The Silurian System overlies the Cambro-Ordovician with angular unconformity. In the northeast and central parts of the region sedimentation continued from Silurian into Devonian time, but in the southwest part a disconformity apparently exists. The Early Silurian sea transgressed from northeast to southwest. Deposition of terrigenous sediments was followed by deposition of sediments of volcanic origin derived from the south-southeast. Llandoverian sediments are characteristic of a rapidly sinking trough, Wenlockian sediments are indicative of a shallow-marine environment, and Ludlovian and 'Skalian' sediments suggest a more rapidly sinking trough.