Nicholas (1915) described the geology of St. Tudwal's Peninsula. He termed the oldest strata, exposed in the western cliffs of the peninsula, the 'Hell's Mouth Grits'. Bassett and Walton (1960) described the sedimentology and provenance, and Young et al., redescribed the rocks and formalized the formational names, newly retitling the 'Hells Mouth Grits' as the Hells Mouth Formation. Correlated lithologically with upper part of the Rhinog Formation in the Harlech Dome, the grit beds exposed at St Tudwal's are very uniform in lithology and thickness when traced along the outcrops, with a gradual thinning southwards. They exhibit the characteristic textures and structures of greywackes but differ from the normal type in being relatively well sorted and commonly laminated. Intercalated mudstones are more variable both in thickness and in lithology and contain laminated mudstones rich in sponge remains. The sandstones have sharply defined bases, often bearing sole structures and occasionally loaded. Sandstone dykes cut down from the bases of some beds and extend through up to 0.6 m of underlying siltstones. The sandstones may form sheets up to 4 m thick, although a bed thickness of up to 1m is more usual, and have been interpreted as turbidites deposited by currents from the northeast.