In The Course of his classic work on the Llandovery graptolite stratigraphy of central Wales, O. T. Jones (1909) mapped a large portion of the Ashgill inlier north of Pont Erwyd. Jones erected three sub-divisions of the Ashgill series; in descending stratigraphic order:
- Bryn-glas Group
- Drosgol Group
mudstone, grits and conglomerates
- Nant-y-moch Group
mudstones and flags
The Group was next described by Jones & Pugh (1935) in a regional survey of west central Wales which established that these strata are confined to the Plynlimon inlier. The original fossil finds of Jones were augmented by the discovery of Orthograptus ex. gr. truncatus (Lapworth) at other levels. A thickness of 718 m (base not exposed) was given (1935, p. 264) which suggests that the entire area of culmination had been studied.
During 1965–67 the writer mapped the Plynlimon inlier on the scale of six inches to one mile, as part of a regional study of the Ashgill basin facies in west central