One of the most dramatic environmental changes in the Mesozoic history of Europe was the switch from terrestrial to marine deposition marked by the Rhaetian Transgression, 205 Ma. Beginning with this event, the Mendip Hills, composed primarily of uplifted and folded Lower Carboniferous limestones, were flooded in a stepwise manner from the Late Triassic to mid Cretaceous. The basal Rhaetian beds at the eastern end of the Mendips (Hapsford Bridge, Vallis Vale) lie directly on Carboniferous limestone, which was bored, indicating it functioned as a hardground. Bored pebbles were then eroded, transported, encrusted with bivalves, and deposited in marine muds in the lower parts of the Westbury Formation. At certain levels also, suspended mini-conglomerates within finer-grained sediments suggest continuing storm activity. The Hapsford Bridge Rhaetian bone bed includes microvertebrate remains of four species of sharks and two species of bony fishes, all of them typical of Rhaetian-aged bone beds. The invertebrate fauna is especially rich, including bivalves and echinoids, as well as trace fossils. Unusual elements are barnacles and a possible belemnite.

欧洲中生代历史上最戏剧性的环境变化之一是从陆上沉积向海洋沉积的转变，其特征为205 Ma的Rhaetian海侵。从这一事件开始，从三叠纪晚期到白垩纪中期，逐步充斥了主要由隆起和折叠的下石炭统石灰岩组成的门迪普山。Mendips东端的基础Rheatian床（Hapsford桥，Vallis Vale）直接坐落在无聊的石炭系石灰石上，表明它起着坚硬的作用。然后将钻孔的卵石侵蚀，运输，用双壳类动物包裹并沉积在韦斯特伯里组下部的海相泥浆中。在某些水平上，细颗粒沉积物中悬浮的微型团块也表明持续的风暴活动。Hapsford桥Rhaetian骨床包括四种鲨鱼和两种骨鱼的微脊椎动物遗骸，所有这些都是Rhaetian时代骨床的典型特征。无脊椎动物的动物特别丰富，包括双壳类和类棘突类动物以及微量化石。不寻常的元素是藤壶和可能的贝伦石。