Abstract

The President (SIR PETER KENT) welcomed this contribution with the comment that a Permo-Triassic age would be in line with evidence elsewhere of an early breakdown of the Atlantic continental margin, long pre-dating the lateral translation marked by magnetic stripes etc. He was however somewhat concerned that other authorities (also familar with other New Red Sandstone outcrops in the Hebrides) had not accepted such a late date, and he asked whether there was direct evidence from palynology or other studies.

He also commented that thicknesses of the order estimated for the Stornoway conglomerates (4 km) were usually associated in other places with relatively local contemporaneous trapdoor type faulting.

Dr Audley-Charles remarked that studies of clast composition in the thinner English Permo-Trias deposits have been interpreted as indicating deepening and widening levels of erosion. The present interpretation of the Stornoway Formation as approximately 4 km thick and consisting mainly of rudites would seem to require deep sub-aerial erosion of the adjacent landmass contemporaneous with accumulation of these rudites. Had the authors made any statistical study of clast composition? Such a study might reveal evidence of systematic changes in clast composition through the accumulated succession, resulting from erosion having removed successively deeper levels of the nearby landscape from which these rudites were derived. Would the authors accept that the lack of such composition changes could be used as an argument for the Stornoway Formation being much thinner than they had estimated?

Professor J. Watson had examined the Stornoway Conglomerate in the field

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