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.

[1] The formation extends from Dinas Mawddwy in the north-east to Tywyn in the south-west.

[2] The Garnedd-Wen Formation is the upper strata of rock across a wide part of western Mid Wales.

In particular, it forms the upper rock beds of the Tarren Hills on the south side of the Bala Fault, west of Talyllyn Lake.