The Age of the Hirnant Beds

Abstract

The age of the Hirnant Beds has for a long time been a matter of uncertainty, occurring as they do at the junction of the Ordovician with the Silurian. They were originally included by Sedgwick in his “Upper Bala”, and in the Memoir of the Geological Survey on North Wales they are grouped with the Bala, and therefore in a modern classification would be relegated to the Ordovician. In 1879, however, Ruddy stated that the grits representing the gritty facies of these beds in the northern part of the area were the same as those coloured Lower Llandovery on the Geological Survey map, and he then held this view of their age, though later, in 1885, the placed them at the top of the “Caradoc or Bala Series”. Hughes also regarded the Hirnant Beds as representing the base of the Silurian, though he regarded the Ashgill Shales of Fairy Gill and elsewhere as Ordovician. Marr has also always regarded the Ashgill Shales of Fairy Gill as “Upper Bala”, and has included them in his Ashgillian division at the top of the Ordovician. It is perhaps significant that Salter in his Catalogue includes the fossils from both sets of beds in the “Upper Bala”, but divides this into two: (a) The Hirnant Limestone and Llanfyllin Beds. (b) The Llandovery (Lower Llandovery of the Survey).