Geology of Dorset: The Cretaceous rocks
In the third part of his series, John Chaffey looks at the rocks responsible for some of Dorset’s finest coastal scenery and downland
Published in September ’12
The Cretaceous rocks take their name from the Latin creta, meaning chalk. The Chalk, so familiar along sections of the Dorset coast, and in the great swathe of The Dorset Downs, belongs to the Upper Cretaceous. The Lower Cretaceous encompasses a much wider range of older rocks including the valuable limestones and shales of the Purbeck Beds, the sands, clays and grits of the Wealden Beds, the sands of the Lower Greensand, the Gault Clay, and the tough sandstones of the Upper Greensand.
Sea levels were relatively low after the end of the Jurassic Period. In early Cretaceous times, coastal lagoons were the main sites of deposition. Later in Wealden times Dorset received the deposits of a huge river, which appears to have flowed into the area from the west. Sea levels then began to rise, and marine deposition was characteristic of the rest of Lower Cretaceous times in Dorset. Upper Cretaceous times saw sea levels some 200 metres higher than today, when the Chalk was deposited over much of the present day position of the British Isles.
The oldest rocks of the Cretaceous System are the Purbeck Beds. The boundary between the Jurassic and the Cretaceous lies somewhere in the lowest part of the Purbeck succession, although its exact position is still debatable. In the east of the Isle of Purbeck, the Purbeck Beds are displayed in the well-known exposures in the cliffs of Durlston Bay and underlie much of the southern plateau of Purbeck to the west, where they are widely quarried for building stones. Beyond, there are narrow outcrops in Worbarrow Bay. Mupe Bay, Lulworth Cove and at Durdle Door. They form a thin veneer above the Portland Beds over much of the Isle of Portland.
The Purbeck Beds were laid down in shallow lagoons and coastal swamps similar to those at the head of the Persian Gulf today. Most of the rocks are limestones and shales; temperatures were high enough to promote high rates of evaporation, witnessed by the presence of gypsum and salt in the rock sequence. Desiccation cracks appear on some rock surfaces where muds were exposed to long periods of sunlight, and ripple marks on bedding planes indicate shallow water deposition. Most remarkable is the Fossil Forest to the east of Lulworth Cove, where the remains of tree stumps of forerunners of present day conifers can be seen in their positions of growth in the underlying fossil soils or ‘Dirt Beds’. The ‘Dirt Beds’ are well seen on both the east and west coasts of Portland, and a smaller but more accessible suite of fossil tree stumps can be seen in the disused Kingbarrow quarry just to the south of the Portland Heights Hotel. The famous Purbeck Marble is not a true marble such as the Italian Carrara Marble widely used in sculpture, but a freshwater limestone full of small snail shells. It takes a good polish and is widely used in churches in Dorset and elsewhere, particularly in Salisbury Cathedral.
The main outcrop of the Wealden Beds (which take their name from the area of Kent and Sussex where they outcrop) is in the Isle of Purbeck. They outcrop in the east in the cliffs in Swanage Bay, and they underlie the Vale of Purbeck westwards to Worbarrow Bay, where they form the characteristic red, brown and yellow cliffs that fringe its inner edge. To the west the Wealden Beds outcrop in Mupe Bay and Lulworth Cove, where they are found on the eastern and western sides of the Cove. The most westerly coastal outcrops are in Man O’ War Bay and in Durdle Cove, where they are extremely narrow. Inland they outcrop westwards in the small lowlands around the villages of East Chaldon and Upton.
Sands, sandstones, clays and grits make up the rock sequence of the Wealden Beds. They were laid down by a large river flowing from the south-west into present day Dorset. In the bands of grit, well seen on the eastern side of Lulworth Cove, there are large fragments of quartz, derived from the granites of Dartmoor. Some of the sands contain fragments of black lignite, formed of fossil plant remains. Beds with ostracods (tiny crustaceans) and freshwater snails are quite common. Harder sandstone bands form distinct ridges in the Purbeck outcrops of the Wealden beds, such as Windmill Knap west of Swanage, and Corfe Common.
Although the Lower Greensand is important geologically in Dorset, since it represents a return to marine conditions resulting from a rise in sea level, its outcrops are limited and have little impact on landscape and scenery. It is called ‘Greensand’ because of the included mineral glauconite, which is dark green in colour, weathering to black on exposure. There are overgrown exposures at the northern end of Swanage Bay in Punfield Cove, and much narrower and ill-defined outcrops in Worbarrow Bay, Mupe Bay and Lulworth Cove.
