Fold
Horizontal movements are produced by forces of
compression and tension. Folding is the bending of rock strata due to
compression. Folding on a large scale results in mountain building generally
referred to as orogeny.
Up thrown part of a fold is called anticline. Down
thrown part of a fold is syncline. The side of the fold is a limb. The top of
the fold is the crest. The plane which bisects the angle between two limbs is
called the axis of fold or axial plane. The fold is formed by the plate
movements.
The type of fold depends on the nature of the rock,
the intensity of compression forces, etc. The types of the fold can be many but
we will deal with five of the following.
1.
When
compressional force is equal from both sides, the angle of the limb is same on
both sides. Such a fold is called symmetrical
fold.
2.
When
compressional force is more from one end, one limb is steeper than the other.
Such a fold is called asymmetrical fold.
3. Isoclinal folds are similar to symmet-rical folds, but these folds both have the same angle and are parallel to each other. 'iso' means 'the same' (symmet-rical), and 'cline' means 'angle,' so this name literally means 'the same angle.'
So isoclinal
folds are symmetrical and aligned in a parallel fashion.
4. When one limb of the fold is pushed over the other limb of the fold, it is called as over turned fold. Limbs are seldom horizontal.
5.
When
one side of the fold is pushed so much that it lies positioned over the other,
such a fold is called recumbent fold.
When plates converge, the weak rocks and sediments
lying between two plates get squeezed and folded. Parallel folds form long
chains of fold mountain ranges with high peaks. The fold mountains are
characterised by peaks and valleys. The tops of anticlines become the peaks and
synclines become the valleys. Intermontane plateaus (plateau surrounded by the
mountain ranges all sides) may be found between the high ranges. Example,
Tibet.
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