Geological History
Devon’s ‘red county’ description is attributed to the local red soils which are restricted to this region of the country. 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 stretched 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.
The 1970s work was considered to be preventing natural processes in breach of the 1984 SSSI and later international designations therefore plans were drawn up to change the defensive approach. The Dawlish Warren Beach Management Scheme is designed to flood Greenland Lake and other low-lying terrestrial habitats in the event of any breach. The new wall near the visitor Centre is designed to prevent any further tidal encroachment from such a breach. The majority of the work was implemented in 2017, replacing all the groynes, removing most of the gabion baskets along the length of the Warren and reshaping the Dune Ridge after groyne 10 with an internal sand filled geotube. Further work was undertaken to recharge the beach with materials from offshore increasing its height by up to 2m.
Unfortunately much of the sand had already washed away from the beach by the time it was officially opened in December 2017, and it is now lower in places than before the work. The Dune Ridge has also fared badly, the removal of the gabions accelerated the erosion with most of the new Ridge being washed away and the geotube exposed by October 2019. This increased erosion has also removed large areas of Warren Point that had been exhibiting ideal natural processes.
Despite efforts to preserve the spit, climate change and rising
sea levels may cause the eventual demise of the terrestrial habitats here
sometime later this century.
References:
Durrance, E & Laming,
D.J.C. 1982. The Geology of Devon. University of Exeter.