The base of the Silurian System and Rhuddanian Stage is defined at 1.6m above the base of the Birkhill Shale Formation, in a section at Dob's Linn, near Moffat in the Southern Uplands of Scotland, U.K., and coincides with the first appearance of the graptolite Akidograptus ascensus, defining the base of the A. ascensus Biozone.
[Note: The previously published assignment as coincident with the local base of the Parakidograptus acuminatus Zone is now known to be incorrect, and this had caused conflicting and confusing correlations.
See also Silurian Times No. 14 (2006) at http://www.silurian.cn/newslettersshow.asp?id=38&subid=5
The GSSP for the Silurian is located in a section at Dob's Linn in an artificial excavation created just north of the Linn Branch Stream (U.K. National Grid Reference NT 1962 1584).
Two lithological units (formations) occur near the boundary. The lower is the Hartfell Shale (48m thick), consisting chiefly of pale gray mudstone with subordinate black shales and several interbedded meta-bentonites. Above this is the 43m-thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate gray mudstones and meta-bentonites.
Incoming of graptolites Akidograptus ascensus and Parakidograptus praematurus at 1.6m above the base of the Birkhill Shale Formation.
Chitinozoa and conodonts are known from the section, but are neither well preserved nor very age diagnostic.
Graptolite FAD Akidograptus ascensus