(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.
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