Teaching Earth Sciences - Earth Science Teachers' Association

Figure 5 Plunge refers to the dip of the fold axis. In a non-plunging fold, it is
horizontal.
Figure 3 A symmetric fold has limbs of equal length: the fold axes are
perpendicular to the enveloping surface. In an asymmetric fold, the limbs are
unequal, and the axial planes are oblique to the enveloping surface.
Figure 4 Fold attitude is defined by the orientation of the axial plane and the
younging directions of the limbs.
Figure 6 The inter-limb angle of a non-plunging fold can be calculated by
subtracting the sum of the dips of the two limbs from 180˚.
described as ‘upright’. In 4b, the axial plane has moved
away from the upright, but the younging direction in both
limbs is upwards – the fold is therefore said to be ‘inclined’.
4c shows a fold in which the axial plane is so inclined that
the younging direction in one limb is now downwards,
allowing the fold to be defined as ‘overturned’. In 4d,
the axial plane is more or less horizontal and the fold is
‘recumbent’. It is the inclined fold that is most frequently
mis-identified as asymmetric.
Plunge refers to the inclination of the fold axis: a plunging
fold has an inclined axis (Figure 5b); if the axis is horizontal,
the fold is described as non-plunging (Figure 5a). The
attitude of the axial plane is irrelevant.
The inter-limb angle, as the name implies, is the angle
between the limbs. In a non-plunging fold, it can be
calculated by adding together the dips of the two limbs,
and subtracting them from 180˚: the remainder is the interlimb
angle, as shown in Figure 6. This gives 5 categories of
fold (Figure 7):
• gentle folds have an inter-limb angle of
120˚ – 180˚.
• open folds, in which the inter-limb angle is
70˚ – 120˚.
Figure 7 Five categories of fold can be recognised based in inter-limb angles.
• close folds with an inter-limb angle of 30˚ – 70˚.
• tight, in which the onter-limb angle is less than 30˚
• isoclinal (literally, “equal dip”) if the limbs are
approximately parallel.
In some systems of classification, the ‘gentle’ and ‘open’
categories are merged, as are the ‘closed’ and ‘tight’
for inter-limb angles greater than, and less than 90˚
respectively.
Figure 8 illustrates the difference between cylindrical folds,
which are persistent and have straight hinge lines (8a), and
non-cylindrical folds, which are impersistent (8b).
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