| Ceiswyn Formation | |
| Type: | Group |
| Age: | Hirnantian |
| Period: | Hirnantian |
| Prilithology: | Mudstone |
| Otherlithology: | Siltstone |
| Region: | Mid Wales |
| Unitof: | Ogwen Group |
| Underlies: | Nod Glas Formation |
| Overlies: | Craig-y-Llam Formation |
| Thickness: | Typically 1400m (4,600feet), up to 1550m (5,090feet) at the western end |
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 Bala Lake to Cardigan Bay near Tywyn.[2]
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.
Harnagian-Soudleyan trilobites have been found in the rocks of the Ceiswyn Formation near Dinas Mawddwy.[3]