Ceiswyn Formation
Geologic formation in Wales
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The Ceiswyn Formation (also known as the Ceiswyn Beds) is an Ordovician lithostratigraphic group (a sequence of rock strata) in Mid Wales.[1] The rock of the formation is made up of interleaved beds of silty mudstones and siltstones with some sandstones and tuffs also present in small amounts. The formation runs diagonally across Mid Wales from close by Llyn Tegid (Bala Lake) to Cardigan Bay near Tywyn.[2]
| Ceiswyn Formation | |
|---|---|
| Stratigraphic range: Hirnantian | |
The cliffs of Graig Goch which expose the Ceiswyn Formation | |
| Type | Group |
| Unit of | Ogwen Group |
| Underlies | Nod Glas Formation |
| Overlies | Craig-y-Llam Formation |
| Thickness | Typically 1,400 m (4,600 ft), up to 1,550 m (5,090 ft) at the western end |
| Lithology | |
| Primary | Mudstone |
| Other | Siltstone |
| Location | |
| Coordinates | 52°39′39″N 3°53′57″W |
| Region | Mid Wales |
| Country | Wales |
| Type section | |
| Named for | Nant Ceiswyn |
Previous names for the rocks that form this unit include the Nant Hir Mudstone(s), the Nant Hir Group and the Allt Ddu Formation.[2]
Outcrops
The formation is exposed in a number of locations in Mid Wales where glacial valleys cut across it. It is especially visible in the cliffs of Graig Goch.[citation needed]
Fossil content
Harnagian-Soudleyan trilobites have been found in the rocks of the Ceiswyn Formation near Dinas Mawddwy.[3]
