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The Skala horizon of Podolia has been under discussion as a possible candidate for the stratotype of the fourth Silurian series (primary subdivision) in the international standard chronostratigraphical scale. After a brief survey of the history of investigation, the section through the basal boundary of the Skala horizon is fully documented. The lithostratigraphy, biostratigraphy, and correlation of the Skala horizon are then discussed.

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The Global Stratotype Section and Point (GSSP) for the base of the Silurian System was defined in 1985. Since that time, a number of researchers have suggested that this section has serious deficiencies for use as a GSSP. As a result, in 2000, the Subcommission on Silurian Stratigraphy (SSS) proposed undertaking a formal restudy of this GSSP, which was subsequently approved by the International Commission of Stratigraphy (ICS). The result of this restudy was the formal proposal that the current GSSP, at 1.6 m above the base of the Birkhill Shale, at Dob's Linn, Scotland, should be maintained as the same locality and stratigraphic level, but the biostratigraphical definition of the boundary should be revised. The previously defined, basal Silurian graptolite zone, the Parakidograptus acuminatus Zone, should be subdivided into a lower Akidograptus ascensus Zone and a higher P. acuminatus Zone. The base of the A. ascensus Zone, marked by the first appearance of A. ascensus, should be regarded as the biostratigraphic mark for the base of the Silurian. This proposal has now been formally approved by the SSS and ICS, and ratified by International Union of Geological Sciences. This is the first GSSP to undergo formal restudy and redefinition, and the proposal also included a recommended procedure for restudy of GSSPs. A report outlining the results of this work The Rhuddanian Stage, the lowest stage of the Silurian System, is named for the Cefn-Rhuddan Farm in the Llandovery area. How