The bedrock geology of Ceredigion (formerly Cardiganshire) in west Wales consists wholly of a considerable thickness of Ordovician and Silurian age sedimentary rocks of marine origin.
Rocks of Ashgillian age are also found in the north of the county, to the northeast of Ponterwyd and to the east of Furnace where the mudstones and sandstones of the Bryn-glas, Drosgol and Nant-y-Moch formations are exposed.
Folded and faulted Llandovery age sandstones and mudstones of the Trefechan and Mynydd Bach formations, combined as the Aberystwyth Grits Group, are exposed in the coastal cliffs southfrom Borth to Cwm Ty-du southwest of New Quay.
The mudstones of the Derwenlas and Rhayader Mudstones formations, combined as the Claerwen Group occur in a band of country from the coast north of Llangrannog in a broken and wavering line east via Talgarrog to Llanybydder on the county boundary and also as an inlier at Pontrhydfendigaid, bounded to the north by a branch of the Wyre Fault running along the Ystwyth valley.
Aligned at an angle to these are a prominent set of faults entering the county via Cwm Ystwyth and continuing west-southwest along the line of the Afon Wyre to the coast at Llanrhystud.
The former derives from cumulative snowfall on the Welsh mountains whilst much of the latter had a more distant provenance and moved onshore from Cardigan Bay.
[5][6] A prominent offshore feature which is interpreted as a glacial legacy is Sarn Gynfelyn, a shingle spit which stretches straight out from the shore at Wallog.
Peat deposits at Cors Caron near Tregaron represent the most intact raised bog in Wales and is amongst the largest such features in Britain.
The remains of mines can be found scattered in the vicinity of Nant-y-moch Reservoir and also at Goginog, Cwmsymlog and Pen-bont Rhydybeddau and the Pont-rhyd-y-groes area.
Underpinning the wildlife interest of the Dyfi National Nature Reserve is the assemblage of post-glacial geological features including sand dunes, mud flats and raised bog.