Ceiswyn Formation

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]

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]