Geology Group Diary (34)

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    The Geology Group met at Merlin's Bridge village hall at 10.30am on Wednesday 12 September 2018. The topic for this month was BRITISH JURASSIC GEOLOGY.

    A shallow sea on the western margin of the Tethys Ocean spread across Britain during the Jurassic Period (201-145 Ma) when the area lay between 30° and 40° N. A succession of alternating marine clays and limestones were laid down but due to several axes of uplift the Jurassic strata is thinner over the swells and much thicker in the intervening basins. The three most prominent uplifted areas of pre Jurassic rocks are the Mendip, Moreton and Market Weighton axes. Outcrops of Jurassic rocks extend in a diagonal belt from the Dorset coast up through the Cotswolds to the North Yorkshire Moors. In the early 19th C William Smith, a canal engineer, was the first to work out the Jurassic sequence of strata using ammonites as zone fossils. In 1815 he produced the first geological map of England and Wales.
    LOWER JURASSIC (201-174 Ma)
    The Lias clays are exposed in the cliffs around Lyme Regis and Charmouth in west Dorset. During the 19thC  quarrymen used the term Lias  (a corruption of the word layers) to describe the sequences of alternating thin limestones and bluish grey clays that reach a thickness of over 130 metres. They represent the sediments deposited on the margins of the Tethys Ocean which spread over much of the Permo-Triassic landscape of Britain. The Lias sea contained a rich fauna of ammonites and marine reptiles such as Ichthyosaurs and Plesiosaurs made famous by Mary Anning, the Victorian fossil collector who lived in Lyme Regis. Dark bituminous shales also occur in the Lias which suggest that the sea deepened from time to time. The shales contain iron pyrites, a mineral formed where there is a lack of oxygen in deep waters. The Bridport Sands occur at the top of the Lias sequence and mark a change in environmental conditions as sandy sediment was washed into the area.
    Liassic sediments also outcrop along the coast between Staithes and Robin Hood’s Bay in East Yorkshire.  They underlie the Cleveland Hills which form the northern part of the region bordered by Teesside to the north and the River Esk to the south. The Cleveland Ironstone Formation (Middle Lias) can be seen to the east of Staithes where it was worked in the 19th C for use in the iron and steel industry on Teesside. There are 4 main seams of bedded ironstone exposed on the coast and these can be traced inland where they thicken and outcrop in the escarpment around Guisborough. The harbour at Port Mulgrave was opened in 1856 to handle the output of the coastal mines. The iron ore occurs as an oolitic mudstone rich in siderite and chamosite and was deposited under shallow marine conditions; evidence includes a prolific shell fauna, ripple marks, trace fossils and cross stratification.
    The Upper Lias contains the famous Jet Rock which is a bituminous shale horizon containing discrete  masses of the carbonaceous mineral jet. This appears to have been formed from drifted logs of wood which sank into the anaerobic muds on the sea floor and then became compressed under the weight of overlying sediments. The Whitby jet derives from Jurassic plants similar to the Chilean pine or Monkey Puzzle tree. Jet was worked by the Romans and was also popular in Victorian times particularly for mourning jewellery.
    The Alum Shales occur at the top of the Upper Lias. These pyrite (FeS2) rich shales formed the raw material for the alum industry that began in the 17thC and continued up until the 1870s. The alum was used in tanning, dyeing and in medicine. The shales were quarried mostly on the cliff tops where the treated waste could be easily dumped on to the shore below. Calcination required alternate layers of brushwood and shale to be slowly heated over a period of a year or more. During calciation sulphuric acid was produced by the oxidation of the pyrite and it reacted with the aluminium silicates in the shale to form aluminium sulphate, The burnt shale was then steeped in water in tanks to extract the aluminium and iron sulphates from the shale. The resulting liquid was then concentrated by repeated boiling, evaporation and crystallisation of the salts. Whilst the iron salts remained in solution the  alum salts ( hydrated  sulphate of potassium and aluminium) would crystallise out. The exact time to cease heating was determined by floating an egg on the liquid!
