66 to 89 million years ago (MYA).
Upper chalk. The Chalk Group exposed within the Jurassic Coast World Heritage Site (WHS) comprises up to 400 metres of chalks composed dominantly of calcareous nannofossils. Traditionally, the succession is divided into the Lower, Middle and Upper Chalk, although this classification is inappropriate in east Devon. On the coast of Purbeck the Chalk forms spectacular cliffs at Bat’s Head and White Nothe between Worbarrow Bay and Lulworth Cove and at Ballard Head. In East Devon it is about 100 metres thick, and is seen within the cliffs between Lyme Regis and Branscombe, and occurs as isolated outliers between Branscombe and Sidmouth. The Upper Chalk consists of variously nodular, flinty, slightly marly and smooth white chalks which display conspicuous bedding that is picked out by lines of flints, marls and beds of harder chalk. Diverse calcitic faunas of bivalves, brachiopods, belemnites, corals, echinoderms, bryozoans, serpulid worms, fish and a few reptiles have been found in the Upper Chalk within the Jurassic Coast WHS. The sections in the vicinity of Beer, and between White Nothe and Ballard are particularly valuable as reference sections for the chalks of southern England.
90 to 94 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.
95 to 100 MYA
The Lower Chalk comprises marly chalks, often rhythmically bedded, which are rather sparsely fossiliferous in the field, but which contain abundant microfossils typical of the Cenomanian Stage. The basal bed of the Lower Chalk (also known as the Basement Bed or Glauconitic Marl) locally contains an abundant macrofauna of beautifully preserved phosphatised ammonites and other molluscs which are very important for inter-regional correlation. The Lower Chalk shows progressive onlap to the west with a basal conglomerate of the chalk facies younging from West Purbeck (Middle Cenomanian) westwards to Devon (late Cenomanian). The Lower Chalk is absent in west Dorset and east Devon, where it is replaced by a succession of thin, fossiliferous, sandy limestones called the Beer Head or Cenomanian Limestones.
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.
111 to 112 MYA
The Gault Clay and the Upper Greensand formations within the Jurassic Coast WHS are generally sandy in character. At Swanage and in the Lulworth area the Gault is a poorly-fossiliferous silty clay and the sand content increases both westwards and up-succession. In east Devon the Gault is inseparable from the overlying Upper Greensand. Ammonites are often abundant: over 100 species have been recorded from the Gault and Upper Greensand in Purbeck. Microfossils are quite rare, with much of the formation decalcified and badly weathered.
113 to 125 MYA
The Lower Greensand, Gault Clay and Upper Greensand are seen at outcrop within the Jurassic Coast WHS between Sidmouth and Swanage. These exposures display key features of earth history during over c. 15 million years during the Aptian and Albian Stages of the Cretaceous. They represent complex changes in sedimentary environments during the early stages of one of the greatest periods of continental flooding by the oceans that the planet has experienced, culminating in the deposition of the overlying Chalk. The Lower Greensand Formation consists of a grey-black sandstone with ironstone levels. These horizons are fossiliferous, with marginal marine bivalves including Eomiodon, Cuneo corbula, and the gastropod Cassiope, and including ammonites such as Deshayesites spp. The formation includes, at Swanage, the Punfield Marine Band with a unique estuarine fauna, which becomes fully marine on the Isle of Wight.
126 to 139 MYA
The Wealden Group, of early Cretaceous age, comprises a succession of mudstones and sandstones laid down in non-marine (fluvial) environments. Exposures of Wealden strata are of international interest because they provide data on terrestrial environments during the early Cretaceous, a time of major environmental change. The Wealden exposures within the Jurassic Coast WHS are the most stratigraphically extended succession seen within Europe at a single locality. The formation is exceptionally thick, up to 600 metres at Swanage, and thins rapidly to zero west of Lulworth Cove.
140 to 144 MYA
The Purbeck Group consists mainly of fossiliferous thin-bedded limestone and mudstone. It has long been recognised as a distinctive unit, and it differs obviously from the massive Portland limestone beneath not only by the thin-bedding but also by the generally non-marine fauna, rarity of thick-shelled molluscs and absence of ammonites. The lagoonal and lacustrine fauna with vertebrates of continental origin contrasts with the marine faunas of the underlying rocks. The separation from the non-marine clastic Wealden Group above is lithological. Up to 246 numbered beds have been described in some detail, and five geological members can be mapped in the field. Many are named and the ostracod and gastropod content of each is known.
145 to 146 MYA
The Portland Sand and Portland Limestone formations represent the final shallowing on the last major sedimentary rhythm of the Dorset Jurassic (J5) which commenced with the deeper facies of the Oxford Clay; in the overlying Purbeck Group lacustrine and non-marine environments are finally developed. Well-exposed on the Isle of Portland and in Purbeck, the area was recognised as a key area in earliest days of investigation and in 1829 Brongniart coined the term Portlandian by which rocks of this age have been known internationally. The excellent Portland Stone has been quarried and exported nationally and internationally for over three centuries.
The Portland Sand comprises impure siltstones and with oyster beds grading up from the Kimmeridge Clay below; levels are full of rhaxellid sponges. It contains an important ammonite fauna, which provides the last link with Russian faunas before restriction of the Boreal seas. The overlying Portland Limestone has a lower unit full of cherty levels and a Basal Shell Bed, which has yielded rich bivalve and gastropod faunas. The Portland Freestone above is the famous building stone of shelly limestones and oolitic levels with local stromatoporoid occurrences. The giant ammonite faunas have been described at least since 1668; the biostratigraphy has been well described (Wimbledon and Cope, 1978).
147 to 157 MYA
The Kimmeridge Clay is represented in Dorset by a succession of mudrocks of variable organic carbon content, together with minor amounts of carbonate, siltstone, fine-grained sandstone and ironstone. The formation is remarkable in that it has some very high total organic carbon values averaging around 10 per cent but reaching up to about 60 percent. In contrast most mudrocks have a total organic carbon content of 0-2 percent. The individual mudrock beds of variable organic carbon content are generally tens of centimetres thick and reflect the changing chemical, physical and biological conditions in seawater at that time. The beds contain an abundant well-preserved fauna of mainly ammonites, bivalves, gastropods and vertebrates. It is the standard section for the Boreal Lower Tithonian (Bolonian) ammonite zones and is probably the only section world wide with good exposure of the Rotunda and Fittoni Zones. Recent work on samples from the Kimmeridge Clay Formation in Dorset presents the only radiometric date for the marine Tithonian world-wide. The Eudoxus to Fittoni Zones are exposed uninterrupted between Brandy Bay and Chapman’s Pool. Further sections, particularly of the lowermost zones, occur at Ringstead Bay, Osmington Mills, Black Head and West Weare Cliffs on the Isle of Portland.