District Wide Assessment Reduced
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5 DISTRICT WIDE APPRAISAL DISTRICT WIDE APPRAISAL 5.0 DISTRICT WIDE APPRAISAL 5.1 Geology & Physical Geography 5.1.1 The landscape interest and diverse range of character areas across the Mendip district are inextricably linked to the underlying geology and geomorphology. The distinctive limestone ridge is identified as one of the defining special qualities of the Mendip Hills AONB [5.1]. Lynchcombe NR | | 29 DISTRICT WIDE APPRAISAL KEY Geology Overview | | 30 DISTRICT WIDE APPRAISAL 5.1.2 Within the Statement of Significance of the AONB’s KEY Special Qualities it is noted;- BRITISH GEOLOGICAL SURVEY LOCALITY AREAS “The diverse and visible geology that ranges from 1 Black Down 6 Priddy GEOLOGICAL SSSI 2 Charterhouse 7 Harptree Smitham Hill REGIONALLY IMPORTANT GEODIVERSITY SITE Devonian to Jurassic in a relatively small area, making (RIGS) one of the best areas in the country to appreciate 3 Draycott & Westbury-Sub-Mendip 8 Beacon Hill the relationship between geology, landscape, natural 4 Ebbor Gorge 9 Mells and Wadbury Valley history including the iconic Cheddar Gorge”. [5.1] 5 Wookey Hole 10 Great Elm and Vallis Vale 5.1.3 This interest extends the full length of the Mendip ridge well beyond the AONB boundary. From the early exploitation of the lead and coal resources through to the modern extent of limestone quarrying within the district, the geology has also influenced the patterns of settlement and infrastructure within the district. With the natural exposure of strata and the mineral extraction operations the district geology has helped shape the understanding of many aspects of geological processes. Within the district there are a large number of geological SSSI and RIGS* sites highlighting this interest. 5.1.4 Comprehensive explanations of the Mendips geology and mineral history has been collated into an on- line resource by the British Geological Survey (BGS); this includes detailed descriptions of many of the key geological feature sites. Through this resource and an active programme of public events the local geology is promoted as an aspect of visitor interest for the district; the Mendip Rocks Festival started in 2010 and has become an annual event. [5.2] Spot Location Map-Geological Interest This image illustrates the distribution of the BGS feature areas and designated geological interest sites.*RIGS – Regionally Important Geological (and Geomorphological) Sites. | | 31 DISTRICT WIDE APPRAISAL For a simple overview the description of the Mendip district can be broken down into five main areas; 1. The broad Mendip Plateau to the west of the B3139 (Bath Road from Wells to Emborough) 2. The narrower section of the Mendip ridge to the east of the B3139 including Beacon Hill ridge extending to Frome 3. The North Somerset Coal Fields on the north side of the Mendip ridge around the 4 Nettlebridge, Holcombe, Coleford to Mells Valleys 1 3 4. The high ridge encircling the deep valleys of the Wheel and Buckland Brooks and landscape extending to district boundary along the lower Frome Valley. 2 5. The gently rolling hills and valleys to the north and south of Frome extending around the south eastern side of the district to the Fosse Way 5 6. The south western area of low lying 6 landscape with intermittent ridges that extends from the Fosse Way to the district boundary. Context Map-Geology with Topography Illustrates a simplified geology map. For a simple overview the description of the Mendip district can be broken down into five main areas as identified on Geological Areas (and Geomorphological) Sites. | | 32 DISTRICT WIDE APPRAISAL AREA 1 - THE MENDIP PLATEAU 5.1.7 A major period of uplift or ‘mountain building’ tremendous forces involved the rocks were thrust occurred during the Triassic period, 251 – 200 million up and over other younger rocks and left with many years ago, following the end of the Carboniferous fault lines. The formation of a steeply folded range era (the mountain building period referred to as the also created heavily fractured material susceptible Variscan Orogeny [5.3]). During this the Devonian to rapid erosion of the upper exposed levels. The and Carboniferous sediments were pushed up into strata that remain as the hills seen today are the rings a series of steep folds. In the Mendips these have of exposed carboniferous limestone surrounding the a distinctive form known as Periclines where the older sandstones. Four main characteristic Periclines folds drop down at either end taking the form of remain as Black Down, North Hill, Pen Hill and 1 an upturned boat. The resultant mountain range Beacon Hill. may have reached up to 1500 m and with the BLACK DOWN 5.1.5 Among the oldest rocks exposed within the district are the Devonian deposits laid down as thick sequence of sediments across an area of desert some 416 to 359 million years ago. Often seen as reddy brown NORTH sandstones with the colour arising from iron oxides HILL deposited in dry conditions. As the parent material arose from the erosion of an earlier mountain range PEN HILL (from the Caledonian era) the series also contains areas of pebble conglomerate. These rocks occur as the ridge of the Black Down hill as hard and insoluble BEACON HILL core to the hills relatively resistant to erosion. 