Fold classification, strike, true and apparent dip

Classification of folds

1. On the basis of convexity: anticline (upfold, older beds in core), syncline (downfold, younger beds in core), monocline.
2. On the basis of axial plane: symmetrical (upright, vertical axial plane), asymmetrical (inclined), overturned (both limbs dip the same way), recumbent (horizontal axial plane), isoclinal (parallel limbs).
3. On the basis of plunge: non-plunging (horizontal axis) and plunging (inclined axis) folds.
4. On the basis of shape: open, closed, tight, chevron, box, dome and basin.

Strike

The strike is the bearing of the horizontal line on an inclined plane (line of intersection of the plane with a horizontal plane). It is perpendicular to the dip direction.

Relation between apparent dip and true dip

The apparent dip is the dip measured in a direction oblique to the strike. It is always smaller than the true dip:

$$\tan\delta_a = \tan\delta \times \sin\alpha$$

where $\alpha$ is the angle between the strike and the direction of apparent dip. When $\alpha = 90°$, $\delta_a = \delta$; when $\alpha = 0°$, $\delta_a = 0°$.