Rocks are commonly folded. Folding occurs slowly over a long period of time under conditions of heat and pressure. When rocks are deeply buried they become hotter and subject to stresses which produce folds.

Folds are only recognisable in rocks which have layers such as bedding or banding, and the folds are visible as distortion of the bands. Folds may be any size from microscopic to many kilometres across. Very large folds often cannot be ‘viewed' as folds because they are too large and discontinuous. Their existence is deduced from careful geological mapping, such as in the Cowal area, or in some countries - usually desert countries where there is not much soil cover - folds can sometimes be imaged from satellite.

Folded rocks Ardgour

Folded rocks in a stream bed, Ardgour, Western Highlands of Scotland


Folding at outcrop scale. Locality unknown.


Giant folds in Morocco, satellite image, artificial colours.
These folds are many kilometres in width.

The three rock samples shown here were collected from the shore near Strachur.

plate 12

A. Mica schist with numerous folds exhibited by thin schistose layers B. Folding of quartzite.
Note the fold maintains roughly the same thickness all the way round the arch of the fold.
Folding of quartzite with thin micaceous interlayers. Note how the crest of the fold have thickened and
the limbs have thinned.

Sample 12A (Plate 12, sample A) is mica schist with numerous layers of slightly different composition which exhibit abundant small folds.
Sample 12B is a quartzite which has rounded arch-shaped folds. These folds maintain the same thickness around the crest of the fold. There is a thin interlayer of schist between the quartzite layers.
Sample 12C is a quartzite rock with thin interlayers of schist. The folds thicken at the crest and are narrower on the limbs.

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