ARCHAEOLOGISTS have discovered a chain of more than 50 coastal forts, up to 4000 years old, on an island in the Outer Hebrides.

The first of the promontory forts on Lewis was found three years ago, but in a field study carried out this year by students from Edinburgh University, some 45 more have been identified.

Archaeologists are baffled by aspects of the chain and believe that scores more exist throughout the Western Isles. But the forts, some of which occupy spectacular sites 300ft above the sea, could have been part of early Viking defences, when the Outer Hebrides were part of the Norse Empire.

The best known evidence of Viking links with the islands is the discovery of the 12th-century Lewis Chessmen and one of the forts at Crowlista is opposite where the 78 walrus ivory chessmen were found in 1831.

The islands first attracted the attention of Norse raiders in 800AD and the Hebrides remained part of the Norse empire until 1266. But archaeologists have been unable to date the age of the fortifications, which could range between 2000BC and 1000AD.

Norse pottery has been found at some sites, while neolithic evidence has been unearthed at others.

The study, mainly financed by Historic Scotland - and led in the field by Mr Mike Church - revealed 38 forts on the west side of Lewis. The stone and turf structures, some of which are eight feet tall and contain several houses, are spaced just more than a mile apart.

Archaeologist Chris Burgess, project director, said most of the fortifications would have been used as defensive retreats but some may have also had religious significance.

He did not believe the forts were part of an organised protective network, rather the result of individual communities building fortified shelter for times of danger.

Similar chains of fortifications have been found in the Northern Isles and the east coast of Scotland, and Mr Burgess said the discovery on the Western Isles would help shed more light on their importance and the type of people who used them.

Some of the structures are built into the cliff with straight walls erected across the neck of the headland. .

``The size of the forts varies, but some were capable of holding whole communities with as many as five houses,'' said Mr Burgess.

``What has been found is very exciting and has made a more complete picture of what was there. But we still know comparatively little at this stage.

``Until further research is carried out we just do not know what exact period they date from.''

A report is being submitted to Historic Scotland and, if more funding is made available, a survey in Harris may be mounted in 1998.

The same archaeologists have also discovered the most extensive bronze age site in the Western Isles at Guinnerso in Uig on Lewis.

Until now, little evidence of bronze age settlement on Lewis has been unearthed, but at Guinnerso a pottery factory, together with evidence of dwellings, fish traps, field systems, burial cairns and drains have been discovered.

It is believed the site may have been occupied for 3000 years and a further study may be carried out in 1998.

Another large bronze age site may have also been discovered at Barvas.