Sir Peter Kent writes: In discussing the structural control of Jurassic deposition in Britain, Hallam (1958), Sellwood & Jenkyns (1975, 1976 this Journal131, 373-88, 132, 230–2) and Hudson (1976 ibid.132, 227–30) have referred to two aspects—firstly the nomenclature and secondly the relation of Mesozoic to deep structure. The problems raised can be clarified by reference to the regional evidence.

To the writer the word ‘swell’ implies either a long smooth wave—as at sea— or a symmetrical protuberance as in a swelling. Either has some resemblance to a sine curve. Defined in these terms ‘swells’ are rare in the British Jurassic, for among the many cases of contemporary positive areas few approximate to a sine curve in section. The construction of measured sections, especially if drawn to true scale, will demonstrate that in case after case the crestal area is nearly plane. Among well documented examples may be quoted the Market Weighton block, the East Anglian massif (Palaeozoic as well as Jurassic) and the Mendip area. The use of the term ‘swell’ is in line with the former descriptive term ‘anticline’ for such a structure: both names are inappropriate. ‘Positive area’ or ‘structural high’ are better but a new term is perhaps needed.

In contrast to the flatness of the positive structures, the intervening Mesozoic basinal areas present profiles which (with many irregularities) more nearly approximate to a catenary curve: they are not simply flat bottomed depressions. The high-low relationship thus cannot be reduced to

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