UPSC ESE 2025 Civil Engineering Paper

Depending upon the orientation or attitude of the fold, the hinge line may be horizontal or inclined. If the hinge line is inclined to the horizontal, it is said to plunge, and the fold is called a plunging fold. If the hinge line is horizontal, plunge is zero; such folds are called non-plunging folds.
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\caption{(a)}
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\caption{(b)}
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Figure: Plunging and non-plunging folds. (a) Plunging fold. The plunge is the angle between the hinge line and its horizontal projection line.
(b) Non-plunging fold in which the hinge line is parallel to a horizontal line.