Discussion of ‘A late Ordovician/early Silurian non-depositional slope and perched basin along the Tywi Anticline, Mid Wales’ by D. M. D. James

In Volume 26, 7-26 of the Journal, Dr D. M. D. James described the stratigraphy, sedimentology and structure of the late Ordovician to earliest Silurian succession in the core of the Tywi Anticline between the Sugar Loaf and the Wye Valley. He has used these new data to consolidate his previous model of a non-depositional slope, established by Ashgill progradation of the shelf margin, separating basinal from shelf facies during the latest Ashgill and early Silurian (James 1983). Over the past four years, we have mapped the northern region of the Tywi Anticline, north-east of the Chwefru Valley, as part of the survey of the 1:50000 Sheet 179 (Rhayader) (British Geological Survey (a) in press). Our results (Barron et al. 1989; Davies et al. 1991; Smith et al. 1991; Waters et al. in press; Wilson et al. in press) lead us to question aspects of James's Ashgill stratigraphy and the viability of his non-depositional slope model.