The Geology Of Dorset: The Palaeogene rocks
In the fourth part of his series, John Chaffey looks at the rocks of the early Tertiary period
Published in December ’12
The Tertiary Period is now often referred to as two separate divisions, the earlier Palaeogene, and the later Neogene. The Palaeogene rocks of Dorset only appear in the east of the county, where they are preserved in a huge downfold in the strata, known as the Frome syncline, which is part of the much larger Hampshire Basin. They are first encountered to the east of Dorchester, where they occur as a feather edge on the underlying Chalk. Eastwards they increase steadily in thickness and extend as far as the Hampshire border. To the north and the south they are bounded by the outcrop of the Chalk. In the north the Chalk of the Dorset Downs dips gently beneath the Palaeogene rocks and underlies the Palaeogene rocks at depth (in the Wytch Farm borehole, near Wareham, at depths of 500 feet) before reappearing to the south in the Purbeck Hills and the downland to the west beyond Lulworth. Here, in the Purbeck Hills, the Chalk is dipping steeply to the north: in some places it is almost vertical. Only two divisions of the Palaeogene are represented in Dorset, the highest strata of the Palaeocene and most of the Eocene. Towards the end of the Eocene, southern England began to be affected by the distant effects of the Alpine orogeny or mountain-building episode. Folding affected the newly deposited rocks then, but the main folding occurred later about 20 million years ago. This was responsible for many of the important structures found along the Dorset coast and inland, such as the famous Lulworth Crumple exposed in Stair Hole at Lulworth, and the spectacular Ballard Down Fault seen in the cliffs to the south of Old Harry.
After the Chalk had been folded, tilted and uplifted, there followed a long period of erosion by rain and rivers. The latter carried sediment down to the open sea which now lay some distance to the east of Dorset. This sea encroached towards the west from time to time, but for most of the Eocene, Dorset was the site of large tidal estuaries of rivers that flowed into the area from the west. Sediments were laid down in migrating river channels, swamps and lagoons, and along coastal beaches and sandbars. During this time a low lying plain was gradually built out towards the east. Dinosaurs and ammonites had become extinct at the end of the Cretaceous, probably caused by the impact of a large extra-terrestrial body with the Earth. Palaeogene times saw the proliferation of mammals, and smaller invertebrate groups such as molluscs. Remains of the rich flora of Palaeogene times are common in the Palaeogene beds of Dorset and include leaves and fossil wood preserved as black lignite.
The oldest surviving Palaeocene rocks are the Reading Beds, although these have now been subsumed into the overlying London Clay formation on modern maps of the British Geological Survey. At the southern end of South Beach Studland is one of the most important sites in Dorset geology. At one time it was thought that the Reading Beds could be seen resting on an eroded surface of Chalk; the junction was regarded as an unconformity, representing a long period of erosion separating the uplift of the Chalk, and the deposition of the Reading Beds. Recent work by the Geological Survey now considers this junction may well be a fault, and that the overlying beds are younger than the Reading Beds, possibly being the Creekmoor Clay of the Poole Formation. Elsewhere the Reading Beds are made up of coarse sands and sandstones that form a prominent escarpment overlooking the lowest parts of the Chalk, particularly west of Bere Regis at Black Hill, and north of Morden, and around Chalbury, north of Wimborne. The overlying London Clay is seen in the cliffs to the north on South Beach, but, because it was deposited in relatively shallow water, occurs as grey sands, rather than the stiff clay that is found further to the east in the Hampshire and London Basins. Dark iron-rich sandstones from within the London Clay have been used as a building stone in churches at Lytchett Matravers and Almer.
Above the London Clay in the Palaeogene sequence is the Poole Formation. This consists of sands and sandstones alternating with beds of clay that have been widely worked in the Isle of Purbeck and the Poole area. These are mostly riverine or estuarine deposits, with occasional beds of marine origin. The beds of the Poole Formation are well exposed along the eastern coast of Purbeck in South Beach Studland and Redend Point to the north.
The exposures, known as the Broadstone Sand, on South Beach Studland show remarkable red and yellow sands, with concentric bands of deposition, which occurred in a water-saturated sediment. On the northern side of Redend Point the sandstone contains some unusual iron-rich pipes, the origin of which is not clearly understood. Above are grey clays that
are very rich in fossil leaves and lignite (fossil wood) fragments.
