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 / ﻿52.6608°N 3.8993°W﻿ / 52.6608; -3.8993
Region| Mid Wales
Country| Wales
Type section
Named for| Nant Ceiswyn

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]

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]

## References

1. ↑ W. T. Pratt; D. G. Woodhall; Malcolm Fletcher Howells; M. J. Leng (1995). Geology of the Country Around Cadair Idris. H.M. Stationery Office. ISBN 978-0-11-884509-0. https://books.google.com/books?id=lhXwAAAAMAAJ.
2. ↑ 2.0 2.1 "Ceiswyn Formation". BGS on-line lexicon of rock units. British Geological Survey. http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=CSW.
3. ↑ Richard A. Fortey; D. A. T. Harper (2000). A Revised Correlation of Ordovician Rocks in the British Isles. Geological Society of London. pp. 23–. ISBN 978-1-86239-069-0. https://books.google.com/books?id=ekD5uKAw_nYC&pg=PA23.

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