5.1.6 These were overlaid in the Carboniferous period (359 – 299 million years ago) with sediments from a warm shallow sea. Varying conditions during this period created deposits ranging from mudstones, muddy limestones through to deep accumulations of very pure limestones. Devonian - Sandstones & Conglomerates Carboniferous Limestone Series | | 33 DISTRICT WIDE APPRAISAL 5.1.8 Where they outcrop the inner core of the harder here the plateau is notable for the absence of streams Devonian sandstones shed rainwater to the and surface water bodies. The area also contains boundaries of the adjoining limestones. As a soluble some of the most significant cave systems with several rock deposit the limestones allow the percolation extending to depths over 150m and many km of of water through the strata which allows further accessible passage. dissolving and erosion of the rock below ground. The Mendip landscape exhibits a range of ‘Karst’ 5.1.11 Within the MDC area the notable caves include the formations such as sink holes, swallets, caves very popular Swildons Hole, St Cuthbert’s Swallet and resurgence springs and streams. The basic and Eastwater Caven located in a cluster of caves around the village of Priddy. component of limestone, calcite or calcium carbonate is dissolved by rainwater (slightly acidified by carbon 5.1.12 Swildons is perhaps one of the best known cave dioxide) but is also readily recrystallized as purer systems with opportunities for moderate adventure forms of the mineral and as cave formations such through to extreme exploration. The cave has now as stalagmites and stalactites and other flowstone been explored and surveyed to a length of 9.2km features. and depth of 167m. [5.4]. The lower parts of the cave require diving equipment to get to the end 5.1.9 Another aspect of the intense pressures and processes of the mountain uplift was the creation of mineral beyond some 12 sumps, but the connection to follow deposits within the earlier strata. During the the stream water through to its eventual outflow at movement of the earth’s crust heated water from the Wookey Hole remains elusive. St Cuthbert’s Swallet surrounding depths was forced into the Mendip rock also an extensive and demanding cave entered via a formations. With this various dissolved elements and challenging climb within a very narrow rift but with compounds were carried up into fissures and fault the reward of chambers with spectacular formations. lines to then crystallise as veins of mineral. These are Eastwater offers a sporting trip entered via an unstable found through-out the hills as ores including those boulder ruckle that is really not for the faint hearted. of lead, zinc, iron and copper. These outcropped 5.1.13 While it may be unusual for a Landscape Character to the surface in areas leading to early human Tratmans Temple - Swildon’s Hole - Photographer : Donald Tomson Assessment to identify aspects below the surface, exploitation in the form of shallow mine workings it is important to recognise the extensive and very from well before the arrival of the Romans. The varied cave systems of the Mendips as an essential Statement of Significance : The AONBs Special Qualities natural outcrops and the spoils resultant from the part of the natural environment. This is reflected in various eras of extraction have created unusual soil “Caves, for their wildlife, geological, archeaological the AONB Special Qualities in respect of both the conditions giving rise to some unique habitat and importance including Aveline’s Hole - the oldest environmental interests and recreational value of flora collections within the district. cemetery site in Britain, and Goughs Cave - one of the the caves set out within the Statement of Significance most important Palaeolithic sites in Europe that provides AONB Management Plan:- 5.1.10 On the broader part of the Mendip ridge on the a breeding site for Lesser and Great Horseshoe bats” western side of the district the Devonian and [AONB Management Plan Jan 2019 - Para 1.4 page 11 - 7th bullet point - n.b. Carboniferous outcrop is the largest of the Periclines. caves named located outside of MDC area but representative of many others within the district]. With the depth of the limestone cross section exposed | | 34 DISTRICT WIDE APPRAISAL AREA 2 - THE BEACON HILL RIDGE 5.1.16 Smaller cave systems are found around the sides pattern. The juxtaposition of much younger Jurassic of the hills on the eastern Mendips.