164 million years ago (MYA).
Oxford clay. The boundary between the Callovian and Oxfordian Stages is well exposed on the coast at Ham Cliff and is well-known for its ammonite faunas and contribution to Oxfordian biostratigraphy. Important individual features include the well-preserved aragonitic ammonites and nodule beds. In particular the Red Nodule Bed exposed at Furzy Cliff is associated with a beautifully preserved three dimensional ammonites and bivalves. The Dorset Oxfordian sections have contributed to significant publications and ongoing work in many fields. The Oxford Clay (Callovian) is hydrocarbon-rich and has produced fine faunas of marine invertebrates and reptiles. Callovian: upper Macrocephalus to Lamberti Zones.
The Kellaways Formation and Lower Oxford Clay are poorly exposed along the Fleet shore from East Fleet to near Tidmoor Point. They represent the initiation of the fourth major sedimentary rhythm (J4), which ends with the carbonates of the Corallian Beds (upper Oxfordian).
The Cornbrash is exposed at Shipmoor Point, Berry Knap and near East Fleet. This thin limestone comprises a shallow-water facies with abundant brachiopods, bivalves and other fauna. The faunas show a major change within the Cornbrash at which the Bathonian/Callovian boundary is drawn. Lower Cornbrash (Barry Member), Bajocian: Discus Zone: Upper Cornbrash (Fleet member), Callovian: Macrocephalus Zone.
Forest Marble.Discontinuously exposed just west of West Bay, between Burton Bradstock and Abbotsbury Castle, and along the shores of the Fleet. The overlying sequence from the mudstones of the Fuller’s Earth through mudstones and calcareous sands of the Forest Marble to the limestones of the Cornbrash represent the third major sedimentary cycle (J3) of the Jurassic and the Bathonian Stage.
The Frome Clay Limestone forms the uppermost reservoir in the Wytch Farm Oilfield. It includes richly fossiliferous horizons and fossil oyster beds. Bathonian: Zigzag Zone to lower Discus Zone.
Fullers Earth. In the English Channel Basin the formation is predominantly mudstone with carbonaceous material at some levels and shell beds.
170 to 171 MYA
The Inferior Oolite is exposed along the coast on top of the Bridport Sands between West Bay and Burton Bradstock. This thin limestone unit of Aalenian and Bajocian age is a condensed unit but shows a very rich fauna of invertebrate fossils. It is characterised by local unconformities and discontinuities but a remarkably full ammonoid biostratigraphy is known. It is especially famous for the work by Buckman (1860-1929) on the fossil brachiopods and ammonites, both from along the coast and outcrops inland. Buckman used the ammonite sequence to establish a zonation, which can be correlated in many other areas of the world. Following the pioneering studies of Albert Oppel (1831-1865), Buckman’s detailed ammonoid work established a new precision, which led to modern chronostratigraphy. Although much criticised in his day, the detailed work of Buckman has been replicated in recent years by Callomon and Chandler and others. The topmost unit of the Bridport Sands and up to the Yellow Conglomerate is assigned to the Aalenian Beds (4-7 at Burton Bradstock), and Beds 8-16c are assigned to the Bajocian.
172 to 174 MYA
Bridport Sands. Best seen between West Bay and Burton Bradstock but also below Thorncombe Beacon. The base of the Aalenian and Middle Jurassic lies in the topmost bed. These microrhythmic sandstones have been much studied at outcrop because they form the middle reservoir of the Wytch Farm Oilfield. The change in age of the sandstones of the upper Lias southward from the Midlands was first documented by Buckman and is an example of diachroneity much quoted in textbooks. Toarcian: Levesquei Zone.
175 to 182 MYA
Down Cliff clay: Best seen in Down Cliff east of Eype Mouth. Thought to belong to the Toarcian Levesquei Zone.
Beacon Limestone: Best seen immediately east of Eype Mouth. These thin condensed limestones comprise the top unit of cycle J1. They show evidence of contemporary fault movement. The Marlstone includes condensed faunas of the Pliensbachian Spinatum Zone and these pipe down into the beds below. The overlying Junction Bed (sensu stricto) comprises condensed faunas of the Toarcian Falciferum to earliest Levesquei Zones. Dyrham Formation (29-176 metres).
Dyrham Formation. Thorncombe sands: Best seen below Thorncombe Beacon this unit comprises cross-stratified sands with bioturbated levels. Topmost Pliensbachian.
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.
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.