Geology Group Diary (05)

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    The Geology Group met at 10.30 am on Wednesday 13 January at Merlin's Bridge Community Centre.
    Here are the notes for the session on the topic:
    The Vale of Severn is for the most part formed of Lias Clay (Lower Jurassic 205-180 Ma) and one of the best places to examine these rocks is at Hock Cliff near Fretherne on the shores of the River Severn. Here bands of argillaceous (clayey) limestone reinforce the thin muddy shales. These strata were deposited in a shallow muddy sea in which belemnites, ammonites and sea lilies (crinoids) were flourishing, but the most famous fossil found at Hock Cliff is Gryphaea arcuata, an extinct type of oyster , also known as the ‘Devil’s toenail’. The Lias Clay was used extensively for brick making in the 19thC and many small  brick works’ quarries have yielded a wide variety of fossils. Blockley Quarry near Moreton in Marsh is a well known fossil collecting site. Ammonites such as Liparoceras cheltiense, and bivalves
    The low undulating clay vale is in places punctuated 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 (180 – 169 Ma). These limestones are formed of tiny ooliths around which calcium carbonate has been deposited by currents in shallow shelf seas (similar to the Bahamas shelf today). 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. Much of Regency Cheltenham was built of stone from the Inferior Oolite. The Devil’s Chimney 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 (169 – 159 Ma) 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.

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