River Parrett - Geology

Geology

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 which have occurred, 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 (0.3–0.8 m) over 100 years.