General classification of folds

When the stress or compressional force are acted on the rock strata then the rock strata tends to bend or curve as a result of permanent deformation which is known as folds. As rocks are considered to be elastic and if the maximum pressure is applied on it, there arise two cases. Either it breakdown / slip off or it bends. So, in this case those rocks which posses ductile nature are capable to bend when the stress is applied which give rise to the folds.

 

Before knowing anything about the types of fold we need to know about its parts.

1)         Crests and Troughs:
These are the convex and concave portion of folds respectively forming a series of wavy undulated structure.
 
2)         Flank / limb :
It is the stretched part of the fold lying between series of crest and trough.
 
3)         Axial plane :
It is an imaginary plane which divides the fold into two equal parts (symmetry).
 
4)         Hinge :
It is the line of maximum curvature of fold.
 
5)         Plunge :
It is the angle where fold axis makes with the horizontal.
 
TYPES OF FOLD
1)         Syncline :
A fold which is concave upward is called syncline. Here two limps dip towards each other and the younger rocks are in the center of fold.
 
2)         Anticline :
A fold which is concave downward is called anticline. Here two limbs dip away from each other and the older rock are in the center of fold.
 
3)         Symmetrical fold:
It is the fold where the axial plane is vertical.
 
4)         Asymmetrical  fold :
It is the fold where the axial plane is inclined.
 
5)         Overturned fold:
It is the fold where the axial pane is inclined and both limbs dip in same direction but in different angle. It is also called over fold (>90).
 
6)          Recumbent fold:
It is the fold in which axial plane is horizontal.
7)         Isoclinal fold:
The fold in which the two limbs dip in the same direction and at the equal angle.
 
8)         Chevron fold :
It is the fold in which hinge are sharp and angular.
 
9)         Homocline :
It refers to the bed that dips in a single direction.
 
10)           Monocline :
When the fold dips steeper within a horizontal strata or (gently dipping strata) forming step like structure is called monocline.
 
 
11)           Closed and open fold:
When the deformation is maximum and the flowage of mobile bed occurs then it is called closed fold. And when the flowage has not taken place yet is called as open fold.