Abstract

Dr S. G. Matthews said it was surprising that the authors made so little reference to fold plunge. They used a plunge feature in S Cornwall to separate Zones 11 and 12, but there is no mention of plunge in N Cornwall and S Devon (Zones 6 and 7) where geological maps have long shown different senses of plunge. In N Devon and W Somerset (Zone i) they suggest that fold axes are subhorizontal and trend E-W. Shearman (1960) referred to varying plunge in this region and Webby’s (1965) work in the Brendon Hills and the Quantocks confirmed this.

The WSW plunging folds on the south side of Exmoor involve an U. Devonian to U. Carboniferous sequence and if one of these folds acts on the base of the U. Carboniferous greywackes then it has the character that the authors prescribe for Zone 2. The same fold in an up-plunge direction, where the Pilton Beds out-crop, has dominant slaty cleavage, a characteristic that Sanderson and Dearman ascribe to Zone i. Such situations are frequently mentioned in the German literature under the name Stockwerktektonik. This should not be translated as ‘stockwork’ but as the development of structural styles in successive levels in a major structure. A Stockwerktektonik situation exists in N Cornwall with Zones 4–6 forming a recumbent fold zone with different structural effects in the different major lithologies and so plunge should again be significant, tending to run the boundaries of zones (or major lithological types) around major closures

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