Editing Geology of the Llanidloes area: Introduction

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This Sheet Explanation provides a summary of the geology of the district covered by Geological 1:50 000 Series Map Sheet 164 (Llanidloes), published in 2010 as a Bedrock and Superficial Deposits edition.  The district mostly lies within the county of Powys, but includes small parts of Ceredigion in the extreme west and south-west.  Much of the western part of the district is occupied by the deeply dissected uplands of the Cambrian Mountains, a designated Area of Outstanding Natural Beauty.  In this area the land rises to 740 m on the flanks of Plynlimon (Pumlumon Fawr), the highest summit in the range.  It falls away towards the eastern part of the district into rolling countryside that includes the important catchment of the River Severn (Afon Hafren) and its tributaries, the largest of which are the rivers Carno, Trannon, Cerist, Clywedog and Dulas.  A major reservoir (Llyn Clywedog) occupies the upper reaches of the Clywedog valley, its purpose being to regulate river discharge and groundwater levels within the catchment.  The south-western part of the district is drained by the River Wye (Afon Gwy) and its tributaries, that flow south-eastwards via Llangurig.  The sources of both the Severn and Wye are situated on the eastern flanks of Plynlimon within the western part of the district.
 
This Sheet Explanation provides a summary of the geology of the district covered by Geological 1:50 000 Series Map Sheet 164 (Llanidloes), published in 2010 as a Bedrock and Superficial Deposits edition.  The district mostly lies within the county of Powys, but includes small parts of Ceredigion in the extreme west and south-west.  Much of the western part of the district is occupied by the deeply dissected uplands of the Cambrian Mountains, a designated Area of Outstanding Natural Beauty.  In this area the land rises to 740 m on the flanks of Plynlimon (Pumlumon Fawr), the highest summit in the range.  It falls away towards the eastern part of the district into rolling countryside that includes the important catchment of the River Severn (Afon Hafren) and its tributaries, the largest of which are the rivers Carno, Trannon, Cerist, Clywedog and Dulas.  A major reservoir (Llyn Clywedog) occupies the upper reaches of the Clywedog valley, its purpose being to regulate river discharge and groundwater levels within the catchment.  The south-western part of the district is drained by the River Wye (Afon Gwy) and its tributaries, that flow south-eastwards via Llangurig.  The sources of both the Severn and Wye are situated on the eastern flanks of Plynlimon within the western part of the district.
  
The town of Llanidloes is the main centre of population, with smaller settlements at Llangurig, Carno, Trefeglwys, Caersws and Staylittle; the Newtown conurbation impinges on the eastern part of the district.  Much of the district is given over to beef and dairy farming, although sheep are reared in the remote upland areas in the west and extensive forestry plantations have been developed in places. The Ordovician and Silurian rocks of the district have been exploited locally, in the past, as a source of building material and, recently, commercial quantities of sandstone aggregate have been excavated at Penstrowed Quarry [SO 0680 9100].  The district includes part of the Central Wales Mining Field from which substantial volumes of lead and zinc ore were extracted during the 19th and early 20th centuries. A number of former mine sites are still visible, notably along the Van, Nant-y-ricket, Dylife, Dyfngwm and Llanerchyraur lodes (Jones, 1922<ref name="Jones 1922">Jones, O T. 1922. Lead and zinc: the mining district of north Cardiganshire and west Montgomeryshire. Memoir of the Geological Survey of Great Britain, Special Report on Mineral Resources. No.&nbsp;20.</ref>; IGS, 1974), and the historic Bryntail Mine, below the Clywedog Dam has been restored as a site of industrial archaeological interest.
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The town of Llanidloes is the main centre of population, with smaller settlements at Llangurig, Carno, Trefeglwys, Caersws and Staylittle; the Newtown conurbation impinges on the eastern part of the district.  Much of the district is given over to beef and dairy farming, although sheep are reared in the remote upland areas in the west and extensive forestry plantations have been developed in places. The Ordovician and Silurian rocks of the district have been exploited locally, in the past, as a source of building material and, recently, commercial quantities of sandstone aggregate have been excavated at Penstrowed Quarry [SO 0680 9100].  The district includes part of the Central Wales Mining Field from which substantial volumes of lead and zinc ore were extracted during the 19th and early 20th centuries. A number of former mine sites are still visible, notably along the Van, Nant-y-ricket, Dylife, Dyfngwm and Llanerchyraur lodes (Jones, 1922<ref name="Jones 1922">Jones, O T. 1922. Lead and zinc: the mining district of north Cardiganshire and west Montgomeryshire. Memoir of the Geological Survey of Great Britain, Special Report on Mineral Resources. No.&nbsp;20.</ref>; <span style="background:#ffff00">IGS, 1974</span>), and the historic Bryntail Mine, below the Clywedog Dam has been restored as a site of industrial archaeological interest.
  
