# Geomorphic Processes

The earth’s surface constantly keeps on changing under the inuence of a number of forces which are divide d in two broad categories known as internal or Endo genetic forces and external or Exo genetic forces

Endogenetic forces are of two types viz:- diastrophic (or slow) forces and sudden forces such as earthquakes and volcanoes. Diastrophic forces are further divided into (a) epeiroganic (upward and downward) forces and (b) organic (tension and compression forces.

Diastrophic Forces – As mentioned earlier, diastrophic forces are slow forces which originate within the earth’s interior and are effective both in the horizontal and vertical directions. They are responsible for folding and faulting in the earth’s crust.

#### Folding

Bending of horizontal layers of rocks is known as folding .Simple folds have two limbs along the slope. It is called an anticline when the limbs incline in different directions from the top. It is syncline when the limbs incline to the same direction into the bottom of the troughs. In simple words, the up-folds are called anticlines while down-folds are known as syncline. Folds are of the following types.

Following are the main types of folds

(a) Symmetrical Folds

When both the limbs of a fold are equally bent and they are inclined at almost equal angles to the horizontal, it is called symmetrical fold.

(b) Asymmetrical Folds

This type of fold is formed when one limb of the fold is pushed too far so that one side of the fold is long and gradual in slope while the other is short and steep.

(c) Monoclinal Fold

When the compressional forces act in such a way that one limb of the fold is vertical is called monoclinal fold.

(d) Isoclinal Fold or Overfold

It is formed when the fold is pushed over one side so that both the sides of the fold are inclined in the same direction.

(e) Recumbent Fold

When the isoclinal or overfold goes to such an extent that one limb is pushed over the other and the two limbs are almost parallel to each other lying in a roughly horizontal posture, it forms recumbent fold.

(f) Overthrust Fold

In case of acute folding, the pressure of the compressional force is so great that a fracture occurs in a fold, and one limb slides forward over the other limb along the thrust plane. This results in overthrust fold. The overthrust limb of the fold is known as Nappe.