Geology Group diary (13)

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    We gathered at Merlin's Bridge Village Hall at 10.30am on Wednesday 12 October 2016. The topic this month was:
    The geology of East Yorkshire consists mainly of Jurassic and Cretaceous strata. The major structural feature of the region is the Cleveland anticline that runs roughly W-E through the Cleveland Hills to Robin Hood’s Bay. This was uplifted in Miocene times (like the Wealden anticline). To the south of the axis the strata youngs and dips gently towards the Vale of Pickering.
    Lower Jurassic (205-180 Ma) Liassic sediments are well exposed along the coast between Staithes and Robin Hood’s Bay.  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!
    Middle Jurassic (180-159 Ma) sandstones and shales form the North York Moors to the south of the River Esk. Here the poor acidic soils support only heather and rough grassland. Contrast this with the richer farmland of the Cotswolds that are formed of oolitic limestones of the same Middle Jurassic age. Why is this?  Jurassic sedimentation varied considerably across England and was largely deltaic in Yorkshire as opposed to the shallow marine conditions that existed in the south of England. This was because the underlying basement exhibited a basin and swell topography where areas of subsidence accumulated thick deposits of Jurassic strata whilst areas of uplift were covered with much thinner deposits. The three main swells are Mendip, Moreton and Market Weighton; note that above the latter the Jurassic cover is almost completely removed. The Middle Jurassic of Yorkshire begins with a distinctive marine unit known as the Dogger Formation which contains conglomerates, sandstones and ironstones and is often characterised by the presence of large concretions or ‘doggers’. However, most of the Middle Jurassic is made up of deltaic sands and mudstones in which are preserved abundant ferns, conifers, cycads and horsetails (Equisetites). These plants must have been growing on the delta flats and alongside distributary channels in sub tropical climatic conditions. Roseberry Topping, an isolated hill near Great Ayton, is a good example of an outlier capped by deltaic sandstones in which occurs the earliest known plant bed in the Middle Jurassic. The famous Cleveland Dyke runs close to Roseberry Topping in a direction WNW-ESE for nearly 50 kms across the North York Moors. The dyke is formed of basaltic andesite and contains phenocrysts of plagioclase feldspar. It was intruded in Tertiary times from the igneous complex on the Isle of Mull. The igneous rock makes an excellent roadstone and so the dyke has been quarried in several localities around Great Ayton where it forms a deep cutting.
    Upper Jurassic (159-150 Ma) rocks form the high plateau of the Tabular Hills on the southern margin of the North York Moors. Since the strata is dipping south there is a well defined escarpment along the north side of the Tabular Hills which are capped by Corallian oolitic limestones and calcareous sandstones. The scenery here is similar to the Cotswolds with rich brown soils and dry stone walls. The undulating plateau is dissected by streams flowing down the dip slope to join the River Derwent in the Vale of Pickering which is underlain by Kimmeridge Clay. The impressive ruins of the Cistertian abbey at Rievaulx stand in an isolated valley on the Corallian dip slope near Helmsley.
    Glacial Drainage. During the late Pleistocene the North York Moors were surrounded by ice  from the North Sea and the Vale of York. The eastern end of the River Esk was blocked by ice and a proglacial lake was formed in the Esk valley. Eventually the water level built up to the point where it overflowed southwards across a col at Newtondale cutting a deep meltwater channel. The water poured into the Vale of Pickering where a proglacial lake had formed due to ice blocking the River Derwent’s outlet to the North Sea. Overflow from the lake spilled out through the Kirkham Abbey gorge, thus diverting the river south to the Humber; a classic example of glacial drainage diversion.

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