7 Charmouth to Thorncombe Beacon

 

Cretaceous

101 to 109 million years ago (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.

110 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.

Jurassic

183 MYA

Dyrham Formation. Thorncombe sands: Best seen below Thorncombe Beacon this unit comprises cross-stratified sands with bioturbated levels. Topmost Pliensbachian.

184 MYA

Dyrham Formation. Down Cliff sands: Best seen below Golden Cap and Thorncombe Beacon. These are laminated sandstones with the Starfish Bed yielding the fossil brittle star Palaeocoma egertoni at the base. The Margaritatus Stone forms the top unit. Pliensbachian: Margaritatus Zone.

185 MYA

Dyrham Formation. Eype clay: Best seen around Eype Mouth. This is a light grey micaceous clay with nodules and the extraordinarily fossiliferous Day’s Shell Bed within it, which has yielded about sixty species, mostly of molluscs, close to the top. (Pliensbachian: Margaritatus Zone).

186 to 187 MYA

Dyrham Formation. Three tiers: Well exposed below Stonebarrow and Golden Cap, this unit comprises three well-cemented levels within a fine-grained sandstone. Pliensbachian. 

188 MYA

Charmouth mudstone. Green Ammonite Member: Seen below Stonebarrow and Golden Cap, this unit comprises marine shales, which become more silty and less calcareous upwards. Named after the colour of calcite filling ammonite moulds. Pliensbachian: Davoei Zone. Taken as the top unit of the Lower Lias although the topmost bed, Beds 39-41, are referred to the basal Middle Lias, Margaritatus Zone.

189 MYA

Charmouth mudstone. Belemnite Marls: This light grey unit is more calcareous (marly) than the preceding unit. It is characterised by small-scale cyclicity showing as light and dark couplets thought to be due to climatic modifications controlled by precessional orbital changes. The sedimentology and geochemistry of the cycles has been studied in detail. The Belemnite Stone represents the topmost unit. Pliensbachian: Jamesoni and Ibex Zones.

190 MYA 

Charmouth mudstone. Black ven marls: This unit is best seen in Black Ven and below Stonebarrow. It comprises blue-black mudrocks mostly in the form of calcareous shales with occasional thin limestones and nodules. It is famous for ammonites preserved in translucent yellow calcite. (Late Sinemurian: Turneri-Raricostatum Zones).