The Geology Of Dorset: The Jurassic rocks
In the second of his series, John Chaffey looks at the rocks responsible for some of Dorset’s finest coastal scenery
Published in March ’12
All of the fine coastal landscapes of West Dorset are carved in Jurassic rocks, and the austere cliff architecture of the Isle of Portland owes its character to younger Jurassic strata. The cliffs east of Weymouth as far as White Nothe display the fascinating geology of the Oxford Clay and the Corallian rocks. Beyond, the Portland Beds contribute to the spectacular scenery between Durdle Door and Mupe Bay east of Lulworth Cove. In Purbeck, Kimmeridge Clay forms the dark unstable cliffs between Brandy Bay and Chapman’s Pool. Portland Limestone forms the bulwark of Purbeck’s southern cliffs facing the Channel.
Inland, in West Dorset, Lower Jurassic clays underlie the damp pastures and copses of the Vale of Marshwood. Younger Jurassic rocks form the landscape northwards from Bridport through Melplash and Netherbury to the Somerset border where deep sunken lanes display the golden yellow sands and sandstones, known as the Yeovil Sands. Although the Middle Jurassic limestones of the Inferior Oolite are only a few feet thick at the coast, they thicken considerably around Sherborne where they yield an attractive and easily worked building stone. Younger Middle and Upper Jurassic rocks outcrop over much of North Dorset to the north-east of Sherborne. Oxford Clay and Kimmeridge Clay underlie much of the Blackmore Vale, but it is the rubbly limestones of Cornbrash and the wider range of sandstones and limestones of Corallian that form the ridges of higher ground on which most of the villages of the vale are sited.
Dorset’s Jurassic rocks appear to have been deposited in the relatively shallow sea of a continental shelf area at approximately 30?north as it slowly drifted northwards away from the Equator. Variations in the depth of the sea, result from either subsidence of marine basins or global changes in sea level. Deepening of the water led to the accumulation of vast thicknesses of clay; shallowing of the water led to a change from deposition of clay through sand to limestone in the clearest conditions, for example in the sequence from Kimmeridge Clay to Portland Sand and then Portland Limestone.
The oldest of the Lower Jurassic (Liassic) rocks are the clays, shales and limestones of Blue Lias, well exposed on either side of Lyme Regis in Monmouth Bay, and in Church Cliffs. There appears to be a rhythmic repetition of the thinly bedded shales and limestones. Several ideas have been put forward to account for these variations: episodic changes in sea level may have led to a greater input of carbonate sediment, changes in the amount of insolation (received sunlight) resulting from variations in the tilting of the earth’s axis, causing global climatic change, and affecting deposition of sediment. The Blue Lias layer is particularly rich in ancient marine life including ammonites, bivalves, brachiopods and reptiles such as ichthyosaurs, and plesiosaurs. Above this is Charmouth Mudstone, well exposed in the cliffs on either side of the Charmouth Heritage Centre. It consists of a huge thickness of shales and mudstones, much prone to landslides in the cliffs. Its fauna includes ammonites and belemnites (related to present day squids), both of which have given their names to particular beds – the Green Ammonite Beds and the Belemnite Marls. It was in Charmouth Mudstone that the first remains of Scelidosaurus, a plant-eating dinosaur were found in 1858.
Middle Liassic rocks are exposed in the cliffs from Golden Cap eastwards as far as Eype Mouth, and include clays, sands and thin limestones. Included in the sequence is the famous ‘Starfish Bed’, a thin sandstone with the remains of a brittle starfish. One of the most distinctive of the Upper Liassic rocks is the Bridport Sands, exposed in East Cliff, West Bay, and in Burton Cliff at Burton Bradstock. The sequence is one of golden yellow sands, with certain layers stiffened by deposition of calcium carbonate to form a remarkable ribbing to the cliff profile. The Bridport Sands appear to have been deposited in shallow water, possibly in a huge sandbar that slowly migrated south in Lower Jurassic times.
