The rocks in Ceredigion were laid down on the floor of a deep-sea basin during the Silurian and Ordovician period some 505 to 406 million years ago. These sedimentary rocks – mudstones, siltstones and sandstones – were uplifted and emerged above the sea during a later mountain building period. This caused them to be folded and faulted. The eroded coastline provides a unique opportunity to see these rocks and their structures.
Over the past 800,000 years, Ceredigion witnessed a number of glaciations. This last glacial period remained until about 15,000 years ago. During the last ice age there were two glacier systems of particular significance for Ceredigion - one was centred in mid Wales and the other being the Irish Sea ice sheet.
The shore line of Ceredigion is faced largely by rock cliffs, but in some places the rock face occurs a short distance inland, hidden by glacial deposits; a mixture of stones, gravel, sand and clay. Low boulder clay cliffs are found along the coast between Aberaeron and Llanrhystud whilst other sections at Morfa Bychan and north of Clarach this material forms cliffs of up to 45 metres in height.
Offshore, the ‘sarnau’ of Cardigan Bay owe their existence to the last Ice Age. The sarns (also described as causeways) are an important feature of Cardigan Bay. They consist of glacial material, probably deposited by a glacier of Welsh ice after the retreat of the Irish Sea glacier. There are five sarns in Cardigan Bay, three of which are off the Ceredigion coast - Sarn Cynfelyn, Sarn Dewi and Sarn Cadwgan. Sarn Cynfelyn extends out to sea from Wallog beach for approximately 12 kilometres. These sarns are thought to be the inspiration for a local legend; the story of ‘Cantre'r Gwaelod’.