page_title: Polden Hills
section_title: Geology
hierarchical_section_title: Polden Hills / Geology
caption_attribution_description: English: The viewpoint, Walton Hill, Polden Hills, Somerset
context_page_description: Polden Hills in Somerset, England are a long, low ridge, extending for 10 miles, and separated from the Mendip Hills, to which they are nearly parallel, by a marshy tract, known as the Somerset Levels. They are now bisected at their western end by the M5 motorway and a railway, the Bristol and Exeter Railway, part of the Great Western Main Line.
context_section_description: The Poldens are a low narrow ridge of late Triassic / early Jurassic mudstones (often referred to as clay) alternating with limestones. The south-southwest facing scarp is formed by a succession of rocks, lowermost of which are the mudstones of the Mercia Mudstone Group (formerly known as the Keuper Marl), overlain in the west by moree mudstones of the latest Triassic age Blue Anchor Formation (formerly ‘Tea Green Marls’), a thin unit recorded from Puriton Hill east to the vicinity of Stawell.  Elsewhere the overlying layer is that of the earliest Jurassic (Rhaetian) age Westbury Formation (formerly known as the Rhaetic clays or Westbury Beds) consisting of mudstones and limestones. Stratigraphically above these are the mudstones and limestones of the Blue Lias, forming the north-northeasterly directed dip-slope of these hills.  Blue Lias is believed to have been quarried on the Polden Hills in the 15th century; and was quarried in Puriton from the early 19th century until the 1973, when the local cement works closed. The outcrop is intersected by two sets of normal faults aligned roughly WSW-ENE and WNW-ESE.  At the east end of the range, Dundon Hill forms an outlier of the higher formations. The low ground to both north and south is covered by thick alluvial deposits and peat.
caption: Walton Hill, Polden Hills
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