The oldest rocks, of Silurian age, occur in the heart of the county between Usk and Pontypool; the outcrop of these shales, limestones and sandstones, surrounded as it is by outcrops of younger rocks, is referred to as the Usk Inlier.
The oldest rocks, of Silurian age, occur as a broad, northeast to southwest aligned anticline in the heart of the county. The central portion of this zone, between Usk and Pontypool, comprises the outcrop of the older shales, limestones and sandstones, surrounded as it is by outcrops of younger rocks, is referred to as the Usk Inlier.