Offa's Dyke repairs use 400 tonnes of airlifted stone
More than 400 tonnes of crushed stone will be flown by helicopter to the Brecon Beacons to restore an eroded footpath on Offa's Dyke.
Brecon Beacons National Park Authority plans to fly in the material to Hay Bluff which is 677m (2,220ft) above sea level.
The stone will help create improvements to more than two miles (3km) of path to limit further damage.
The airlift of one tonne loads will take place over the next five days.
The sections of worn and eroded footpath are up to two miles from the nearest road and due to its steep gradient and ground conditions, can only be reached by foot or by helicopter.
The path lies between Hay-on-Wye in Powys and Pandy in Monmouthshire and is one of the most popular trails used by walkers who visit the area.
'Protected and maintained'
Wardens and staff from Brecon Beacons National Park Authority are being helped by Black Mountains Uplands volunteers during the operation.
The crushed stone has been selected because of its durability and compatibility with the existing stone along the footpath.
Richard Ball, access officer for Brecon Beacons National Park Authority said: "Changeable weather conditions, the remote location and the altitude of the site meant that using a helicopter was the most appropriate option for this access work.
"Most of the upland area in the Black Mountains is protected as a Site of Special Scientific Interest because of its upland habitats which are really special and we need to take care not to damage the site whilst completing the works.
"The area is also important economically in terms of farming and tourism, and whilst this brings important income to the area it also costs money to ensure that it is protected and maintained."
The project is funded by Natural Resources Wales, Natural England and Brecon Beacons National Park Authority.