The ptygmatic fold in nature, South Sinai, Egypt

The outcrop of a ptygmatic fold in nature

• Photo description for geosciences specialists:

- Definition:
The ptygmatic fold is described as an irregular, lobate fold, usually found where single competent layers are enclosed in a matrix of low competence. Typically, ptygmatic folds do not maintain their orthogonal thickness (i.e., they are similar folds). Characteristically, their axial planes are curved.

- Reference:
Mitra, S., Datta, J. Ptygmatic structures: an analysis and review. Geol Rundsch 67, 880–895 (1978). https://doi.org/10.1007/BF01983242.

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• Photo description for non-specialists:

- Definition:
Primary folding in migmatites is caused by the high-temperature and high-pressure processes to which the migmatites owe their origin and composite character.

- Terminology:
1) Folding is a famous geological process by which the layers are shown to be bended, such as when you bend a paper in a systematic, geometric, or symmetric pattern.
2) Migmatite is a type of metamorphic rock that is formed when the main and original rocks are exposed to medium- and high-grade metamorphic environments, where the rocks will be changed to another one with different characterizations.
3) High-temperature and high-pressure processes are the main effective factors that facilitate the metamorphism process.

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⁃ Taken and published by Geologist Metwally Hamza,
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