Folds In Geography

Folds are manifestations of compressional stress representing undulated rock deformation. A fold can be considered a structural feature formed when planar surfaces are bent or curved and rocks are deformed.

The term deformation refers to changes in a body’s shape, position, or orientation resulting from applying differential stress. In fold, rocks are deformed by compressional stress, forming wavy undulations on the earth’s surface.

In style and intensity, they differ from place to place and rock to rock. For example, their size may range from minute wrinkles in a small piece of metamorphic rock to huge fold mountains like the Himalayas.

Regarding their inter-limb angle, they may be very gentle to very tight categories. The folding types depend on the rock’s strength, magnitude and direction of stress under the specific condition of pressure and temperature.

Generally, folds are recognized by rock outcrop patterns, direct field observations, photo-geological methods and structural studies. Satellite imageries are gaining importance in identifying the fold patterns across the earth’s surface. For example, satellite imagery (Google Earth) shows the beautiful pattern of folds along the Fink River in Australia.

Basic Fold Geometry

The basic fold geometry includes: Hinge, Limb, Hinge Line, Axial Plane, Interlimb Angle, and Wavelength of the Fold, Crest, Trough and Fold Profile Plane.

Hinge: It represents the zone of maximum curvature of the surface. The figure shows that the hinge separates the two limbs.

Limb: It is the area between two hinges. A point in a limb where the sense of curvature changes is recognized as an inflexion point. It is clear from the above diagram that in a fold, there are two limbs and one hinge.

Hinge Line: It is a line of intersection with a bedding plane. It denotes the line of the greatest curvature in a folded surface. It is important to note from the figure (below) that the hinge line may be straight, which is designated as a ‘Straight Line Hinge’ or cylindrical fold. The adjacent figure also exhibits that the hinge line may be curved. In this case, it is designated as a ‘Curved Line Hinge’, which recognizes a non-cylindrical fold.

Axial Plane: The plane which bisects the angle between the two limbs is called an axial plane. The yellow colour in the above diagram shows the axial plane. It should be noted that the axial surface does not necessarily divide the fold into equal halves.

Interlimb Angle: The angle made by the limbs of the fold (profile plane) is designated as the fold angle or interlimb angle.

It is also important to note that the smaller the interlimb angle, the greater the intensity of folding.

Wavelength: The distance between the two hinges or zone of maximum curvature is called the wavelength of the fold. From the diagram (below), it is clear that the amplitude of the fold is half the height of the structure measured from crest to trough.

Crest and Trough: the uppermost and lowest points of a fold are called the crest and trough, respectively.

Fold Profile Plane: It signifies the shape of the fold with reference to a plane which is perpendicular to the hinge line.

Classification of Folds

A fold can be classified in several ways.

(A) Folds Based on Interlimb Angle:

We already know that the angle made by the limbs of the fold is designated as the interlimb angle. The classification of folds based on interlimb angle signifies the tightness and openness of the fold. In this way, geologists can also infer the amount of deformation through the present classification. We can generally identify four types of fold based on the interlimb angle. They are as follows:

Gentle Fold: In this case, the interlimb angle remains between 120° to 180°
Open Fold: In this case, in general, the interlimb angle ranges between 60° to 120°
Tight Fold: in this case, the interlimb angle ranges between 10° to 60°
Isoclinal Fold: It denotes the special fold where the interlimb angle is nearly 0°

(B) Based on Symmetry of the Fold:

Symmetrical Fold: When the axial plane bisects the fold at a right angle or perfectly vertical in an ideal situation, it is regarded as Symmetrical Fold.

Asymmetrical Fold: The diagram shows that the inclination or presence of a dip in a fold makes the fold asymmetrical.

Recumbent Fold: The diagram shows that when we find the axial plane of the fold in a horizontal position, we classify that type of fold as ‘Recumbent Fold’. It is an overturned fold. They are well exposed in the Alps (Billings, 2016). The figure shows that naturally, under the immense pressure of overlying rock in the recumbent Fold, the strata in the inverted limb region are much thinner than the beds in the normal limb, which is thicker than the inverted limb. The sub-features of this fold are as follows:

Arch-bend: The figure shows that the curved part of the recumbent fold between the normal and inverted limb is known as an arch-bend.
Shell and Core: The outer part of the fold is called the shell, and the inner part is recognized as the core of the recumbent fold. Notably, the core is composed of crystalline rocks surrounded by shells of sedimentary rocks.
Digitations: The figure shows that the fingers, like the subsidiary fold attached to the recumbent fold, are called digitations.

Isoclinal fold: We already know that an isoclinal fold denotes a fold where the interlimb angle is nearly 0°. Isoclinal is a Greek word which means “equally inclined”. The figure shows that in this fold, two limbs dip at equal angles and in the same direction. It should be noted that many recumbent folds are also considered as isoclinal.

Overturned Fold: In this fold, the axial plane is inclined and limbs of the fold dip in the same direction but at different angles.

Folds Based on Appearance in Cross Section:

Anticline: The figure shows that convex-shaped up-folded rock beds are designated as anticlines, i.e., a type of fold which is convex upward. The figure also exhibits that they may be symmetrical and asymmetrical in nature.

Accordingly, they are also named as symmetrical anticlines and asymmetrical anticlines. Likewise, based on the interlimb angle, they may be in the form of a gentle anticline and a steep anticline.

Syncline: The diagram shows that it is a fold which is concave upward and generally dips towards the axial plane.

Anticlinorium: It is a large anticline containing subsidiary folds of smaller size. One can find an example of this category of fold in Scotland’s mountainous area.

Synclinorium: It denotes a large syncline containing subsidiary folds of smaller size. They are found in the central part of the Aravali Ranges.

(D) Special Types of Folds

Chevron: The diagram shows that they are angular folds in which hinges are very sharp. They are also known as zig-zag.

Box Fold: The picture shows that it is a type of fold in which the crest is broad and flat. They are also known as Coffer Fold.

Fanfold: When the upper shell of the Anticlinorium is eroded due to exogenic forces, the remnant outer shell is regarded as fanfold. They are found in the Chota Nagpur Plateau region of our country.

Kink Bands: They are the narrow bands of beds ranging from a few inches to a few feet wide. The beds acquire a dip which is either steeper or gentler than the adjacent bed.

Similarly, there are many other types of folds, such as: Drag fold, monocline, Homocline, Diapir Folds, Plunging Fold and Dome.

Basic Fold Geometry