They exhibit the characteristic textures and structures of greywackes but differ from the normal type in being relatively well sorted and commonly laminated.

The sandstones have sharply defined bases, often bearing sole structures and occasionally loaded.

The Hells Mouth Formation has yielded fossils dating back to the Lower Cambrian period, including the trilobites Hamatolenus (Myopsolenus) douglasi Bassett et al. (1976, p. 627, pl.1, figs 1 - 8; pl.

The fauna is of late lower Cambrian age and falls within the upper part of the protolenid-strenuellid Zone [6] of the Comley Series of British nomenclature.

East of Trwyn y Fossle acritarchs from the upper part of the Hell's Mouth Formation are often numerous but, at best, moderately well preserved (Young, et al., 1994, p. 338.