Fold Classification

Folds are classified on the basis of several geometric factors:

• Tightness of folding
• Orientation of axial plane
• Thickness of folded beds

The tighness of folds can be described as open (limbs dip gently), tight (limbs dip steeply) or isoclinal (limbs are parallel).

The orientation of the axial plane relative to the horizontal together with the orientation of fold limbs allow subdivision into upright (axial plane vertical, limbs symmetric), overturned (axial plane moderately inclined, one limb overturned), or recumbent (axial plane near horizontal, one limb inverted).

Thickly-bedded, brittle units tend to form concentric folds with the bed thickness preserved normal to bedding surfaces. Thinly-bedded, clay-rich units have a tendency to develop a foliation parallel to the axial plane and form similar folds with the vertical distance between top and bottom of the unit preserved through the deformation.