At the close of the Permian, central Britain was part of a shimmering, landlocked desert underlain by the Precambrian basement rocks of the Worcester Basin. The hot arid conditions persisted into the Triassic period and prolonged continental deposition resulted in the accumulation of a deep layer of sedimentary rocks. Renewed erosion caused by further rifting of the Worcester Basin resulted in the deposition of the coarse grained sandstones and gravels of the Bromsgrove Sandstone Formation.
The rocks comprising the Bromsgrove Sandstone Formation are fluvial in origin and were deposited by long, braided streams meandering across the Worcester Basin. Although some of the transported material originated from local sources, the presence of ‘exotics’ shows that the sandstones were deposited by northward flowing streams which drained North-Western France during the Triassic period. A small outcrop of the Bromsgrove Sandstone Formation can be found in Malvern town centre.
As the topography became more subdued and erosion was overtaken by deposition, broad inland mudflats were established over large parts of the area, depositing the rocks of the Mercia Mudstone Group. The presence of evaporates such as nodules of gypsum within the Mercia Mudstone Group suggests that shallow playa lakes covered a large part of the Worcester Basin.
The Mercia Mudstone Group crops out to the east of the area. Topographically it is characterised by low-lying scarps and gentle dip slopes. The mudstones are easily eroded to produce the low relief landscape of the Severn Valley, with the occasional band of sandstone forming slightly more prominent features.
The Post-Triassic period
Late Triassic and early Jurassic rifting associated with the break up of Pangaea resulted in the opening of a number of small shallow seaways. By the middle Jurassic period, Pangaea had stated to separate into the two separate continents of Gondwana (Southern Hemisphere) and Laurasia (Northern Hemisphere). This continental rifting heralded the opening of the North Atlantic Ocean. The arid continental deposition characteristic of the Triassic was gradually replaced by marine deposition within shallow inland seas which covered most of Europe during the early Jurassic period. By the middle Jurassic, a shallow sea was firmly established over the area, giving rise to the limestones of the Inferior Oolite Group. Broad Down provides an excellent view across the Triassic Severn Vale to the Jurassic outlier of Bredon Hill (SO964403) which forms part of the Cotswold escarpment.
The structural and depositional history of the Malvern Hills is poorly understood after Triassic times. It is likely that the Malvern Hills formed part of a landmass until the early Cretaceous period. However a prolonged period of non-deposition and intense sub-aerial erosion has stripped away the Jurassic and Cretaceous sediments, producing a major gap in the stratigraphic record of the Malvern Hills and surrounding area. However, approximately 23 million years ago, a period of global cooling resulted in the formation of the continental ice caps of Antarctica and small ice sheets within the Northern Hemisphere. This gradual progression towards cooler climatic conditions paved the way for the climatic instability experienced during the Quaternary Period.