 
The district is underlain by a succession of Late Ordovician (Ashgill) to Silurian sedimentary rocks, over 5&nbsp;km thick, deposited between 450 and 420 million years ago in the Early Palaeozoic Welsh Basin ([[Media:P930911.jpg |'''Figure P930911''']]).  The basin developed on a fragment of the ancient supercontinent of Gondwana, known as Eastern Avalonia (e.g. Pickering et al., 1988<ref name="Pickering 1988">Pickering, K T, Bassett, M G, and Siveter, D J. 1988. Late Ordovician–early Silurian destruction of the Iapetus Ocean: Newfoundland, British Isles and Scandinavia&nbsp;—&nbsp;a discussion. Transactons of the Royal Society of Edinburgh: Earth Sciences, Vol.&nbsp;79, 361–382.</ref>), that drifted northwards to collide with the continents of Baltica and Laurentia during the Late Ordovician and Silurian (Soper and Hutton, 1984<ref name="Soper 1984">Soper, N J, and Hutton, D H W. 1984. Late Caledonian sinistral displacements in Britain: implications for a three-plate collision model. Tectonics, Vol.&nbsp;3, 781–794.</ref>; Soper and Woodcock, 1990<ref name="Soper 1990">Soper, N J, and Woodcock, N H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine, Vol.&nbsp;127, 527–542.</ref>; Woodcock and Strachan, 2000<ref name="Woodcock 2000">Woodcock, N H. 2000. Late Ordovician to Silurian evolution of Eastern Avalonia during convergence with Laurentia. 168–184 in Geological history of Britain and Ireland. Woodcock, N H, and Strachan, R A (editors). (Oxford: Blackwell Science.)</ref>).  To the east and the south of the basin lay the Midland Platform, a relatively stable shallow marine shelf that was subject to periodic emergence.  The basinal sediments are predominantly deep marine turbiditic facies that were introduced into the district by density currents from southerly, south-easterly and north-westerly quadrants.  Coeval shallower-water ‘shelfal’ sediments were deposited north and east of the district, and locally impinge on its northern margins.  Thickness variations within the major sedimentary units suggest that, at times, syndepositional fault movements were an important control on their distribution.  During late Silurian (Ludlow) times, shallowing of the basin occurred, and sandstones, variably interpreted as a turbiditic (Cave and Hains, 2001<ref name="Cave 2001">Cave, R, and Hains, B A. 2001. Geology of the country around Montgomery and the Ordovician rocks of the Shelve Inlier. Memoir of the British Geological Survey, Sheet 165 (England and Wales).</ref>) or storm-generated facies (Tyler and Woodcock, 1987<ref name="Tyler 1987">Tyler, J E, and Woodcock, N H. 1987. The Bailey Hill Formation: Ludlow Series turbidites in the Welsh Borderland reinterpreted as distal storm deposits. Geological Journal (Thematic Issue), Vol.&nbsp;22, 73–86.</ref>), were laid down over the eastern part of the district and adjacent areas.  The shallowing was a result of tectonic reconfiguration of the basin, a precursor to the late Caledonian (Acadian) Orogeny that affected the region during the late Early Devonian, around 400 million years ago.
 
The district is underlain by a succession of Late Ordovician (Ashgill) to Silurian sedimentary rocks, over 5&nbsp;km thick, deposited between 450 and 420 million years ago in the Early Palaeozoic Welsh Basin ([[Media:P930911.jpg |'''Figure P930911''']]).  The basin developed on a fragment of the ancient supercontinent of Gondwana, known as Eastern Avalonia (e.g. Pickering et al., 1988<ref name="Pickering 1988">Pickering, K T, Bassett, M G, and Siveter, D J. 1988. Late Ordovician–early Silurian destruction of the Iapetus Ocean: Newfoundland, British Isles and Scandinavia&nbsp;—&nbsp;a discussion. Transactons of the Royal Society of Edinburgh: Earth Sciences, Vol.&nbsp;79, 361–382.</ref>), that drifted northwards to collide with the continents of Baltica and Laurentia during the Late Ordovician and Silurian (Soper and Hutton, 1984<ref name="Soper 1984">Soper, N J, and Hutton, D H W. 1984. Late Caledonian sinistral displacements in Britain: implications for a three-plate collision model. Tectonics, Vol.&nbsp;3, 781–794.</ref>; Soper and Woodcock, 1990<ref name="Soper 1990">Soper, N J, and Woodcock, N H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine, Vol.&nbsp;127, 527–542.</ref>; Woodcock and Strachan, 2000<ref name="Woodcock 2000">Woodcock, N H. 2000. Late Ordovician to Silurian evolution of Eastern Avalonia during convergence with Laurentia. 168–184 in Geological history of Britain and Ireland. Woodcock, N H, and Strachan, R A (editors). (Oxford: Blackwell Science.)</ref>).  To the east and the south of the basin lay the Midland Platform, a relatively stable shallow marine shelf that was subject to periodic emergence.  The basinal sediments are predominantly deep marine turbiditic facies that were introduced into the district by density currents from southerly, south-easterly and north-westerly quadrants.  Coeval shallower-water ‘shelfal’ sediments were deposited north and east of the district, and locally impinge on its northern margins.  Thickness variations within the major sedimentary units suggest that, at times, syndepositional fault movements were an important control on their distribution.  During late Silurian (Ludlow) times, shallowing of the basin occurred, and sandstones, variably interpreted as a turbiditic (Cave and Hains, 2001<ref name="Cave 2001">Cave, R, and Hains, B A. 2001. Geology of the country around Montgomery and the Ordovician rocks of the Shelve Inlier. Memoir of the British Geological Survey, Sheet 165 (England and Wales).</ref>) or storm-generated facies (Tyler and Woodcock, 1987<ref name="Tyler 1987">Tyler, J E, and Woodcock, N H. 1987. The Bailey Hill Formation: Ludlow Series turbidites in the Welsh Borderland reinterpreted as distal storm deposits. Geological Journal (Thematic Issue), Vol.&nbsp;22, 73–86.</ref>), were laid down over the eastern part of the district and adjacent areas.  The shallowing was a result of tectonic reconfiguration of the basin, a precursor to the late Caledonian (Acadian) Orogeny that affected the region during the late Early Devonian, around 400 million years ago.

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