The Cardigan Bay Area of Special Conservation (SAC) has both rocky and 'biogenic' reefs, and along with the extensive sandbanks provide a rich habitat for wildlife. The rocky reefs extend north from Cardigan Island almost unbroken to Ynys Lochtyn, just north of Llangrannog, whilst another section extends north as far as Aberarth. The reefs provide a home to a variety of species including the rare mantis shrimp and other crustaceans, molluscs and colourful algae. Biogenic reefs are created by the animals themselves, such as the honeycomb reef worm sabellaria alveolata.
Another feature of Ceredigion coast are shingle ridges, 'causeways' or 'sarnau'. The river Ystwyth has been diverted north by the shingle beach at Tanybwlch, while the village of Borth is built just above the tide line along the brow of a shingle ridge. Sarn Ddewi and Sarn Cadgwan are hidden under the waters of southern Cardigan Bay, but Sarn Cynfelin, which is protected as part of the Pen Llyn a'r Sarnau Special Area of Conservation, is visible at low tide and extends out to sea for almost 11km from Wallog, between Clarach and Borth. These 'sarnau' are moraines formed by receding ice sheets at the end of the last ice age, and their built causeway like appearance may be the foundation of the legend of the lost land of 'Cantre'r Gwaelod'.