The Middle Jurassic sequence commences with Inferior Oolite. Limestones known as oolites consist of millions of minute spherical ooliths, in each of which calcium carbonate has been deposited around a nucleus of a sand grain or a tiny shell fragment. On the coast the Inferior Oolite forms a capping to the cliffs of Bridport Sand between West Bay and Burton Hive. Fallen blocks on the beach reveal a rich ammonite fauna and elliptical brown iron rich structures known as ‘snuff boxes’ whose successive layers have been added by iron-oxidising bacteria. It has been extensively quarried and used in many of the fine buildings in Sherborne including Sherborne Abbey. Above the Inferior Oolite is Fuller’s Earth, which has a very high clay content and has been used for degreasing sheep’s wool, but more recently in plasticine and in drilling muds. It was deposited in deeper water than the Inferior Oolite. At the coast it forms a thin capping to the Inferior Oolite east of Freshwater. There are exposures immediately inland from the coast in West Dorset and also inland in a belt that runs from Chedington through Purse. In North Dorset a local variation known as the Fuller’s Earth Rock forms a distinct bench in the escarpment south of Sherborne at Dancing Hill. The highest beds in the Fuller’s Earth (now known as Frome Clay) are well exposed in the cliffs immediately to the west of the promenade at West Bay.
Forest Marble, succeeds the Fuller’s Earth, but is not a true marble, rather a shelly limestone that takes a good polish. Laid down in relatively shallow water, it forms the coastal escarpment running from Abbotsbury to Burton Bradstock and Bothenhampton: inland it runs in a belt from Corscombe north-eastwards to Henstridge. Forest Marble was widely used as a building stone, particularly in Bridport’s rope factories, and for paving stones. Above Forest Marble is the thin Cornbrash; it was originally used as a rubble stone in building but all the quarries are now disused and the only exposures are along the shores of the Fleet in south Dorset.
Cornbrash is followed in the Middle Jurassic succession by Kellaways Beds: shales and siltstones which were widely used for brickmaking near Weymouth. Exposures of the overlying Oxford Clay appear in the Weymouth Lowland, along the shores of the Fleet and also to the east of Weymouth at Furzy Cliff. A long, curving outcrop is found in north Dorset, running from the Melburys north-eastwards. Oxford Clay consists of considerable thicknesses of grey mudstones which were deposited in deep water and are richly fossiliferous, yielding a wide variety of ammonites and bivalves.
Dorset’s Corallian rocks occur in both the south and the north of the county. In the Weymouth Lowland they outcrop from Abbotsbury in the west to Osmington and Ringstead in the east, and again in the south around Wyke Regis. In North Dorset they form an important belt between Mappowder and Marnhull, near where the fine Todber Freestone is still quarried. Corallian seems to have been deposited in three cycles, each of which began with mudstones, continued through sandstones and ended with limestones, best displayed in coastal exposures between Osmington and Ringstead.
The finest coastal exposures of vast thicknesses of clay and shales are east and west from Kimmeridge itself, but there are other outcrops in western and northern Dorset, where Kimmeridge Clay underlies parts of Blackmore Vale. The best-known bed is the famous oil shale – the Blackstone Band – much worked in the past around Kimmeridge for use in the production of alum, salt and glass, although none of the industries prospered for long. Distinctive limestone bands form the treacherous Kimmeridge Ledges.
Shallowing of the water led to the deposition of the overlying Portland Sand and finally of Portland Limestone; the sequence continues into the Lower Cretaceous where the Purbeck Beds accumulated in coastal lagoons. Both the Portland Sand and the Portland Limestone are exposed on the south coast of Purbeck, and more or less continuously on the coasts of the Isle of Portland. The Portland Sand consists of marls and sandy limestones, although the latter are often dolomites, with a high magnesium content.
The Portland Limestone, arguably Dorset’s most familiar rock, is widely used as building stone, both in Dorset and in many other locations, such as St. Paul’s Cathedral in London. Shallow water conditions encouraged the development of oolitic limestones, readily worked and of fine appearance. On Portland three main beds are quarried : the highest, The Roach, is a highly fossiliferous limestone, below is the Whit Bed, an exceptionally pure and attractive-looking limestone and below that is the Base Bed, another softer limestone. Amongst the fossils of the Portland Limestone is the huge ammonite, Titanites.