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axial plane
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Diagram showing four types of geological folds in layered rock, from left to right: symmetrical fold, asymmetrical fold, overfold, and recumbent fold, with the inclination of the axial plane decreasing across the sequence. Red lines indicate the axial planes and axes, and the layers are shaded in brown tones.
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axial plane

geology
Also known as: axial surface
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Diagram showing four types of geological folds in layered rock, from left to right: symmetrical fold, asymmetrical fold, overfold, and recumbent fold, with the inclination of the axial plane decreasing across the sequence. Red lines indicate the axial planes and axes, and the layers are shaded in brown tones.
folding Folds, progressively decreasing from left to right in the inclination of the axial plane

In geology, an axial plane is an imaginary plane that divides an anticline or syncline as symmetrically as possible. Anticlines are folds convex upward, while synclines are concave upward. The axial plane can be vertical, horizontal, or inclined at any angle. The intersection of the axial plane with a stratum forms the fold's axis. Folds are undulations in the Earth's crustal rocks, formed when originally flat sedimentary layers are warped. These folds vary greatly in size and can be caused by compressional forces, gravity, or glacial action.12

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