Folding

• The bending of rock strata due to compressional forces acting tangentially or horizontally towards a common point or Plane from the Opposite direction is known as folding.
• It results in the crumbling of strata into folds.
• Folds occur in a series of trough and crest alternating with each other.
• The upfolds are termed anticline and the downfold are termed the syncline.
• On the crest of the anticline, along the axis from where the limbs dip away, is a zone of tension and, therefore, of weakness.
• The rivers flowing here breach the anticline and gradually erode the material and deepen the valley. Where the anticline is fully eroded the ridge is replaced by a valley. This is termed anticlinal valley.
• It is an example of what is commonly known as inversion or relief or topography. So are the synclinal ridges.
• The sides of both these features correspond to very steep, scarp slopes. Synclines correspond to valleys.
• Between two anticlinal valleys the synclinal portion stands higher than the valleys. This is termed a synclinal ridge.
• The different types of folding are as follows:

• A monocline results when horizontally laid beds clip and then flatten out producing simple flexure.
• In a symmetrical fold, both the limbs are equal and inclined at same angle.
• Asymmetrical fold is one where one limb in a fold structure is steeper than the other.
• An overturned fold is formed when one limb occupies the normal position, while the other bends more than 90°.
• An isoclinal fold results from the continued lateral compression upon an overturned fold and crowding it upon the adjacent overfold. Here both the limbs dip at equal angles in the same direction.
• Recumbent fold is literally a fold lying down, resulting from the continuation of pressure. The axial plane and both limbs of a fold lie roughly and horizontally.
• Nappe results when the pressure exerted upon a recumbent fold is sufficiently great to cause it to be torn from its roots and to be thrust forward.

FAULTING

• Faults are formed due to the combined effect of both the compressional and tensional forces, but the tensional forces play a significant role in it.
• Excessive stresses and strains produce fracturing and is usually accompanied by dislocation.
• Such relatively rapid movement produce crustal blocks (Horsts) and Rift valley or depression (Graben).
• Horsts and grabens are found in association with one another. The plane along which the rocks are displaced is called fault plane.
• The different types of faults are as follows:

• In Normal Faults, the rock blocks are displaced in opposite directions and one of the blocks moves downwards. The surface gets stretched during a normal fault. The fault plane is usually in between and in the vertical.
• In Reverse Faults, the rock blocks move towards each other and one block over-rides the other. During the reverse fault, the faulted area gets compressed. These are also called Thrust faults. The fault place in this case is usually inclined at an angle between 40° and the horizontal (0°).
• When a series of faults occur in any area in such a way that the slopes of all the fault planes of all the faults are in the same direction, the resultant faults are called as Step faults. The Rhine valley in Europe is situated in such step faults.

• When the rocks are compressed from two opposite directions, the rock blocks move forward or backward instead of upward or downward. In this process, strike-slip or transcurrent faults are formed. San Andreas fault in California is an example of such fault.
• The part above the fault plane is known as the hanging block and the one below it is termed the foot block.
• The surface of the hanging block on the fault plane is the hanging wall and that of the foot block on the fault plane is the foot wall.
• Displacement is of two types, the vertical displacement termed throw and horizontal displacement termed as heave.