One of the natural skills of people brought up in rural communities is the ability to build strong, long-lasting drystone walls, according to an expert in the craft.

Drystone dyke instructor, folk singer and musician Dave Goulder of Rosehall, Sutherland, has just tested nine candidates on the Hebridean island of Harris for their initial certificate with the Dry Stone Walling Association. ''It's in their hands,'' he said.

''All nine fellows got great marks. Their work was quite excellent, and this is true of most people brought up in rural communities.''

He has just completed a training video for the Association, the first in its 25-year history. It details the tools and techniques of the ancient craft of building enclosing walls, using modern day computer graphics to demonstrate construction and composition methods.

Mr Goulder, who moved to Scotland 36 years ago from Derbyshire, says there are five different rock formations from the east coast to the west, with all sorts of minor intrusions and variations.

He said: ''From the lovely soft sandstone of the Black Isle to Helmsdale, there is the heavy granite of Rogart and the Mound to schisty whin around Rosehall and the limestone of Inchnadamph.

''Then on the west coast there is Torridonian sandstone and Lewisian gneiss, not counting volcanic Skye, with its marble and basalt.

Mr Goulder, who is an instructor and an examiner for the Dry Stone Walling Association, says the different rock formations explain why there are variations in the style of drystone walling from place to place.

''There are quite a lot of Galloway dykes here in the north, and that would be because of the Highland Clearances and the arrival of shepherds and sheepmen from the Borders, bringing with them their own patterns.''