The road, which would pass through an area of Heritage Coast, may provide a test of the trust's willingness to use its full legal powers to protect its land.
The bypass, on the A35 between Bridport and Charmouth, will cost about pounds 35m. The proposal is being examined by a public inquiry at which the National Trust will present its evidence tomorrow. If the inquiry finds in favour of the bypass, the trust can appeal to Parliament to prevent its land being appropriated. The trust has legally declared most of its land to be inalienable but it has not used the power this gives to protect it against incursion by roads for many years and is reluctant to do so. David Bett, regional director of the National Trust, said: 'This is a very special power which the National Trust would never use lightly.'
The route for the bypass suggested by the Department of Transport will run almost parallel to the existing road and only 250 yards from it as it goes through National Trust land and passes Morecombelake, near Charmouth.
Many village residents object to the bypass because it would be another road with faster traffic. This section of the proposed road would cut across the St Gabriel's valley, a natural bowl between four hills just north of Golden Cap, the highest point on the southern coast of England. Mr Bett said: 'It is an intimate landscape of small lush fields and the proposed road would cut an artificial engineering line across them. The existing road follows a much better line along the bottom of the hill where it meets the fields.'
The trust owns 10 miles of coast between Charmouth and Eype together with hills and valleys extending up to a mile and a half inland as far as Morecombelake itself. The estate was put together from 30 acquisitions, several of them gifts, made as part of Enterprise Neptune, which was launched in 1965 to acquire unspoilt coastline for permanent preservation and make it available for easy access by the public. The proposed route for the bypass would require three cuttings and three bridges but the most controversial section is where it would cross National Trust property at a boggy field called Moor meadow. Technically known as a seepage marsh, the meadow is a home of some unusual beetles and flies including a crane fly which preys on slugs. Delicate mauve blossoms of milkwort and the rich yellow flowers of kingcup can be seen in the meadow.
George Elliott, the National Trust warden at Golden Cap, warns about what may happen if the road is built across Moor meadow: 'It is a Dorset wet flush and it could double the cost of the road to build across it - engineers have only just started to look at the problem.
'In the 1940s we had the biggest landslip in Europe here covering 700 acres, demolishing the road between Charmouth and Lyme Regis.
'The engineers are planning to build on a series of ancient landslips at Moor meadow and something similar could happen. They are planning to cut back to form a platform and put a plastic mat on top of it with boulders over that so the water can find its way through. But when running green sand is mixed with immature sand on blue Lias it will all begin to move when the water table rises - then the acidity will change and the chalk becomes like toothpaste. It will all squeeze out and the hill will come down on top.'
The Department of Transport's own advisory committee on landscape warned of such geological problems in 1986 and also recommended against any route south of the existing road because it would be 'unacceptably damaging to the landscape of the West Dorset Heritage Coast and to the environment of Morecombelake'. This advice was ignored by the department and the committee has since been disbanded.
Despite its opposition to a bypass at Morecombelake, the trust supports the proposal for a bypass a few miles down the road at Chideock, which is part of the same scheme. The proposed route will go north of the village of Chideock relieving congestion and enabling people to enjoy walking in the village, where 54 old stone buildings are listed.
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