Sarnau are glacial moraines (ridges of rock moved and built up by the action of glaciers) and are composed entirely of boulders, cobbles, and pebbles mixed with various grades of sediment. The reefs are surrounded by sediment plains on all sides and are exposed to tidal currents and wave action. The reef communities are characterised by a large number of species able to withstand the mobile nature of the reef substrate.
The reefs lie mostly in shallow water (less than 10m deep) although they extend into deeper water at their south-westerly extremities. These shallow rocky reefs are dominated by seaweeds resistant to scour and sand cover and which form dense seaweed beds across the reefs.
Hidden beneath the water and extending out to sea from Wallog, between Clarach and Borth, this causeway is only revealed at very low tide. It was formed by glacial moraine left by receding ice sheets at the end of the last ice age and runs for eleven kilometres out to sea, with just one small gap. It ends at an underwater reef historically known as Caerwyddno but now more commonly known as Patches.
Sarn Cynfelyn is the most southerly of three such formations that extend out into Cardigan Bay and which are features of the Pen Llŷn a’r Sarnau marine Special Area of Conservation. The longest and most northerly of these is Sarn Badrig at Shell Island near Harlech, while Sarn y Bwlch can be found close to Tywyn. Two other smaller sarns are located further south in Ceredigion.
These Sarnau, together with the sunken forest at Borth may well have been the inspiration for the origin of the legend of the lost land of Cantre'r Gwaelod - the Lowland Hundred.
As with all such myths there are several versions of the tale.
Cantre'r Gwaelod was a rich lowland kingdom that was ruled by Gwyddno Garanhir in what is now Cardigan Bay. It was protected from the sea by a series of dykes and sluice gates. However the appointed steward Seithennin - a heavy drinker in some versions of the legend - neglected the mighty ramparts and one night there was a great storm which breached the walls and swept over the kingdom.
It is said that if you listen closely, the church bells of the lost city can still be heard from Ynys-las and Aberdyfi. However the best time to listen is a matter of some conjecture. Some say a quite evening, some a Sunday morning while others say in times of danger.