Recognition of Folds:
It is not easy to recognize the folds in the field. All the limbs of the fold are seldom visible and therefore, a systematic study is required for identifying folds. The features which help in recognizing folded strata are as follow.
- In some areas, folds are easily traced by the topography. Areal photographs are used for this purpose.
- The repetition of outcrops of rocks suggests the presence of a fold. In such cases, care should be taken to eliminate the possibility of a fault.
- If the fold is of open type, the reversal of dip direction is enough to identify folds. In anticlines, the oldest rock bed will occupy an axial position and in synclines, the youngest bed will occur there.
- Plunging folds, as a rule, give rise to curved outcrops the apex of which is called a ‘’closure’’ or ‘’nose’’.
- In the case of the overturned and isoclinal folds where all the limbs dip in the same direction, detailed observations are necessary to identify synclines and anticlines. The features which aid in finding out the top and bottom of the rock bed and hence the synclines and anticlines are drag folds, rock cleavage, cross-bedding, oscillation ripple marks, graded bedding, and mud cracks.
- Drag Folds: These folds show characteristics, which are similar to major folds. As a rule, the axial plane of a drag fold is roughly parallel to that of the major fold of which it is a part. This provides a very useful means of knowing the general symmetry of the major fold.
- Rock Cleavage:
The flow cleavage or slaty cleavage is formed due to the parallel orientation of platy and flaky minerals. This cleavage develops parallel to the fold axis and therefore it can be used in the study of major folds.
- Cross-Bedding: In cross-bedding, the laminae of the rock are tangential to the true bedding at the bottom but form sharp angles at the top.
- Graded Bedding: In a graded sedimentary bed, the coarser grains are always at the bottom.
- Mud Cracks: In mud cracks, the tapering end indicates the bottom.