Folds are classified into two main types namely anticlines or up-folds and synclines or down-folds.
- Anticline Folds:
An anticline consists of beds bent upwards with limbs dipping away from each other.
- Syncline Fold
A syncline consists of beds bent downwards with limbs dipping towards each other.
- Symmetrical Fold and Asymmetrical Fold:
A symmetrical fold is a fold whose axial plane is vertical and the limbs dip equally. The axial plane in this case divides the fold into two equal halves. If the two limbs dip at different angles the fold is an Asymmetrical fold.
This is a fold in which only one limb is bent. This is a case when a rock-bed bends abruptly and resumes the original attitude at the lower level.
- Plunging Fold or Pitching Fold:
This is a fold whose axis is at some angle with the horizontal. The inclination of the fold axis with the horizontal is called plunge of the fold.
- Isocline or Carinate Fold:
This is a fold whose limbs dip at the same angle in the same direction. The two limbs in this case are parallel. The axial plane may be vertical, inclined or horizontal.
- Overturned Fold:
This is a fold whose limbs dip unequally in the same direction.
- Recumbent Fold:
This is a fold whose limbs are bent back on themselves almost horizontally.
- Zigzag Fold or Chevron Fold:
This is a fold having a sharp angular crest or trough.
10. Supratenuous Fold:
This is a fold whose beds are thinner at the crest and thicker at the trough. Such folds are formed due to contemporaneous sedimentation, compaction over irregular surfaces uplift folding, sinking etc.
These folds are produced by tangential pressures which lift up the beds slowly and vertically at the crests. The thick troughs are formed due to sinking and large accumulation of sediments.
- Dome Fold or Quaquaversal Fold or Pericline:
Dome fold consists of a set of rock beds lifted centrally giving the feature of a dome. The area of rock bed lifted may be circular or oval shaped. In a vertical section through the summit, the fold exhibits an anticlinal feature. For this reason this fold is also called a compound anticline. After the domes are eroded, the younger rocks appear surrounding the older rocks.
- Basin Fold or Centrocline:
Basin fold consists of a set of rock beds which are sunk down centrally giving the feature of a basin. The area of the rock bed sunk may be circular or oval shaped. In a vertical section taken centrally the fold exhibits a synclinal feature. For this reason this fold is also called a compound syncline.