Abstract

D. M. D. James writes: The palaeocurrent data for the Rhuddnant Grits presented by Clayton (1993) fills a major gap in knowledge of transport directions within the Welsh Silurian Basin and is much to be welcomed. The analysis of possible causes for the observed orientations of the palaeocurrents ventures boldly onto somewhat speculative quantitative ground, as Clayton fully acknowledges, and is thus doubly welcome.

More data could usefully have been included on the nature and estimated height of the slope bounding the Rhuddnant Grits sub-basin and interpreted as induced by syndepositional activity on the basement faults of the Central Wales lineament. Major thickening of the grits across the lineament occurs in the turriculatus Biozone (British Geological Survey 1993, section 1); in contrast the grits of the crispus Biozone show negligible thickening across it and Clayton does not indicate from which biozone the palaeocurrent data was measured. It would also be helpful to indicate the criteria by which the thickness variation is considered to be syndepositional rather than infill of static topography.

The generation of solitons on a transverse slope (i.e. at right angles to current motion) is presumably favoured by large differences between head and body heights and/or densities of the turbidity current and a rapid longitudinal transition between the head and body such that time is available after passage of the head to allow the laterally ponded material to accelerate to speeds capable of causing bed shear stress in the ripple field (i.e. an observable result) when interfering with

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