THE SEDIMENTARY HISTORY OF SOME TRIASSIC CONGLOMERATES IN THE VALE OF GLAMORGAN, SOUTH WALES

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Abstract
SUMMARY: From a study of maximum particle size distribution and sedimentary structures it is concluded that the Triassic conglomerates of the Vale of Glamorgan are alluvial fan deposits. A number of alluvial fans are sufficiently well preserved to be mapped, differences in the shapes of the maximum particle size isopleths, lithology and sedimentary structures leading to the interpretation that both stream flood and stream deposits are present. From details of succession an alternation of stream flood and stream deposits is demonstrated, and this is probably the result of the alternation of wet and dry periods.Variations in sphericity and in particle shape through the parameters of size and distance show that an original population of roughly 50% round, 30% disc, 15% rod and 5% blade shaped particles in the size range 5‐125 mm median diameter was subjected to stream and stream flood transport which carried disc shaped particles futher (or more swiftly) than others of the same sizes. The postulated means of trans‐ port was suspension for discs of 5‐95 mm in diameter and traction for round and rod shaped particles of equal and larger sizes.

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