The acute F 1 folds in the Isle of Man possess Caledonoid axial planes and plunge predominantly to the north-eastern or south-western quarters at a low angle. Synchronous F 1 cross-folding is locally developed and plunges steeply to the north-west or south-east. The axial planes of these acute structures have the same steep Caledonoid orientation as their orthodox associates and they share the same first axial-plane cleavage (S 1 ). The generation of the cross-folding is related to original culminations and depressions in the Caledonoid hinges of the large-scale F 1 folds. Comparison is made with early cross-folding in other parts of the Caledonides.

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