The hills and valleys of Elenydd are carved into a suite of
mudstones and
sandstones largely of
Silurian age though some
Ordovician strata is present locally within the core of the Rhiwnant Anticline which runs northeast–southwest through the southern Elenydd. Parallel to but northwest of this structure is the complex
Central Wales Syncline.
Turbidite mudstones of the lower Silurian Cwmere Formation occur around Caban-coch and Carreg-ddu reservoirs. The Caban Conglomerate Formation forms such summits as
Drygarn Fawr. To their north and west are the similar
lithologies of the
Derwenlas Formation which can be seen for example beside the dam of Claerwen Reservoir. The
Rhayader Mudstones overlie these and are to be found further north and west again forming such summits as Esgair Penygarreg, Moelfryn and Craig Dyfnant. North and west of Craig Goch Reservoir are the Rhuddnant Grits (Formation), the Pysgotwr Grits (Formation) and the Glanyrafon Formation, each of which comprises both sandstones and mudstones. They give rise to the knolly country around Teifi Pools and to such summits as Esgair y Llwyn in the east. There has been vein mineralisation within the area resulting in the development of the
Central Wales Mining Field which centres on
Cwmystwyth.
Lead,
zinc and
silver have been the prime metals derived by mining these areas over centuries. A legacy of the last ice age, there are extensive tracts of country covered by
glacial till, particularly in many of the broader valleys. Similarly extensive spreads of
peat have developed across parts of the plateau surface in the postglacial period. Some of the valleys are floored with
alluvial deposits. == Nature conservation designations ==