Sarn Badrig, also known as St. Patrick’s Causeway, starts from Mochras point on Shell Island (or Mochras) and extends out in to Cardigan Bay for about twenty-four kilometres (fourteen miles, some nine of which are exposed at neap tides). It is one of three such ridges of rock and shingle that can be found on this west Welsh coast. It is thought that these causeways were created at the end of the last Ice Age by the receding ice sheets leaving glacial deposits. Sarn Badrig is the longest and the most northerly of the Cardigan Bay sarnau, and it is a maritime navigational hazard. The other local causeways are Sarn-y-Bwlch 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. The physical remains of the preserved sunken forest at Borth, further down the Cambrian coast, may have suggested that a great tragedy had overcome a community there at some point in the past, and so the story may have been born from that.
Access to Shell Island is via a sign posted road, immediately south of the bridge over the River Artro in Llanbedr. The road continues to a causeway across to the island, and is passable much of the time, but not at high tide. The island is a very popular campsite for wild camping.