I. Introduction. In the counties of Radnorshire, Montgomeryshire, Shropshire, and Herefordshire, rocks of Ludlow age have been indicated over considerable areas on the Geological Survey maps. 1 There are also several large outliers of Old Red Sandstone strata in this region, and both sets of rocks are crossed from Church Stretton on the north to beyond Presteign on the south by the great Church-Stretton Fault (see fig. 1, p. 222). This fault marks approximately the boundary between the shelly calcareous type of Silurian sediment in Shropshire and the argillaceous type in Wales. The Shropshire succession of Ludlow rocks has been studied in detail (most recently by Miss G. L. Elles & Miss I. L. Slater 2 ), and the ground has been well known to geologists since the original work of Murchison. 3 On the other hand, the country immediately to the west of the great fault is little known, and no detailed account of the uppermost Silurian and Old Red Sandstone rocks has been published. Scattered references certainly exist, but they deal with the general characters of the district rather than with special features and definite localities. It was, therefore, suggested by Dr. A. H. Cox that I should undertake an examination of the area, with the view of determining the sequence and distribution of the uppermost Silurian strata. These rocks are here of especial interest, since the district occupies an intermediate position between the Shropshire (Ludlow) district with its calcareous type of sediment, on the one side, and the Welsh area, where