The Gramscatho Group and the Mylor Slate Formation are the two main lithostratigraphic groups that outcrop in south Cornwall. Their mutual relationship has been disputed for some time because of poor palaeontological control and the effects of polyphase deformation. A study of the lithofacies exposed around Hayle suggests a breakdown of the generally valid lithostratigraphy. A transitional zone occurs in which Gramscatho and Mylor lithofacies are seen to be contemporaneous and are interpreted as a rise-slope association. On the scale of south Cornwall, the Gramscatho Group comprises basinal and rise deposits whilst the Mylor Slate Formation comprises rise, slope and possibly outer shelf deposits
