Geology Group Diary (14)

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    The Geology Group met at Merlin's Bridge Village Hall at 10.30am on 9 November 2016. A total of 18 members attended. The topic was:
    The Thames Basin is a shallow syncline consisting of Palaeogene (Lower Tertiary, 65-24 Ma) sediments resting on Cretaceous rocks. The basin is bordered by chalk scarps; the chalk of the Chilterns dips south under the Thames valley, whilst the chalk of the North Downs dips north. Most of London stands on London Clay which forms an impervious cap above the Chalk aquifer. This in turn is sealed below by an impervious layer of Gault Clay. Thus rain falling on the chalk hills will slowly percolate through the strata creating a subterranean reservoir within the chalk below London. Artesian boreholes sank in the 19th C were under sufficient hydraulic pressure to supply water to the fountains in Trafalgar Square without the need for pumping. The reason why London has the most extensive underground rail system in the world is because it was relatively easy to excavate tunnels in the soft clay using pick and shovel methods in Victorian times.
    The Chalk is Upper Cretaceous (99–71 Ma) in age. It is 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 during the deposition of the chalk.
    The chalk escarpments and dip slopes of both the North Downs and the Chilterns provide examples of typical chalk scenery. Spring lines occur at the base of the chalk scarps where the underlying Gault Clay causes the water to emerge through springs eg. Silent Pool, Shere, Surrey. Dry valleys are also common where the water table has been lowered since the valleys were first eroded. Clay with flints form a cover on much of the chalk surface. It is thought that it is derived from the weathering of the overlying Reading beds.  Localities where you can study chalk features include Box Hill, nr Dorking; Newlands Corner-Clandon nr Guildford and Beacon Hill (Rowant Aston Nature Reserve), Bucks.
    The sediments within the London basin were deposited during the Palaeocene and Eocene periods (65-33 Ma) when the London and Hampshire basins were subject to alternate marine transgression and regression. Initially the sea advanced from the east over the eroded chalk surface and laid down the Thanet Sands which are best exposed along the North Kent coast at Herne Bay.  Later, as the sea regressed the Reading Beds were deposited by meandering rivers flowing eastwards across mudflats . These continental type beds consist mainly of sands and clays but locally where silica cementation occurs, the sands form sarcen stones. These massive stones are often found on the chalk surface after the unconsolidated Reading beds have been removed by erosion. Sarcens can be seen on the Marlborough Downs on the western margin of the London Basin and they were used to build Stonehenge and other prehistoric monuments. Cemented pebble beds also occur such as the Hertfordshire Puddingstone; a superb example of a natural concrete! The succeeding cycle of sedimentation begins with a marine transgression that produced the London Clay up to 150 metres in thickness.  There are beds of calcareous concretions including septarian nodules at various levels within the London Clay. Numerous bivalves, brachiopods and gastropods provide evidence of the shallow marine environment. Sharks teeth are commonly found as fossils. .However, over 500 plant species have been recorded including mangroves, palms, laurel and magnolia which suggests a tropical climate existed in Eocene times. The plants and seeds were probably brought down by rivers and washed out to sea to be preserved in the silty marine sediments. A return to continental type conditions occurred as the sea shallowed and regressed and the Bagshot Sands were deposited in an estuarine environment. These beds are well developed in the western part of the London Basin giving rise to heathlands much used by the military around Aldershot and Bagshot.
    Around 400,000 years BP during the Anglian glaciation the proto-Thames flowed along the Chiltern plateau and the present Vale of St Albans towards the Essex coast. As the ice sheet moved slowly southwards the St Albans channel was blocked and the drainage was diverted south through the Finchley depression. This outlet was later blocked by the ice front and the Thames established its modern route through London towards Southend on Sea. At this time the Thames meandered across a wide flood plain some 50 metres above present sea level, but as sea level dropped (or alternatively uplift of the land occurred) the river cut down to a lower base level leaving the former flood plain as the Boyn Hill Terrace. By 200,000 years BP base level had dropped again causing the Thames to cut a new deeper channel leaving its former flood plain as the Taplow Terrace. Finally renewed downcutting  produced the Floodplain Terrace which stands  above the present flood plain. The river terraces are economically important for sand and gravel excavation, reservoirs, water parks (eg.Lea valley) and market gardening on the brickearths particularly in Victorian times.

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