Geology Group diary (04)

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    The Geology Group met on Wednesday 9 December. These are the notes for the topic 'Geology in the Weald of Kent & Sussex'.

    This region of South East England is formed of Cretaceous strata, although the Purbeck beds underlie the surface rocks in the central Weald. Structurally, the Wealden uplift is due to the Alpine earth movements that produced an anticline which dips gently away from its east-west axis, although the fold also plunges to the west.

    The High Weald is formed of the Hastings Beds (145 – 137 Ma) which are the oldest Cretaceous rocks in the centre of the anticline. The rocks are mainly sandstones that show cross stratification, ripple marks and plant remains. These were deposited in deltas within the shallow waters of the Wealden Lake. On the shores of the lake grew conifers, cycads and giant horsetails on which dinosaurs such as Iguanadon were feeding. Also there are clay bands containing ironstone nodules, the source of ore for the Wealden iron industry which developed in the 16th C to provide cannon for Tudor ships that were build of oak from the Wealden forests. The area was extensively forested until the demand for charcoal for smelting and timber for ships led to the removal of much of the woodland. Hammer ponds were used to power water wheels that operated bellows and hammers for the iron works. The High Weald was the centre of the iron industry in Britain until the beginning of the industrial revolution when Abraham Darby developed the coal fired blast furnace in 1709 in Coalbrookdale. There were also numerous small quarries that provided local building stone. Bateman’s House, formerly Rudyard Kipling’s home, is a good example of the use of local Ashdown sandstone. The High Weald forms a major watershed separating north and south flowing rivers including the Medway and the Wey, the Arun, the Ouse and the Cuckmere. River capture commonly occurs where some rivers cut back by headward erosion and divert the headstreams of others, thus increasing their drainage system.
    The Low Weald is formed of Weald Clay (137 – 130 Ma) and forms a horseshoe shaped outcrop around the High Weald. The clay vales are poorly drained (impermeable clay) and mainly provide pastureland. Brick making was based on the Weald Clay, particularly during the 19thC bricks were in demand for the London market.

    The Lower Greensand (130 – 125 Ma) lies above the Weald Clay and it has been worn back by erosion to form prominent scarps overlooking the clay vales. Leith Hill on the northern escarpment forms the highest point in the Weald. The sandy acidic soils generally support heathland and coniferous woodland. A spring line marks the base of the greensand where water emerges along the junction with the clay. Note that the greensand is commonly red in colour but the name came from green sands containing glauconite ( an iron mineral) that outcrop on the Dorset coast. The Lower Greensand was deposited under shallow marine conditions as the Wealden Lake was invaded by the sea. Later as the sea deepened the Gault Clay (125 – 100 Ma) was laid down. The clay is one of the most fossiliferous horizons in Britain containing a rich marine fauna of ammonites, bivalves and gastropods. It forms a narrow vale at the foot of the chalk escarpment. At Folkestone Warren rotational landslipping occurs in winter when the overlying chalk is saturated and slides over the impermeable Gault Clay. In 1915 a passenger train was derailed on the coastal railway line which was buckled by a landslide.

    The Chalk (100 – 65 Ma) encloses the Weald on three sides forming inward facing escarpments along the North and South Downs. The present river system was initiated on the chalk cover which has since been eroded over the Weald. The rivers have cut gaps through the chalk scarp; for example, the Wey gap at Guildford and the Ouse gap at Lewes. Note that where the dip of the chalk is steep as on the Hog’s Back, the outcrop is narrow. Where the dip is gentle or horizontal, the outcrop forms extensive undulating downland. Dry valleys are common where the water table has been lowered. The Seven Sisters on the Sussex coast are dry valleys truncated by the sea. A spring line occurs at the base of the scarp along the junction between the chalk and the underlying Gault clay. Anglo Saxon settlements developed along the spring line. Villages with suffixes such as ‘ham, ton and ing’ date back to this period.
    Geologically, the chalk is a fine grained white limestone formed from calcareous mud containing microscopic coccoliths derived from marine plankton. The Cenomanian transgression in late Cretaceous times covered much of southern England and since there was little sand and mud brought down by rivers, the sea remained relatively clear and the chalk sediment was free from impurities. Fossils are fairly common in the Lower Chalk including ammonites, belemnites, echinoids, bivalves and brachiopods. Micraster, the heart urchin is common as are brachiopods such as terebratulids and rhynchonellids. The Upper Chalk is characterised by the presence of flint nodules which may have been precipitated from silica rich ground waters percolating through the chalk. However, recent research suggests that the flint was formed by the sub surface breakdown of siliceous organisms such as sponges, radiolaria and diatoms during the deposition of the chalk.

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