Ceiswyn Formation

The Ceiswyn Formation is an Ordovician lithostratigraphic group in Mid Wales. 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 to Cardigan Bay near Tywyn.
Previous names for the rocks that form this unit include the Nant Hir Mudstone, the Nant Hir Group and the Allt Ddu Formation.

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.

Fossil content

Harnagian-Soudleyan trilobites have been found in the rocks of the Ceiswyn Formation near Dinas Mawddwy.