of the formation
and flow under gravity of large ice masses on the earth's surface.
Glaciology is concerned with the physics of ice masses.
( 1 7 ) Impact geology is the study of the effects of collisions of extra­
terrestrial masses with the earth's surface, and has derived great impetus
from study of similar features of the moon.
Use of terms 'rock* and 'soil'
4 . Unfortunately engineers and geologists have developed distinct uses of these
terms which cannot be reconciled and which will cause confusion in the present
context unless each use is prefixed.
TABLE 1. R E C O M M E N D E D U S E S O F T H E T E R M S
'SOIL' AND 'ROCK'
Current
geological
usage
Recommended
geological
usage
Example of a
typical
section
Recommended
engineering
usage
Current
engineering
usage
(RefstoCP2001:
1957 appx)
W
(«
(c)
<<0
(*)
Soil
(pedology
definition)
Soil
profile
Temperate
weathered
profile
•Drift'
deposits
Uncon­
solidated
deposits
Glacial till
(Pleistocene)
Semiconsolidated
rock with
concretions
Oxford Clay
with
concretions
(Jurassic)
Consolidated
rock
Coal Measure
shales
(Carboni­
ferous)
(Solid)
Rock
Topsoil
(humic)
Topsoil (C170)
(humic)
Engineering
soil
Soil (C161)
(with
boulders)
Bedrock
Rock (CI 55)
(Bedrock)
NOTE: It is recommended that 'soil' and 'rock* should not be used in writing without
qualification.
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Chapter 1
As indicated in Table 1, geologists have accepted the normal pedological
definition of 'soil' meaning the whole weathered profile down to unweathered
rock; they have also classified as 'rocks' all ancient deposits, whether consolidated
or not (because consolidation is an accident of local crustal circumstances).
Engineers on the other hand have used 'topsoil' for the upper humic part of the
pedological soil profile, and 'soil' for everything underneath that is movable by
normal earth moving equipment; 'rock' or 'bedrock' was sufficiently consolidated
to require blasting or comparable effort to break it up. The usage of these terms
is as described in the relevant paragraphs of the appendix to CP 2001: 1957:
C155. Rock. In the engineering sense, hard and rigid deposits forming
part of the earth's crust, such as sandstones, limestones, metamorphic
formations and igneous masses, as opposed to deposits classed as soil.
Geologists define rock as any naturally occurring deposits be they hard or
soft, but excluding topsoil.
CI 61. Soil. In the engineering sense, any naturally occurring loose or soft
deposits forming part of the earth's crust, particularly where they occur
close enough to the surface of the ground to be encountered in engineering
works, but excluding topsoil. The term covers such deposits as gravel,
sands, silts, clays and peats. It should not be confused with the agricultural
or pedological soil which embraces only the topsoil and subsoil as here
defined.
Pedological soil may come within the meaning of the word soil in the
engineering sense when it is excessively deep as in some tropical and con­
tinental regions, but in the British Isles the two conceptions of the word can
be kept distinct.
CI 65. Subsoil. The weathered portion of the earth's crust that lies
between the topsoil as here defined and the unweathered material below.
The term is sometimes used to refer to the soil in the engineering sense (e.g.
subsoil drainage) but its use in this sense is deprecated.
CI 70. Topsoil. The superficial skin of the deposits forming the earth's
crust that has, by processes of weathering and the action of organic and other
agencies, been transformed into material capable of supporting plant growth.
The term thus embraces the upper or humus-bearing horizons of the soil of
pedology.
The differences between the two well established procedures are clear. In the
long term it will be essential to avoid these difficulties of confusion by developing
modifications of both words which add more precision to any use, and to persuade
all users to subscribe to this practice, particularly in any written statement or
record. This persuasion will take time and will only be accepted if it is simple;
the uses shown in the centre of Table 1 are therefore suggested and will be used
where appropriate in this book.
5. Engineer, geologist and prediction. Geotechnical subjects with particular
relation to engineering are discussed in the later chapters of this book; in the early
chapters however, an attempt is made to isolate those elements of the whole of
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Chapter 1
geology which are necessary to a general understanding of 90 per cent of the
problems in this field likely to be encountered by an engineer. This applies
particularly to the direct finding or tracing of either rocks or underground water.
The geologist is trained to predict conditions by means of a very wide range of
observations, by understanding of processes, and by integration of results. It
cannot be too strongly stressed that field observations which have no obvious
direct relevance to engineering, such as the collection of fossils, may be crucial
to a geologist in making an effective assessment of a problem; the geologist will
normally need all the assistance he can get in recording such observations.
REFERENCE LIST—CHAPTER 1
CP 2001: 1957
Site Investigations. Code of Practice 2001, British Standards
Institution, London (under revision).
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