90 to 94 million years ago (MYA).
Middle chalk. The Middle Chalk is present throughout the area, the lower part comprising very fossiliferous hard nodular limestone. In east Devon this yields important ammonite faunas of the latest Cenomanian and earliest Turonian age that are not found elsewhere in the UK, in addition to abundant bivalves of the genus Mytiloides. The Chalk in the Beer area includes the famous Beer Stone, a soft inoceramid-rich chalk that hardens into a fine ornamental building stone on exposure to the air. The overlying, softer, New Pit Formation is locally flinty in east Devon and while poorly fossiliferous in the field, nevertheless contains an highly diagnostic and abundant fauna of planktonic foraminifera, ostracods and calcareous nannofossils, all of which can be used for regional and international correlation.
101 to 109 MYA
The Upper Greensand Formation within the Jurassic Coast World Heritage Site (WHS) is a glauconite-rich succession of sandstones and calcarenites, with thin sandy limestones (usually concentrations of shell debris) and dark brown splintery cherts. In east Devon the chert-rich sandstones reach their maximum thickness (c. 25 metres). In this area, the cherts are characterised by the presence of sedimentary bedding. Trace fossils and other macrofauna visible within the silica concentrations confirm that the cherts are replacement features within the diagenetic history of the sediment. The uppermost sandstones in west Dorset and east Devon are characterised by glauconite-rich cross-bedded sandstones that form a quite distinctive building stone. The macrofauna is dominated by the bivalves, especially Exogyra spp., although gastropods and echinoids are also well known. The Perinflatum Subzone is an important phosphatised horizon at the top of the formation in Punfield Cove. The microfauna is quite restricted due to preservational problems, although the sandstones of east Devon have yielded Orbitolina sefini.
209 to 237 MYA
Mercia Mudstone. Exposed between Sidmouth and Branscombe, between Seaton Hole and Culverhole Point, and at Charton Bay; it succeeds the Otter Sandstone conformably. It is overlain, above an angular unconformity, by westward overstepping Cretaceous rocks; continuity of exposure of the group is broken between Branscombe and Seaton by a combination of faulting and the effect of this unconformity. Between Weston Mouth and Branscombe, and at Charton Bay, exposure is affected by landslip. The group consists largely of red-brown mudstones, with some grey-green or silty beds. Fossiliferous dolomitic sandstones and grey-green mudstones (Weston Mouth Sandstone Member) are exposed around Weston Mouth, and higher beds near Branscombe contain large amounts of gypsum. The highest unit in the group, seen east of Seaton, comprises mainly grey-green sediments (Blue Anchor Formation). The dominant sediments of the group accumulated in playas and sabkhas under subaerial and subaqueous conditions; water was of mixed continental and marine origin, and evaporitic conditions resulted in the formation of gypsum and, elsewhere in the Wessex Basin, halite. The Weston Mouth Sandstone represents a brief estuarine episode. The Blue Anchor Formation represents a transition from dominantly continental to dominantly marine influences. Magnetostratigraphic work indicates that the base of the group is Ladinian (Late-Mid Triassic) in age. Carnian-Rhaetian ages are indicated by palynomorphs from the middle and upper parts of the Group.
238 to 251 MYA
Otter Sandstone. Rests disconformably upon the Pebble Beds and consists largely of fine to medium grained sandstones, some of aeolian origin but most deposited by shallow, northward-flowing, braided rivers. Remains of arthropods, fish, amphibians and reptiles represent the fauna of a range of terrestrial and fresh-water habitats and indicate an Anisian (early Mid-Triassic) age, which is supported by magnetostratigraphic evidence. Internationally important reptile remains.
Budleigh Salterton pebble beds. Exposed at Budleigh Salterton. They rest unconformably upon the Aylesbeare Mudstone Group and comprise gravels and sands deposited in a northward-flowing, braided river system. Some of the pebbles contain fossils indicative of provenance in outcrops of Ordovician and Devonian rocks. At the top of the Pebble Beds a ‘reg’ type palaeosol with numerous ventifacts represents subaerial exposure. The unit has no indigenous fossils; its age is constrained by those assigned to the under and overlying deposits.