Although there are no exposures of the overlying Parkstone Sand along the coast of Purbeck there is one spectacular exposure inland on Godlingston Heath, the Agglestone. The huge mass of gritty sandstone crowns a small knoll about a mile to the west of Studland. Until 1970, the mass, which is said to weigh over 300 tons, rested on a pedestal, but it collapsed in that year to rest in its present uneasily balanced position. Within the yellow-orange sandstone there are bands of quite coarse grit, with fragments of quartz, suggesting an origin in the south-west, where the mineral may have come from granite outcrops. There are various versions of legends that suggest it was thrown by the Devil at Corfe Castle or Studland Church, but he missed! Over a quarter of a mile to the west is the Puckstone, a similar but much less prominent feature.
The clay strata within the Poole Formation contain some of the most important deposits of high quality ball clay in Britain. Ball clay owes its name to the fact that it was worked in the past by using spades known as ‘tubals’: alternatively the name may be derived from the fact that once worked, it readily formed into rough ball-shaped masses. It seems to have originated as a weathering product (kaolinite) of feldspar within the Dartmoor granite. It was then transported eastwards (the clay particles being suspended in rivers), into the Dorset area, and then deposited in shallow lagoons in coastal estuaries. It was initially worked in shallow trenches, later in shallow pits, and more recently in large open-cast pits. In the 20th century it was mined from adits under Creech Barrow, and also near Norden, but the last mines were closed in 1999. The clays within the Poole Formation have also been worked extensively in Poole and Parkstone, mostly for brick-making. The clays were also worked on Brownsea Island, where a tile-making industry flourished briefly in the second half of the 19th century.
Rivers continued to flow across the area of East Dorset during the deposition of the Branksome Sand, which overlies the Poole Formation. Repeated cycles of sandy deposits in the Branksome Sand can be seen in the exposed cliffs to the west of Branksome Chine. Each cycle consists of sands that become progressively finer upwards culminating in a layer of silty clay. These sands were deposited on the edge of the channels of wide, slow-flowing rivers. Occasionally, during a period of increased flow of the river, a deep channel was cut in these sandy deposits, and was subsequently filled by darker sandy clays. Such an infill deposit can be seen at the bottom of the cliffs immediately to the east of Branksome Dene Chine. Leaf beds are common in the Branksome Sand, and include fragments of lignitic wood, that sometimes exhibit boring by marine molluscs. The Branksome Sand forms the face of the cliffs from Poole Head eastwards to Durley Chine, and the lower part of the cliffs as far east as the low-lying land just to the west of Hengistbury Head.
It is likely that the sea later invaded the low lying estuarine plain that existed in the Bournemouth area during the time that the Branksome Sand, which underlies much of Bournemouth, was deposited. Hence the Boscombe Sand consists of beach sands and other sands deposited immediately offshore. Pebble beds are important in the upper part of the Boscombe Sands, and these represent the remains of old beaches. The Boscombe Sand first appears in the cliffs just to the east of Durley Chine and then forms the upper part of the cliff as far as the end of the promenade at Southbourne. It reappears in the bottom section of the cliffs in the Batters, at the western end of Hengistbury Head. Here it consists of buff-or brown-coloured sands, with a remarkable band of dark-coloured bituminous sand. Above are two lines of pebble beds, separated by greyish sand; these are clearly visible in the cliff profile as they decline in height to the east – they reach beach level just to the east of Warren Hill, the highest point of Hengistbury Head.
The uppermost part of the Palaeogene sequence in Dorset is marked by the marine beds of the Barton Clay which forms the middle and upper parts of the cliff at Warren Hill, Hengistbury Head, indicating that the sea flooded into the area again. Here, the Barton Clay is composed of greenish-grey clay, with an important glauconite content, a mineral that gives the clay its distinctive colour, and confirms its marine origin, probably in water rarely disturbed by waves. Within the Barton Clay there are four discontinuous bands of ironstone nodules known as doggers. These were quarried in the mid nineteenth century on the far side of Hengistbury Head, and were also collected from the base of the cliffs. From here they were taken to Southampton in barges and then shipped to South Wales for smelting. Once they had been removed from the base of the cliffs their protective function ceased, and the subsequent erosion dramatically altered the shape of the Head. The extraction of the ironstone ceased in 1870, but the cliffs remained unprotected until the building of the Long Groyne in 1938, which led to the accumulation of sand and shingle at the base of the cliffs, thus affording protection once again. The sequence is completed by the much-gullied Warren Hill Sands at the top of the cliff. Their yellow/gold colour contributes much to the attractive appearance of Hengistbury Head.