page_title: River Parrett
section_title: Geology
hierarchical_section_title: River Parrett / Geology
caption_attribution_description: English: View from the summit of Burrow Mump across the winter-flooded levels toward Aller Hill and the village of Aller.
context_page_description: The River Parrett flows through the counties of Dorset and Somerset in South West England, from its source in the Thorney Mills springs in the hills around Chedington in Dorset. Flowing northwest through Somerset and the Somerset Levels to its mouth at Burnham-on-Sea, into the Bridgwater Bay nature reserve on the Bristol Channel, the Parrett and its tributaries drain an area of 660 square miles – about 50 per cent of Somerset's land area, with a population of 300,000.
  The Parrett's main tributaries include the Rivers Tone, Isle, and Yeo, and the River Cary via the King's Sedgemoor Drain. The 37-mile long river is tidal for 19 miles up to Oath. The fall of the river between Langport and Bridgwater is only 1 foot per mile, so it is prone to frequent flooding in winter and during high tides. Many approaches have been tried since at least the medieval period to reduce the incidence and effect of floods and to drain the surrounding fields.
  In Anglo-Saxon times the river formed a boundary between Wessex and Dumnonia. It later served the Port of Bridgwater, and enabled cargoes to be transported inland.
context_section_description: Close to the source of the river the underlying geology is a thin layer of Fuller's earth clay over Yeovil Sands. The resulting light soil made the area important for the production of flax and for market gardening in the past.
  Burrow Mump, an ancient earthwork owned by the National Trust, is a natural hill of Triassic sandstone capped by Keuper marl, standing at a strategic point where the River Tone and the old course of the River Cary join the River Parrett. It probably served as a natural outwork to the defended royal island of Athelney at the end of the 9th century.
  The Levels and Moors are a largely flat area in which there are some slightly raised parts, called "burtles" as well as higher ridges and hills. It is an agricultural region typically with open fields of permanent grass, surrounded by ditches lined with willow trees. Access to the Levels and Moors is by "droves", i.e. green lanes. The Levels are a coastal sand and clay barrier about 20 feet (6 m) above mean sea level (roughly west of the M5 motorway) whereas the inland Moors can be 20 feet (6 m) below peak tides and have large areas of peat. The geology of the area is that of two basins mainly surrounded by hills, the runoff from which forms rivers that originally meandered across the plain but have now been controlled by embanking and clyces. The area is prone to winter floods of fresh water and occasional salt water inundations, the worst of which in recorded history was the Bristol Channel floods of 1607, which resulted in the drowning of an estimated 2,000 or more people, with houses and villages swept away, an estimated 200 square miles (520 km²) of farmland inundated and livestock destroyed. A further severe flood occurred in 1872–1873 when over 107 square miles (277 km²) were under water from October to March.
  The extraction of peat from the Moors is known to have taken place during Roman times, and has been an ongoing practice since the levels were first drained. The introduction of plastic packaging in the 1950s allowed the peat to be packed without rotting. This led to the industrialisation of peat extraction during the 1960s as a major market in horticultural peat was developed. The reduction in water levels that resulted put local ecosystems at risk; peat wastage in pasture fields was occurring at rates of 1–3 ft (0.30–0.91 m) over 100 years.
caption: View from the summit of Burrow Mump across the winter-flooded Somerset levels toward Aller Hill and the village of Aller, Somerset.
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