The Geology of the District around Llansantffraid ym Mechain, Montgomeryshire

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Abstract
The area to be discussed, lying to the south-west of Oswestry, Shropshire, is bounded on the north and east by Carboniferous and newer rocks. It adjoins on the south-west the Meifod area, described in 1928 by Professor W. B. R. King, and on the south the Welshpool area, dealt with by Dr. A. Wade in 1911. No detailed account of this district has previously been given, but all except the most southerly portion is included in sheet 137 (Oswestry) of the Geological Survey one-inch map, and described in the accompanying memoir (Wedd and others 1929). In the latter, references will be found to published earlier work. As a result of the present reinvestigation of the Llansantffraid district, the map (Pl. XXXIX), differs considerably from the solid edition of the Oswestry sheet. This is particularly so to the north of Llansantffraid, where Ashgillian rocks have been shown to outcrop over a considerable area previously mapped as Caradocian. The limestones south of Llansantffraid have been shown to be the local base of the Valentian. The object of this paper is to describe the stratigraphy of the Llansantffraid area, with particular reference to the zonal classification of the Upper Ordovician rocks and the nature of the Ordovician-Silurian boundary. Mr. B. B. Bancroft worked out a detailed succession in East Shropshire (1928 a , pp. 33–41), and claimed that a part of the same succession was present in the Caradocian rocks at Bala, Meifod, Welsh-pool and Glyn Ceiriog (1928 a , pp. 40–1; 1933, tables). It has been

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