Fold

You must have seen mountains and different kinds of patterns in them. Sometimes wave-like pattern or sometimes V-shaped or zig-zag patterns as well. These patterns are called folds which are formed because of bending due to some stress. There can be 2 fold, 3 fold, four-fold, or multifold in the mountains and these folds form different patterns. In this article, we will be talking about folds, it's meaning or factors responsible for folds, types of folds, etc. 

What is Fold?

It is a structure of wave-like pattern which is formed by bending of the rocks instead of breaking under any compression. These waves can be seen in stratified rocks which were formed because of sedimentary deposits on the flat horizontal sheets. These horizontal sheets are no longer flat at some places and have been warped with time. Sometimes warping can be seen in the rocks as the appearance of the structure changes and sometimes the warping is so much that two layers become parallel to each other. The rocks fold in and fold down and form a pattern. 

The size of these folds can be varied. Somewhere they can be hundreds to thousands of kilometres and somewhere they can be only of few centimetres or even less. Sometimes there can be seven fold or 10 fold mountains or even more. The size and number of folds can be varied.

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The classification of various folds depends upon the appearance and attitude of their axes. The axial plane can be horizontal, vertical, or inclined and it divides the fold symmetrically as possible. An axis is the point of intersection of the axial plane with one of the starta. The Important two-fold which we usually see in the mountains are anticline and syncline. One fold is convex-shaped upwards and another has downwards.  

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Factors Responsible for Folds

Following are the factors which can lead to the classification of folds:

• Folding Tightness

It depends upon the tightness of the folding of the layers as shown in the figure below. It can be open, tight, or isoclinal.

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• Orientation of the Axial Plane

Relative to the horizontal plane along with the orientation of gold limbs, the orientation of the axial plane also leads to different kinds of folds which can be seen in the diagram below. It can be upright, overturned, or recumbent. You clearly can see the difference in folding between all of them in the diagram.

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• Thickness

It depends on the thickness of the folded beds as well. The thick beds form concentric folds whereas thin beds form a foliation parallel to the axial plane.

Types of Folds

Anticline: 

It has an arc-like shape and its core has the oldest beds. The layers of the rocks become older towards the centre and they are more convex upwards.

Syncline: 

It is the opposite of anticline. It is a fold that is convex downwards which is also termed as synformal syncline and has younger rocks at the centre. Some synclines can point upwards due to overturned or formation of an anti-formal syncline.

Symmetrical: 

If the axial plane is vertical, then it is a symmetrical fold.

Asymmetrical: 

if the axial plane is not vertical but inclined, then it is an asymmetrical fold.

Isoclinal: 

these are those kinds of folds in which limbs are parallel to each other and approximately to the axial plane as well. 

Overturned: 

When the folds are highly inclined they are called overturned or overfold.

Monoclines: 

These are kind of folds in which all the layers incline in one and same direction.

Chevron: 

In chevron, folds look like zig-zag and it has repeated patterns with straight limbs and sharp hinges. They formed V-shaped beds which are formed of regional compressive stress.

Slump: 

It is a fold that is formed because of a landslide where sediments were softer. It forms during sedimentation or lithification. 

Ptygmatic: 

It is a type of slump fold where the material which is folding is more vicious than the surrounding material. These are random and disconnected.

Disharmonic: 

These folds are those in which different layers are having different kinds of folds. There can be a combination of various kinds of folds.

Fold Mountains

On the nature of folds, mountains can be divided into two parts which are mentioned below:

• Simple Fold Mountains: 

These are said to be the mountains with open folds where patterns of anticlines and synclines can be found and have wave-like patterns. 

• Complex Fold Mountains:

These are those mountains in which rock strata are compressed so much that it forms a complex pattern.

Features of Fold Mountains

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• These are said to be the most widespread as well as important mountains in the world.

• They are young mountains.

• They can extend up to a great length but they have a small width.

• These are great sources of minerals as well such as tin, copper, gold, etc.

• They are mostly found in the margins of a continent and ocean.

• Fossils here suggest that sedimentary rocks were formed because of the accumulation and consolidation of sediments and silts in the marine environment.

• Usually, they have both concave and convex types of slopes.