This document discusses structural geography and stratigraphy, specifically focusing on folds and folding. It defines folds as bends or curvatures in rock surfaces developed due to stress during formation. It describes the key parts of folds, including limbs, hinges, axial surfaces, and plunge. Folds are classified as anticlines, which are upfolded, or synclines, which are downfolded. Additional classifications are based on parameters like axial plane position, curvature, and plunge. Causes of folding include tangential compression, intrusions, and differential compression. Field observations and engineering considerations of folds are also discussed.
2. Fold and folding
● It is defined as undulation or bend or
curvature which has developed on
rock surface due to the stress created
on time being during its formation.
● It may be on any type of rock in any
shape.
● The ultimate shape of such curvature
depends on nature, magnitude,
direction and duration of forces that
has been induced on them.
● The process involved in formation of
fold is called Folding.
4. Parts of fold
1. Limb :
● Sites for plants of a fold are called as Limbs.
● Each food will have two different Limbs but if the force are in groups
middle Limb will act as common for two adjacent folds.
2. Hinge :
● In a folded layer a point can be found where the curvature is
maximum, and at that point one Limb end and other will start. This
point will called as hinge point.
3. Axial surface :
● When such hinge points are connected throughout the depth of a
folded sequence then such surface may be called as axial surface. It
may be planar or non planar.
5. Cont.., Parts of fold
4. Axial plane :
● It is the Imaginary plane that passes through all the points of hinge in
a folded sequence.
● It may be in vertical incline or horizontal in nature.
5. Plunge of a fold
● The angle of inclination of fold axis with horizontal measure in a
vertical plane is termed as plunge of a fold.
● Fold which is having horizontal plane will have zero plunge.
6. Crust and trough
● Highest point of the uparched fold is defined as trust.
● Lowest point of downarched fold is called as trough.
6. Classification of folds
● Folds usually occur many forms, some may be Complex in their
morphology and some may be very simple.
● Based on the type of modifications, folds can be divided into two
types,
○ Anticline
○ Syncline
7. Cont.., Classification of folds
1. Anticline :
● The strata which is up arched
(convex upward).
● The oldest formed layer will be on
the inner side of fold and
youngest will be on outside.
● Symbolically a syncline may be
indicated by two arrows diverging
away from the central point of the
hinge.
8. Cont.., Classification of folds
2. Syncline :
● The starter which is down
arched (convex downward).
● The oldest formed layer will
be on the outer side of fold.
● Symbolically a syncline may
be indicated by two arrows
pointing towards the central
point of the hinge.
9. Classification of folds based on different parameters
● Position of the axial plane
● Degree of comparison
● Behaviour with depth
● Relative curvature of the outer and inner arcs
● Plunge of fold
● Profile of outer surface
● Mode of Occurrence
● Miscellaneous types
10. Cont..,
Classification of folds based on different parameters
1.Classification based on Position of the axial plane
● Symmetrical folds
● Asymmetrical folds
● Overturned folds
● Isoclinal folds
● Recumbent folds
● Conjugate folds
● Box fold
11. Classification based on Position of axial plane
1. Symmetrical fold
● The fold which is
essentially symmetrical in
both limbs with respect
to Axial plane.
● The limbs will be having
equal length and same
dip in opposite direction.
15. Causes of Folding
Folding my be due to Tectonic forces (which are developed within
the earth body) or non tectonic forces (which are developed due to
superficial process).
1. Folding due to Tangential compression
a. Flexural Folding
b. Flowage folding
c. Shear folding
2. Folding due to intrusions
3. Folding due to differential compression
16. Cont.., Causes of Folding
1. Folding due to Tangential compression
a. Flexural Folding
Factors which affect
folding are ,
● Thickness
● Distance
● Types of the rocks
involved
17. Cont.., Causes of Folding
1. Folding due to Tangential compression
b. Flowage folding
● Incompetent, weaker, plastic
type of rocks such as clays,
shales, gypsum and rock salt
show this kind of folding.
● So in such rocks material
involved will behave as viscous
or plastic mass and tends to
unequal distortion.
18. Cont.., Causes of Folding
C. Shear folding
● The folding is attributed due to shearing stresses rather than simple
compression is called shear folding.
● It is assume that during the sheer folding process numerous closely
spaced fractures develop in the rock in the first stage of process.
● In the further process displacements of blocks will take place which leads
to folded take up bent configuration.
20. Cont..., Causes of Folding
3. Folding due to differential compression
● if the starter is being impacted due to the load which is present in besan of
sedimentation develop, with downward bending.
● If the start a is not homogeneous, then bending may not be uniform in
characteristics.
● As this type of folding is caused due to the superficial causes this is a non
tectonic fold.
21. Field observation of fold
● A fold can be recognised in the field when a surface shows direction,
sense or dip angle variations. Folds are composed of an axial plane, a fold
axis, limbs and a hinge line.
● By considering the anticline and syncline structures, te fold can be
identified.
● By visual observation of parts of fold we can say which category it belongs
to.
22. Engineering consideration of fold
● Folder develop in the areas of work are very important for a civil engineer
because this may make his work more complicated.
● So this folder and their structures need to be studied thoroughly show that an
economical conditions in the ultimate analysis can be reduced.
● During the selection of area for Dam construction, tunnel work, bridges and
Reservoir, the situation of fold plays a vital role.
● General effects of folds on major Civil Engineering projects are as follows,
a. Change in attitude - folding of any type would cause changing dip and
strike off same starter in the areal extent and also in depth.
b. Shattering of rocks- folding is the response of rocks to the stresses
induced on it. Sach in youth stresses are strong enough to break or
shutter the Rocks especially in axial zones, which are having maximum
concentration of the forces.
