The lower Jurassic Blue Lias Formation of southern Britain is a classic epicontinental sea deposit preserving cyclical sedimentation, organic-rich black shale deposits, and is important for its palaeontological finds. This thesis investigates the environment during the time of deposition, integrating results from computer simulations and detailed fieldwork. Results show a cyclically oxygenated marine environment with episodic, high energy deposition. Bioclasts and ichnofauna differ between lithology types showing changes in benthic environment, controlled by temperature- and salinity-induced stratification, with variations up-section affected by large-scale climatic changes and water depth. Carbonate material, sourced from transported muds and aragonitic bioclasts, formed diagenetic limestones whose textures are controlled by porosity and, more recently, weathering through differences in cementation. Stratification is primarily controlled by water depth, with wind flux and heat flux impacting mixing; haloclines are more resistant to mixing than thermoclines. It is very difficult to maintain permanent stratification, so most black shales probably formed under short-term stratification. A new model for aragonite preservation in a cyclically oxygenated carbonate environment is also presented, along with a detailed log showing both the primary lithologies and diagenetic overprint, plus an updated ledge map for the Lyme Regis foreshore.