Geology Group Diary (08)

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    The Geology group met at 10.30am at Merlin's Bridge Village Hall on Wednesday 13 April 2016.

    Here are the notes for the topic 'GEOLOGY OF SHROPSHIRE'

    The oldest rocks in Shropshire are of late Precambrian age. The Uriconian volcanics (566 Ma) form a line of hills extending from the Wrekin south westwards to Caer Caradoc.  These volcanic andesitic lavas were formed in an island arc complex (similar to the Caribbean islands today) above a subduction zone where oceanic crust was forced down at a plate margin. Also rhyolitic ash flows were produced by explosive eruptions from volcanoes lying along the line of the present Church Stretton Fault.

    Adjacent to the Uriconian volcanics are the Precambrian Longmyndian sediments comprising up to 8000 metres of sandstones and shales forming a deep syncline with an inverted western limb. The higher parts of the sequence contain clasts of Uriconian lavas so these sediments must have been deposited in the shallow seas surrounding the volcanic islands. There is a major unconformity between the eastern (older) and western (younger) Longmyndian sediments followed by a period of intense folding and faulting during the late Precambrian. A good locality on the eastern side of the Longmynd is the Carding Mill Valley near Church Stretton where you can examine the steeply dipping mudstones and sandstones that contain various sedimentary structures including ripple bedding, graded bedding and also bands of volcanic ash. These were deposited under shallow marine conditions where volcanic ash would be erupted from time to time during the later phases of Uriconian vulcanicity. Some of the shales contain fossilised rain spots which were probably preserved as the mud flats dried out along the sea shore.

    The Ercall is a small hill on the north east side of the Wrekin. It is significant in that the unconformable junction between the Precambrian igneous rocks and the basal Cambrian sediments is well exposed in the old quarries.  About 560 Ma the Uriconian lavas of the Ercall were intruded by a large mass of granophyre; this is a type of pink granite formed mainly of quartz and orthoclase feldspar. However, banked up against the granophyre are the basal Cambrian sediments known as the Wrekin Quartzite. Since the Cambrian period began around 545 Ma the unconformity represents a time gap of at least some 15 million years. Quartzite is the term used for a metamorphosed sandstone but here the Wrekin quartzite is simply a quartz rich sandstone with a conglomerate at its base. It shows extensive ripple bedding indicative of shallow marine conditions and many of the pebbles in the basal conglomerate are of locally derived rhyolite, tuff and granophyre. The Cambrian shoreline must have surrounded the Precambrian volcanic island arc.

    The Shelve inlier provides a useful sample of Ordovician rocks (495 – 443 Ma) in Shropshire. The inlier lies immediately west of the Longmynd and the Pontesford- Linley Fault. The oldest formation is the Stiperstones Quartzite that dips steeply westwards and forms a spectacular ridge of frost shattered tors developed under periglacial conditions at the end of Pleistocene times. The quartzite is a shallow water deposit that is an exceptionally pure white sandstone with a conglomeritic base. The succeeding Mytton Flags are over 900 metres thick; they are hard, blue-grey flaggy siltstones that display conspicuous rectilinear jointing. These joints formed as the sediments dried out. Geologists consider that during late Devonian times a deep seated granite pluton was the source of hydrothermal fluids that were injected into the joints of the Mytton Flags precipitating minerals such as galena (PbS), sphalerite (ZnS) and barytes (BaSO4). Snailbeach mine was a major producer of lead ore from1845 to 1913.

    Wenlock Edge is formed of a sequence of limestone and shale deposited in mid Silurian times (430-411 Ma). This is an area of classic scarp and vale topography where resistant limestone alternates with softer shale. You will see on the cross section that the Wenlock Limestone forms a scarp overlooking Ape Dale (Wenlock Shale) and the Aymestry Limestone forms a scarp overlooking Hope Dale (Lower Ludlow Shale). The limestone is  full of coral reefs that were formed  in warm shallow seas around 25°S. The bedded limestone wraps round the reefs which are mainly built of corals, sponges and bryozoans, but there are also numerous brachiopods, trilobites and crinoids in the limestone. Wenlock limestone has been exploited as a flux for iron smelting since Abraham Darby first developed coke fired blast furnaces in Coalbrookdale near Ironbridge in the early 18th C. Much later  iron smelting became established in the Dudley area (40 km east of Wenlock Edge) where  the underground mining of  limestone took place. Victorian fossil collectors obtained some beautiful specimens from the Dudley caverns including the famous ‘Dudley bug’, a trilobite with the scientific name of Calymene blumenbachi.

    The Ironbridge Gorge on the River Severn is an example of a glacial overflow channel. At the end of the Pleistocene Ice Age some 15,000 years ago a proglacial lake known as Lake Lapworth occupied much of the lowland north of Telford. The lake was impounded between the retreating ice front to the NW and higher ground to the SE. The preglacial Severn drained northwards into the Dee estuary but as Lake Lapworth began to overflow, it cut two outlets, one at Newport and the other at Ironbridge.  Eventually, the former outlet was abandoned as the Ironbridge gorge was deepened and the Severn drainage was diverted permanently to the Bristol Channel. As the lake was drained several metres of lacrustrine silts and clays were laid down such as those exposed just to the north of Wellington and fluvio glacial sands and gravels were deposited near the entrance to the Ironbridge gorge.

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