Geological History

 

Devon’s ‘red county’ description is attributed to the local red soils which are restricted to this region of the county.  The New Red Sandstone deposits, up to 1100m thick, were laid throughout the Permian period, from 280 million years ago (shortly after the granite intrusion of Dartmoor) to the beginning of the Triassic period some 240 million years ago.  Devon was semi-arid with a very hot climate, large sand storms and rare flash-floods would blow and wash material rich in ferric oxide (which gives the red colour) off the mountains of central Devon into valleys and flood plains stretching from Tiverton to Torbay.  In geological terms the red cliffs of Dawlish and E Devon are best described as “Aeolian and fluvial sands with some interbedded bressia beds…mainly of quartzite and aureole fragments.” They are apparently the best-exposed non-marine Permian rocks in W Europe.

 

Since 240 million years ago the World changed dramatically, continents broke up, the North Atlantic began to form and what is now Britain began to move north.  Sea-levels rose inundating what is now Dawlish Warren for most of the time, leaving Cornwall and W Devon as an island.

 

For the last few hundred thousand years the climate here changed from mostly tropical to alternating glacial and interglacial periods. Sea levels have risen and fallen, flooding and exposing this area.  A reflection of this sea-level range is gained from a quote by Durrance & Laming, “The Exe has two sets of buried channels: an older rock-cut, gravel-filled feature descending to below –50mOD and a younger set cut in both bedrock and gravels…to a level of –30mOD…[which] continues southward off Torbay to a level of at least –46mOD after receiving the buried channel of the Teign as a tributary.”

 

Dawlish Warren was created during the present interglacial period, only about 7000 years ago by post-glacial sea-rises, which attained roughly present levels 5000 years ago.  The Exe from Topsham, which once ran to a coast nearly 20 miles away, now enters the open sea under 5 miles away where at Dawlish Warren the currents of longshore drift and estuarine tidal processes form sandspits, bars and flats.  The Outer Warren and intertidal sandbars are in a state of ‘dynamic equilibrium’ whereby material is constantly shifting within the local realms of Dawlish Bay, but the trend in recent centuries has seen a gradual erosion of the spit, and Warren Point has periodically disappeared or even become a detached island.  Over the last 250 years the spit has reduced from 250m to only 50m wide.

 

The more recent history has been heavily influenced by man, concern over flood defence in the mid 1940s saw the central tidal creek (Greenland Lake), which separated the mobile Outer Warren from the stable Inner Warren, filled in by the Water Board.  A gabion-backbone, which stretches throughout the length of the spit under the dune-ridge, was installed to prevent further sea breaches in the early 1970s, when 17 beach groynes were placed every 100m to impede long-shore drift.  Severe storms encouraged the construction in 1992 of improved seawall defences, including the ‘hard-engineered rock-armour’ which protects the base of the spit and the railway main line.  Despite the best efforts to preserve the spit, climate change and rising sea levels may cause the eventual demise of the terrestrial habitats here sometime late in the 21st century.

 

References:

Durrance, E & Laming, D.J.C.  1982.  The Geology of Devon.  University of Exeter.

 

 

 

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