Geology Group Diary (18)

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    Anonymous

    The Geology group met at 10.30 am at Merlin's Bridge Community Centre (Village Hall) on Wednesday 8 March 2017. The monthly topic was:
    GEOLOGY IN THE WYE VALLEY AND FOREST OF DEAN
    The River Wye meanders down through Ross and Monmouth to join the Severn south of Chepstow. One of the most scenic areas of the river is around Symonds Yat about 8 kms NE of Monmouth. Here the river swings round in an exaggerated loop crossing the junction between the Old Red Sandstone and the Carboniferous Limestone several times and paying no attention to the geological structure. During Pleistocene times when sea level was 200 metres higher than today the River Wye meandered over a relatively flat surface but as sea level began to fall it started to cut down to a lower base level forming a gorge with incised meanders cut into the limestone. Symonds Yat is a limestone hill forming the core of a meander loop. Yat Rock is a good example of the Lower Dolomite that outcrops near to the base of the Carboniferous Limestone sequence. Dolomite is a type of limestone that is formed of calcium and magnesium carbonate (CaMg) CO3. One of the most distinctive rocks in this area is the Quartz Conglomerate. This marks the unconformable base of the Upper Devonian strata and since it is a resistant rock it forms a strong scarp feature particularly around Huntsham Hill and Coppet Hill. Due to the numerous joints in the conglomerate many huge blocks have broken off and fallen down to the River Wye.
    Another interesting feature of the River Wye can be seen near to the village of Newland, about 3 kms SE of Monmouth where there is a large abandoned meander. However, the floor of this fossil meander is about 125 metres above the level of the River Wye so the meander neck must have been cut off probably during the late Pleistocene and the main river then continued to incise its valley down to its present level.
    The Tintern Sandstone overlies the Quartz Conglomerate and it forms the main building stone in Tintern Abbey that stands alongside the River Wye.. However, the Lower Wye re enters the Carboniferous Limestone a few kms south of the abbey and there are some spectacular limestone cliffs at Wintour’s Leap, near Chepstow.
    The Forest of Dean occupies a syncline of Carboniferous rocks between the rivers Wye and Severn. The syncline is asymmetrical with the strata dipping more steeply on the east side than on the west; and the basin is elongated along a N-S axis. This tectonic feature was formed by the Variscan earth movements at the end of Carboniferous times. The Forest of Dean Coalfield is surrounded by Carboniferous Limestone (354-327 Ma) the outcrop of which is wide in the west and narrow in the east where the dip is steep. In the disused  Shakemantle Quarry near Ruspidge the well jointed Lower Dolomite is dipping at about 800W where the outcrop is only 500 metres wide.
    One of the most important minerals found in the limestone is iron ore which occurs as haematite Fe2O3 within solution fissures in the limestone. The iron minerals must have accumulated in the oxidising environment of the Permian deserts, then after torrential rains iron rich waters would have descended into the underlying limestones and entered the joints, widened them by solution action and finally precipitated the haematite. Iron ore was mined throughout the Middle Ages particularly in shallow workings in the west but later deep shafts were sunk into the steeply inclined limestone in the east. The local name for iron workings is ‘scowles ’and today many of these are hidden away in the woodlands.  The ore was smelted in small furnaces using charcoal from local trees but towards the end of the 18thC supplies of timber were running out and the iron manufacturers turned to coke made from local coal. There are still hammer ponds in existence which provided water power to operate the bellows and hammers in the forges. Cannop Ponds were formerly a site of iron smelting.
    A strong unconformity separates the top of the Carboniferous Limestone (which is actually the Drybrook Sandstone) from the Upper Coal Measures (325-300 Ma) that form the Forest of Dean Coalfield. The Millstone Grit and the Lower and Middle Coal Measures are missing so that the basal conglomerate of the Trenchard Group lies directly above the unconformity. Approximately 99% of the strata in the coalfield is sandstone and shale but  there are several thin seams worked by drift mining. The Hopewell Mine that worked the Yorkley Seam is now a Mining Museum. However, the main commercial seam is the Coleford High Delf which has an average thickness of 1.3 metres. This seam lies at the base of the Pennant Sandstone Group which contains some massive red sandstones much used for building stone. Pennant Sandstone from Bixstead quarries near Coleford has been used in many public buildings including Shire Hall, Gloucester and the University of Wales, Aberystwyth. Blocks of sandstone are extracted from underground where the quality of the stone is superior to that near the surface. Many of the quarry faces show cross stratification indicating deposition under deltaic conditions. The casts of large tree trunks such as Sigillaria occur in some of the beds; evidence of extensive forest growth on the deltas.
    Another major unconformity separates the  Upper Coal Measures from the Mercia Mudstones ( Keuper Marl) of the Triassic Period (248-205 Ma)  that are exposed on the west bank of the Severn at Sedbury Cliff near Chepstow. These marls were laid down in mudflats and lacustrine areas of inland drainage in the tropics and they contain few fossils although reptilian vertebra are found. However, the foreshore at Sedbury is strewn with fallen blocks from the thin overlying layers of Rhaetic shales and Lower Lias (Jurassic) limestones. The beach is a well known for fossil collecting as the Lias blocks yield numerous ammonites and the bivalve Qstrea liassica. 

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