Overlying the Lower Greensand is the Gault Clay. This is a sandy clay, which thins as it is followed westwards. Also, as it is traced westwards, it rests unconformably on successively older beds of the Jurassic sequence. In Punfield Cove north of Swanage it rests on the Wealden Beds; at Holworth, near Ringstead it rests on the Portland and Purbeck Beds; in West Dorset it rests firstly on Middle Jurassic Beds at Abbotsbury Castle and then on Lower Jurassic Beds at Golden Cap and Black Ven, west of Charmouth. This indicates that the sea in which the Gault was laid down was extending westwards over an eroded sequence of Jurassic rocks. Eventually it rests on older Triassic rocks in east Devon.
Since the Gault Clay will not allow the downward passage of percolating water, it is often the site of springs, often at the base of the overlying Upper Greensand. Downward percolating water also causes the Gault Clay to become unstable, thus failing to support the overlying rocks, resulting in landslides. Such landslides are common along the coast where Gault underlies the Upper Greensand and Chalk. In Punfield Cove at the northern end of Swanage Bay a massive landslide occurred in January 2001 where the Gault Clay failed and huge quantities of the overlying Chalk collapsed in a rotational slide, which actually pushed the Gault Clay upwards along a section of the beach. Similar landslides are found elsewhere along the Dorset Coast where the Gault has again failed in Worbarrow Bay and in Lulworth Cove. Inland, similar failures have occurred just west of Shaftesbury, where the Gault is responsible for more landslides.
The Upper Greensand overlies the Gault Clay, and follows a similar outcrop pattern. It is a tough sandstone, stiffened and made more resistant to erosion by beds of chert, a type of flint. In Dorset its outcrop is widest around Shaftesbury,where it forms the high plateau on which most of the town is built; the chert beds are particularly well developed here and are very resistant to erosion, hence the high nature of the plateau. Beneath the chert beds in the Upper Greensand sequence is the Shaftesbury Sandstone, much used as a building stone in Shaftesbury and the surrounding villages, such as Ashmore and Compton Abbas. The prominent Duncliffe Hill, to the west of Shaftesbury, is an outlier of the main plateau and is an important landmark in Blackmore Vale. Southwards the Upper Greensand outcrops at the foot of the great Chalk escarpment, and forms a marked bench on which a number of villages were located because of the dry sites that the bench affords and the availability of water from the springs at the junction with the underlying Gault Clay. The outcrop follows the line of the escarpment westwards as far as Seaborough Hill overlooking Crewkerne in Somerset. In West Dorset the Upper Greensand is responsible for many of the dominant summits in the area, such as Pilsdon Pen, Lewesdon Hill, and Lamberts Castle Hill. On the coast it forms the summits of Golden Cap and Thorncombe Beacon to the east. At Golden Cap, the lower part of the Upper Greensand capping is known as the Foxmould, glauconitic sands with beds of sandy limestone, that give the cliff its golden colour. The Foxmould is overlain by resistant chert beds. Inland, Hardown Hill is also capped by the chert beds, which have been widely quarried here, both for roadstone and for use as a building material.
The Chalk, which belongs to the Upper Cretaceous, is the most widely outcropping of all of the rocks of Dorset. From the western end of Cranborne Chase it underlies the rolling expanse of the Dorset Downs bounded on the north by its great escarpment running from Melbury Hill westwards to Winyard’s Gap and finally to Seaborough Hill. The Downs swing round south-eastwards towards the coast, eventually forming the great headland of White Nothe, and the cliffs as far as Bat’s Head and Swyre Head. Beyond, the Chalk outcrop becomes increasingly narrow and to the east of West Lulworth it forms the great hogback ridge of the Purbeck Hills. Across the Dorset Downs the Chalk dips gently southwards, but in its southern outcrops in Purbeck it dips steeply to the north, reflecting the great earth movements of 20 million years ago.
Chalk is an organic marine deposit, made up of the plates (3-5 mm in diameter) of tiny algae known as coccoliths, often eaten by other organisms and consequently falling to the sea floor as faecal pellets known as coprolites. Other organic debris is common, and important fossils include echinoids (sea urchins), belemnites, brachiopods and ammonites. Only the lowest parts of the Chalk, (sometimes known as the Grey Chalk) include terrestrial debris such as clay particles (which give the Chalk a marly texture) or, less commonly, sand. Variations within the Chalk include nodular beds and hardgrounds, the latter forming as nodules immediately beneath the sea floor and then being exposed by erosion to form a rocky pavement. Flint bands, composed of silica derived from the skeletons of sponges living in the Chalk sea, are uncommon in the lowest Chalk, but become increasingly frequent in the higher or White Chalk. Flint bands commonly pick out bedding within