Geology Group Diary (15)

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    The Geology Group meeting took place at Merlin's Bridge Village Hall at 10.30am on Wednesday 14 December 2016. This month's topic was:
    These two counties in North East England extend from the River Tees in the south to the River Tweed on the Scottish border. The Tees, Wear and Tyne drain the western flanks of the North Pennines and the R.Coquet flows off the Cheviots in Northumberland.
    Some of the oldest rocks in the region are the igneous rocks of the Cheviots that are of Lower Devonian age (417-391 Ma). The Cheviots are formed of andesitic lavas, agglomerates and ashes that represent the eroded remnants of a massive Devonian volcano. These volcanic rocks were intruded by a granite pluton (c.380 Ma) which occupies the central summit of the Cheviot hills. The most northerly stage of the Pennine Way crosses the Cheviots before descending to Kirk Yetholm.
    Much of NE England is formed of rocks of Carboniferous age. During Lower Carboniferous times (363-325 Ma) over 1600 metres of Yoredale strata were laid down in the Northumberland basin. (By contrast only 300 metres of limestone was deposited on the Alston Block). The Yoredale sequence contains numerous cyclothems or cycles of sedimentation. Each one starts with a limestone formed under shallow marine conditions, this is followed by siltstones which then passes into cross stratified sandstones laid down in a deltaic environment. Finally, the swampy forested delta top will give rise to the development of thin coal seams. The Scremerston Coal Group within the Yoredale sequence in Northumberland has one 2 metre thick coal seam that has been worked extensively. Thus during the Lower Carboniferous whilst limestones were being deposited over the Pennine blocks, large deltas were spreading across the Northumberland basin to deposit shales, sandstones and coal seams. However, in the Alston Block (North Pennines) the Great Limestone contains a coral band known as the Frosterley Marble. This is a coral limestone that can be polished to look like marble and it is used in the nave of Durham Cathedral as decorative supporting columns.
    By Namurian times (325-315 Ma) the deltas had become more widespread and extended farther south as increasing quantities of sediment were brought down by rivers from the northern land mass. Thick coarse grained cross bedded sandstones known as Millstone Grit were deposited in cyclothems that contained thin coal seams. By Westphalian times (315-300 Ma) cyclic deltaic sediments included beds of shale and sandstone topped by thick coal seams. The Northumberland Durham coalfield supplied ‘sea coal’ to London in Shakespeare’s day; the coal was exported from Tyneside and other coastal ports. By the 19th century the iron and steel, engineering and shipbuilding industries were powered by coal that was mined in the exposed coalfield which extended eastwards beneath the North Sea and formed the concealed coalfield that lay beneath younger Permian strata.
    In late Carboniferous times sheets of molten magma were injected into the rocks of North East England. One of the most famous of these intrusions is the Whin Sill, a 30 metre thick dolerite sill that covers an area of over 500 km2 yet it is exposed at the surface only as a narrow outcrop from the Farne Islands, through Northumbria to Cross Fell on the Pennine escarpment and in Teesdale (High Force waterfall). Hadrian’s Wall follows part of the outcrop of the sill which provides an excellent defensive feature.
    The Permian rocks (290-245 Ma) in south east Durham are represented by dune bedded sandstones reflecting the desert conditions that existed at the time. These cross stratified sandstones rest unconformably on the Coal Measures. However, the Zechstein Sea gradually transgressed across the arid Permian landscape first depositing the Marl Slate, which is a bituminous shale containing fish remains. Next  the Magnesian limestone was laid down and this now forms a well defined escarpment stretching from Sunderland to Teesdale. This limestone is formed of the mineral dolomite or calcium magnesium carbonate. It contains many marine fossils such as brachiopods, gastropods and bivalves. A concretionary form of the Magnesian limestone known as the ‘cannonball limestone’ is well developed in the Roker area of Sunderland. As evaporation took place within the Zechstein Sea, a sequence of repeated cycles of dessication occurred in which salts were precipitated in order of increasing solubility. Starting with the least soluble calcium carbonate CaCO3,then gypsum CaSO4, halite NaCl, magnesium and potassium salts (most soluble). The fluctuating level of the Zechstein Sea would account for the repeated cycles of dessication.

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