Abstract

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

    mudstone

  • Drosgol Group

    mudstone, grits and conglomerates

  • Nant-y-moch Group

    mudstones and flags

These Groups were specified with reference to type areas rather than type sections. Graptolites indicative of the Dicellograptus anceps Zone were found at one locality in the Nant-y-moch Group (Jones, 1909 p. 468–69). The boundary between the Drosgol and Nant-y-moch Groups was not shown on the map. In the area examined which was later found to be the southern portion of the plunge culmination, the Nant-y-moch Group was estimated to be about 305 m thick with a reservation that the true thickness might be half this figure.

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

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