Sarn Cynfelin (Saint Cynfelyn’s Causeway) is the southern most of the three Cardigan Bay sarnau, and begins just below the farmhouse at Wallog, situated on the cliffs between Borth and Clarach, and it extends for fourteen kilometres offshore into Cardigan Bay. Approximately half way along its length, it is bisected by a channel which is about five metres deep. Charted depths of one and a half metres have been recorded; both near the mid channel and, at its western extremity. The other local causeways are Sarn-y-Bwlch and Sarn Badrig. These causeways appear in Welsh legend, see Aberdyfi and Cardigan Bay and The Lost lands of Wales where they were thought to be the remains of ancient dams or dykes protecting the mythical kingdom of Cantref y Gwaelod.