Ordovician Period

(485 – 443 million years ago)

The oldest rocks to be found in the Geopark belong to the period which geologists know as the Ordovician.  This period was named by the distinguished Victorian geologist, Charles Lapworth after the Welsh tribe, the Ordovices that inhabited much of central and north Wales.

Ordovician rocks of the Geopark

The rocks (shown as a cerise areaon the map above) comprise a complex mix of mudstones, limestones and sandstones.  The youngest Ordovician rocks suggest a worldwide fall in sea-level as a major ice age set in.  Geologists recognise six ‘formations’ within the late Ordovician rock sequence.

Rock layer/s (formations) Description Approximate thickness
Cwmcringlyn (highest/youngest) Sandstones with some mudstones Up to 50m
Ciliau Silty mudstones with lime-rich siltstone and thin sandstones Up to 250m
Yr Allt Mudstones and sandstones Up to 300m
Cribarth Sandy mudstones and muddy sandstones Up to 400m
Tridwr Mudstones with thin sandstones Up to 1150m
Nantmel Mudstones(lowest/oldest) Mudstones with thin sandstones Up to 700m

Visit the Ordovician Timechart.

this section of the site is still under development – a photo of Ordovician rocks will appear here