1.Concentric Folds

• Parallel or concentric folds are those where the strata have been bent into more or less parallel curves in which the thickness of the individual beds remains the same.

• From Figure 3.2.1 it can be observed that, because the thickness of the beds remains the same on folding, the shape of the folds changes with depth and, in fact, they fade out. Parallel folding occurs in competent (relatively strong) beds that may be inter bedded with incompetent (relatively weak, plastic) strata.

2.Similar Folds

• Similar folds are those that retain their shape with depth. This is accomplished by flowage of material from the limbs into the crest and trough regions (Fig. 3.2.2). Similar folds are developed in incompetent strata.
• However, true similar folds are rare in nature, for most change their shape to some degree along the axial plane. Most folds exhibit both the characteristics of parallel and similar folding.