Tremadocian
The Tremadocian Age/Stage within the Early Ordovician (ICS 2026/06); about 486.85–477.1 Ma.
Language and route: Welsh place-name, through British stratigraphic English. Welsh place-name, through British stratigraphic English. Name-source: Tremadoc (modern Tremadog), Gwynedd, Wales.
Literal gloss: named after Tremadoc (modern Tremadog), Gwynedd, Wales
The name and its history
Adam Sedgwick, regional name by the mid-1840s; distinct Tremadoc Group discussed 1852 · Adam Sedgwick, regional name by the mid-1840s; distinct Tremadoc Group discussed 1852
- What does the word literally mean?
- Named from Tremadoc (modern Tremadog), Gwynedd, Wales.
- What did the original author mean?
- Sedgwick’s Tremadoc Slates/Group were rock units differentiated from neighbouring Lingula Flags and younger strata in North Wales. The name began as a regional lithostratigraphic label, not a worldwide age.
- Why was the name needed?
- Welsh successions placed between conventional Cambrian and younger fossil-bearing strata needed mapping and classification; their trilobite and graptolite assemblages complicated the Cambrian–Silurian division.
- What evidence supported the original concept?
- Mapping of North Wales, lithological superposition and subsequently described fossil assemblages; the global basal boundary is now tied to conodont Iapetognathus fluctivagus at Green Point, Newfoundland.
- How has geology changed the concept?
- The original British Tremadoc rock division gave its name to the first global Ordovician age; its GSSP lies in Canada, and the modern age is not coextensive with Sedgwick’s mapped group.
Further historical context
*Original use.* Sedgwick’s Tremadoc Slates/Group were rock units differentiated from neighbouring Lingula Flags and younger strata in North Wales. The name began as a regional lithostratigraphic label, not a worldwide age.
*Historical problem.* Welsh successions placed between conventional Cambrian and younger fossil-bearing strata needed mapping and classification; their trilobite and graptolite assemblages complicated the Cambrian–Silurian division.
*Evolution. The original British Tremadoc rock division gave its name to the first global Ordovician age*; its GSSP lies in Canada, and the modern age is not coextensive with Sedgwick’s mapped group.
Editorial qualification
Sedgwick’s 1846 and 1852 references are sometimes conflated; the original regional usage, the later Tremadocian stage and the 2000 GSSP must not share one ‘first named’ date.
Related names
Ordovician · Early Ordovician · Floian · Fossil · Graptolite · Stratigraphy · Lithostratigraphy · Superposition
Sources
- ICS International Chronostratigraphic Chart, 2026/06 — Current nomenclature, rank and numerical age
- ICS official Tremadocian GSSP — International boundary stratotype and correlation evidence
- Hicks (1875), historical analysis of Sedgwick’s 1852 subdivisions — Historical naming, original claim or scholarly review; see confidence note
- JNCC, Tremadoc Series historical review — Historical naming, original claim or scholarly review; see confidence note
- ICS Ordovician Subcommission, full sequence — Cross-check order and stage nomenclature