Primary flow-vector criteria (symmetrical ripple marks) derived from a sequence of sublittoral to possible shoreface sediments of Late Triassic age in South Wales yield well-defined orientations which are nearly identical at a number of widely separated outcrops and attest the prevalence of northwest-southeast oscillatory flow in this region. The bivalves Modiolus hillanus (J. Sowerby) and Rhaetavicula contorta (Portlock) occur abundantly in this sequence and at a few horizons these fossils display preferred orientations which are broadly consistent with the ripple data. However the fossil orientation patterns are not, in general, directly compatible with published experimental data and it is suggested that this discrepancy results from subsequent modification of original wave-formed patterns in response to weak unidirectional (current) flow, possibly induced by tidal or longer period currents, which otherwise has left no discernible evidence in the fine-grained sediments.

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