Geology Group Diary (32)

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    The Geology Group met at 10.30am on Wednesday 13 September 2018 at Merlin's Bridge Village Hall. The topic for the meeting was:
    By the beginning of the Namurian (325-315 Ma) much of Northern England had became a region of swampy delta plains and shallow lakes, with large rivers draining south off the northern ORS continent. The muds, sands  and coal seams were laid down in rhythmic cycles of sedimentation (cyclothems) as sea level fluctuated throughout the Upper Carboniferous. The Millstone Grit so characteristic of the Pennines, represents major deltaic development during the Namurian. And by Westphalian times (315-300 Ma) delta top swamp forests flourished intersected by numerous distributary rivers. The typical Coal Measure cyclothem of shale, sandstone and coal seam represents the changing environmental conditions ranging from the marine submergence of the delta plain, through the development of thick deltaic muds and sands to the growth of swamp forest on the delta top.
    South of St Georges Land, the Rheic Ocean was closing with the resultant formation of the Variscan Mountain ranges that stretched from the Appalachians through the Pyrenees and Atlas Mts to the Urals in Russia. This Variscan Front can be seen in SW England where the Culm Measures (sandstones, mudstones and thin coals) were deposited in a subsiding basin (Devon syncline). Variscan folding and thrust faulting at the end of the Carboniferous can be seen particularly well in the South Wales and Pembrokeshire coalfields. The coastal sections at Broad Haven and Saundersfoot show intense thrusting and folding within the Coal Measures and the chevron folds at Millook Haven in North Devon are classic Variscan features. The intrusion of the granite batholith in Cornwall and South Devon (Dartmoor, Bodmin Moor,etc) is associated with the Variscan earth movement as pressure built up from the south.
    In late Carboniferous times sheets of molten magma were injected into the rocks of the northern Pennines. 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.
    In the Midland Valley of Scotland there was considerable vulcanicity throughout Carboniferous times. The Clyde Plateau basalts are up to a 1000 metres thick and Arthur’s seat in Edinburgh represents an extinct Carboniferous volcano. Salisbury Crags is a massive sill where James Hutton proposed his ideas on the origin of igneous rocks in the late 18th C. 
    THE PENNINES  In the Peak District and  the Yorkshire Pennines the Millstone Grit forms dramatic escarpments where it has been removed by erosion from above the underlying shales and limestone as for example on the Roaches, on Stannage Edge, on Kinderscout or on Ilkley Moor. The term ‘grit’ is commonly used but it is actually a coarse sandstone that was employed extensively in the 19th C to build the ‘dark satanic mills’ and the back to back terraced houses of industrial Yorkshire. The dark exterior colour of Millstone Grit is largely due to industrial pollution since in fresh specimens the rock is a light colour due to the abundance of quartz. The gritstone bands alternate with shales and thin coal seams in a series of cyclothems. Note that cross stratification is often seen in the gritstones, evidence of their deltaic origin.
    The Millstone Grit passes conformably into the Coal Measures which outcrop on the flanks of the Pennines. On the Yorkshire coalfield the strata dips gently eastwards so that the exposed coal measures are eventually concealed beneath a cover of younger Permo-Triassic rocks. Here the cyclothems are fully developed with some seams reaching up to 3 metres in thickness. During Victorian times the exposed field fuelled the industrial revolution, but as these seams were worked out deeper pits were sunk down to 800 metres in the concealed field. Kellingley Colliery in North Yorkshire was a deep mechanised pit developed in the 1960s to feed the Ferrybridge Power station, but the decline of coal fired power stations and the uneconomic cost of deep mining, resulted in the mine was closure in 2015…it was the last deep coalmine in the UK.

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