    The Lias of North Yorkshire represents repeated cycles of marine sedimentation, each cycle containing clays, shales, sandstones and ironstones in upward succession. As the cycle coarsens upwards, the water becomes shallower and oxygenation increases giving rise to a profusion of marine creatures including ammonites, belemnites, bivalves and marine reptiles. Robin Hood’s Bay is a wonderful fossil hunting locality. Dactylioceras and Hildoceras are two well known ammonites from the Upper Lias, the latter is named after St Hilda who founded Whitby Abbey and is reputed to have turned the local sea serpents into stone which are now found as ammonites!
    MID JURASSIC (174-163 Ma)
    The Cotswold Hills overlook the low undulating Vale of Severn that is punctuated in places  by small hills known as outliers, the most famous of which is Bredon Hill. Outliers are small areas of newer rock surrounded by older strata; for example, Bredon Hill is capped by Inferior Oolite but surrounded by Lias Clay. An outlier is really an isolated outcrop left behind as the main escarpment gradually retreats under the influence of weathering and erosion. A closer look at Bredon Hill reveals that it has a steep scarp to the north and the strata dip south with a strong fault along its southern margin. Notice the stepped profile, well seen on the eastern side of the hill. This bench feature is produced by the resistant marlstone, a ferruginous sandstone at the top of the middle Lias.
    The Cotswold escarpment is formed mostly of limestones of the  Inferior Oolite Group that are formed of tiny ooliths consisting of concentric layers of calcium carbonate that has been deposited by circulating currents in shallow shelf seas (similar to the Bahamas shelf today). The colour of the limestone varies from pale buff to orange brown depending on the concentration of iron minerals in the cement. Leckhampton Hill near Cheltenham shows a section through the Inferior Oolite. The main limestone is well bedded and jointed and so makes an excellent building stone since it can be cut easily into blocks; hence it is referred to as a freestone. Much of Regency Cheltenham was built of stone from the Inferior Oolite. The Devil’s Chimney at Leckhampton was left behind by the quarrymen in the 1780s to form a local land mark. Crickley Hill provides an example of a promontory on the Cotswold scarp which it is cut by a deep re-entrant valley followed by the A417 Gloucester road. The cliffs along the face of Crickley hill are formed of the Pea Grit; this is a pisolitic limestone where the grains are much larger than the ooliths in the freestones. Clypeus (echinoid), Trigonia (oyster) and rhynconellids (brachiopods) are common fossils. Highly developed ammonites with intricate suture patterns are also found in the Inferior Oolite.
    The Great Oolite Group  overlies the Inferior Oolite and is found mainly on the dip slope of the Cotswolds. Near the base of the Great Oolite is a famous bed called the Stonesfield Slate. This is not a slate in the geological sense but a sandy limestone that is thinly bedded and so forms flagstones 2-3 cm thick. These were extensively quarried for roofing tiles in the 18C & 19C and can be seen on old cottages and farm buildings throughout the Cotswolds today. However, as long ago as 2000 BC in the Neolithic period  the ‘slates’ were used to build the entrance to the Bellas Knap long barrow! But the Stonesfield Slate was made famous by William Buckland, the great Oxford geologist who first identified the remains of the carnivorous dinosaur  Megalosaurus bucklandii in 1827. There are also leaf impressions of cycad leaves indicating that a rich vegetation must have bordered the coastal lagoons in which the sandy limestones were deposited.
    At the top of the Great Oolite is the White Limestone Formation which can be seen in the old cement works quarry at Kirtlington, a few miles west of Oxford. Here the limestone is hard, white and shelly. The most abundant brachiopod is Epithyris oxonica which makes up much of the rock. However the quarry has also yielded the bones of pterosaurs (flying reptiles), plesiosaurs (marine reptiles) and early mammals.
