Hirnantian
The Hirnantian Age/Stage within the Late Ordovician (ICS 2026/06); about 445.2–443.1 Ma.
Language and route: Welsh toponym → British geological English. Welsh toponym → British geological English. Name-source: Cwm Hirnant, near Bala, North Wales.
Literal gloss: named after Cwm Hirnant, near Bala, North Wales
The name and its history
B. B. Bancroft (1933), regional stage; global basal GSSP ratified 2006 · B. B. Bancroft (1933), regional stage; global basal GSSP ratified 2006
- What does the word literally mean?
- Named from Cwm Hirnant, near Bala, North Wales.
- What did the original author mean?
- Bancroft’s regional Hirnantian was the uppermost Ordovician interval associated with the Hirnant Limestone and a distinctive brachiopod/trilobite fauna.
- Why was the name needed?
- A distinctive terminal Ordovician faunal package had to be distinguished from older Ashgill strata; later global stratigraphers needed to capture a short, correlatable post-Katian interval.
- What evidence supported the original concept?
- Hirnantia-bearing shelly fossils and subsequently global graptolite zones; at Wangjiawan North, China, basal boundary uses Normalograptus extraordinarius.
- How has geology changed the concept?
- The Welsh name survived an expanded global concept now strongly associated with a glacial pulse and biotic crisis, though neither catastrophe is the literal etymology of the name.
Further historical context
*Original use.* Bancroft’s regional Hirnantian was the uppermost Ordovician interval associated with the Hirnant Limestone and a distinctive brachiopod/trilobite fauna.
*Historical problem.* A distinctive terminal Ordovician faunal package had to be distinguished from older Ashgill strata; later global stratigraphers needed to capture a short, correlatable post-Katian interval.
*Evolution.* The Welsh name survived an expanded global concept now strongly associated with a glacial pulse and biotic crisis, though neither catastrophe is the literal etymology of the name.
Editorial qualification
The precise deeper meaning of the Welsh place-name and a universal synchrony of the glacial signal must be handled separately from Bancroft’s naming act.
Related names
Sources
- ICS International Chronostratigraphic Chart, 2026/06 — Current nomenclature, rank and numerical age
- ICS official Hirnantian GSSP — International boundary stratotype and correlation evidence
- Chen et al. (2006), original GSSP document with Bancroft (1933) naming history — Historical naming, original claim or scholarly review; see confidence note
- ICS Ordovician Subcommission, full sequence — Cross-check order and stage nomenclature