Garnedd-Wen Formation

{{Short description|Geological formation in Wales}}

{{Use dmy dates|date=April 2022}}

{{Infobox rockunit

| name = Garnedd-Wen Formation

| image = Tarren y Gesail - geograph.org.uk - 736726.jpg

| caption = The upper slopes of Tarren y Gesail are primarily in the Garnedd-Wen Formation

| type = Formation

| age = Hirnantian

| period = Ordovician

| prilithology = Mudstone

| otherlithology = Sandstone

| namedfor = Garneddwen

| namedby =

| region = Mid Wales

| country = Wales

| coordinates = {{Coord|52.6354|-3.9071|region:GB-WLS|format=dms|display=inline,title}}

| unitof = Abercorris Group

| subunits =

| underlies =

| overlies = Narrow Vein Mudstone Formation

| thickness = {{convert|1000|m|ft}} to {{convert|3000|m|ft}}

| extent = Tywyn to Dinas Mawddwy

| area =

| map =

| map_caption =

}}

The Garnedd-Wen Formation (also known as the Garnedd-Wen Beds) is an Ordovician lithostratigraphic formation (a sequence of rock strata) in Mid Wales. The rock of the formation is a mixture of mudstones, siltstones, fine- to coarse-grained sandstones, greywackes and conglomerates.{{cite web |url=http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=GNWF |title=Garnedd-Wen Formation |work=BGS on-line lexicon of rock units |publisher=British Geological Survey |access-date=11 September 2019}} The formation extends from Dinas Mawddwy in the north-east to Tywyn in the south-west.{{cite book|author=P. J. Brenchley|title=The Geology of England and Wales|url=https://books.google.com/books?id=-6gqvAbdS-MC&pg=PA74|year=2006|publisher=Geological Society of London|isbn=978-1-86239-200-7|pages=74–}}