    The Cotswolds provide a good example of scarpland topography where relatively resistant limestones alternate with softer clays and shales. The clay vale, the scarp and the dip slope are the three essential features of this type of topography. Note that the strata in the Cotswolds dip gently south east and young in the direction of dip. Since the limestone is permeable, water percolates down to the level of the underlying impermeable clay thus producing springs at the junction of the two rocks. Many villages developed in Anglo Saxon times along the spring line at the foot of the Cotswold scarp. Dry valleys are also common in the limestone since the water table has been gradually lowered as the scarp has receded
    UPPER JURASSIC (163-145 Ma)
    The Dorset coast between Weymouth to Swanage provides some excellent exposures of Upper Jurassic rocks. The Oxford Clay outcrops in the core of the Weymouth anticline but it extends in a broad swathe  north west through Oxford to Lincolnshire and beyond. It has yielded aquatic reptilian remains, and a variety of fossil ammonites, belemnites and brachiopods.  Economically it is important as a brick making clay which is high in carbon content thus reducing firing costs. Fletton bricks from Peterborough were used in building much of 19th C London.
    On the coast around St Alban’s Head the Kimmeridge Clay appears below the Portland Stone cliffs. Kimmeridge Bay is the type locality for this formation, consisting mainly of black shales and clays with bands of limestone that form the Kimmeridge ledges. Some of the shales are bituminous and combustible; in fact, the Kimmeridge beds are an important source rock for petroleum under the North Sea. Wytch Farm on the Isle of Purbeck is the largest onshore oil field in Britain
    The Isle of Portland forms the southern flank of the Weymouth anticline and it is highest in the north and dips gently down to sea level at Portland Bill in the south. Although the Portland Stone extends across most of the area 75% of it is covered by the lower Purbeck Beds (quarry overburden). The Portland Stone is a fine grained white limestone that is well jointed and easily dressed into rectangular blocks. It became popular as a building stone in the 17th C when St Paul’s Cathedral was rebuilt after the Great Fire of London. Most of the quarries are on or near the coast so that the stone could be exported by sea. A fossiliferous limestone known as the Roach occurs at the top of the Portland Stone and contains ‘Portland screws and osses ‘eds ‘ otherwise known as Aptylexia (gastropod) and Laevitrigonia (bivalve). Large ammonites such as Titanites have been extracted from the Portland beds.
    The coast from Durdle Door to Lulworth Cove and Mupe Bay is formed of a wall of Upper Jurassic strata that create a barrier protecting the softer Wealden Beds (Lower Cretaceous) from the continuous erosion of the sea which has already broken through in several places. Stair Hole adjacent to Lulworth Cove provides a good example of the way that the sea is excavating the soft Wealden beds behind the Portland/Purbeck wall. The famous Lulworth crumple in the Purbeck beds shows the effect of the Alpine earth movements during the Oligocene. Lulworth Cove itself has been cut back through the sands and clays of the Wealden beds to the high cliffs of the massive Chalk ridge that extends eastwards into the Purbeck hills. Note the exposure of Upper Greensand (at the base of the chalk) near where the road reaches the cove and here the greensand really is greenish in colour due to the presence of the iron mineral glauconite. On the east side of Lulworth Cove is a fossil forest that consists of silicified boles which mark the base of conifers that grew during Purbeck times.
    On the Isle of Purbeck the most important structural feature is the Purbeck monocline produced by the northward thrust of the Alpine orogeny. The chalk ridge of the Purbeck hills can best be seen around Corfe Castle that stands on a knoll between a twin water gap. Here the chalk is almost vertical hence the width of the Purbeck hills is relatively narrow. (Note the rule of thumb is that the steeper the dip of a stratum, the narrower the outcrop). The underlying Wealden, Purbeck and Portland beds all dip steeply north near Corfe but then become horizontal across the Isle of Purbeck forming the southern limb of the monocline. The best place to see the actual curvature of the monocline is in St Oswald’s Bay to the west of Lulworth.
    Swanage was the centre of the Purbeck stone trade in the 18th & 19th C but in Medieval times Purbeck ‘marble’ was much in demand  for cathedral interiors, eg.Westminster Abbey and Salisbury Cathedral. A beautiful example of ‘marble‘columns can be seen in Eldon Memorial Church near Kingston.
    John Downes

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