It is the fifth of the six subdivisions (in ascending order) of the Ordovician System, comprising all those rocks deposited worldwide during the Caradocian Age (458 to 448 million years ago).
Similarly, two divisions, known as the Upper and Lower Hartfell series, are recognized in the southern and central area, in Peeblesshire, Ayrshire and Dumfriesshire.
In the Lough Mask district beds of this age are found, as in Wales, interstratified with volcanic lavas and tuffs.
The series in the type area consists of the Hirnant limestone, a thin inconstant bed, which is separated by 1,400 ft (430 m).
The latter are represented by large contemporaneous deposits of tuff and felsitic lava which in the Snowdon District are several thousand feet thick.