THE GEOLOGY OF THE USK INLIER (MONMOUTHSHIRE)

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Abstract
Calcareous and shelly Upper Silurian beds, about 2000 feet in thickness, outcropping over approximately 28 square miles in the Usk inlier (Monmouthshire) have been investigated. The lithological and faunal succession has been established and found to be constant, with minor exceptions, throughout the inlier. The Wenlock Shale is represented by at least 800 feet of mainly argillaceous beds which become sandy and micaceous in their transition upwards into a thin (40 feet) development of Wenlock Limestone. The Ludlow Series is between 1050 and 1300 feet thick and has been subdivided, mainly on the basis of total fossil assemblages, into seven stratigraphical groups. With the possible exception of a thin conglomerate band in the south of the inlier, there is no sign of any important break in the succession. This contradicts the claim by Gardiner (1917 and 1927) that the Lower Ludlow and Aymestry beds are missing. The presence of the Ludlow Bone-bed has been established and the succeeding Speckled Grit Beds are considered to be Downtonian in age. Structurally, the Usk inlier is a complex pericline whose major axis trends NNE.-SSW. A “Sudetic” phase of embryonic arching, and a possible “Malvernian” phase, preceded the main movements of Armorican age.

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