Sarn-y-Bwlch (Causeway of the Pass) is the middle of the three Cardigan Bay sarnau. It is the smallest of the three sarns, and it extends for about 6 kilometres offshore out from Pen Bwlch point at Tywyn, in a south westerly direction. Its charted depth is as shallow as 0.3 metres (Admiralty chart 1972). The other local causeways are Sarn Badrig and Sarn Gynfelyn. 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.