The beds adjacent to the junction of the Lower Jurassic Blue Lias and Charmouth Mudstone formations are intermittently exposed
in cliff and foreshore sections over a distance of 8 km on the east Devon and west Dorset coast on either side of Lyme Regis.
Comparison of the successions in the highest part of the Blue Lias shows little lateral variation in thickness or lithology, with the
exception of minor thickness changes in the two highest limestone beds. In contrast, the basal beds of the Shales-with-Beef
Member, the lowest part of the Charmouth Mudstone, are laterally variable. Up to five beds of limestone that are present in the
most westerly exposure in Devon are absent at the more easterly exposures in Dorset. This lateral variation does not appear to
be related to contemporaneous fault activity. It is largely due to an unconformity at the base of the Shales-with-Beef which cuts
out successively more of the basal beds when traced from west to east. The strict application of the definition of the Blue Lias
Formation, currently taken at the top of the highest limestone in an interbedded mudstone-limestone succession, would include
beds previously classified as Shales-with-Beef in east Devon
