h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

...there are at least a couple of different styles of dry stone dykes. In Scotland you have the inland variation, where there is a fair amount of shelter and they back fill the center of the wall, making the wall thicker, sturdier. On the western isles where there is little shelter and lots of gales the walls seem flimsier. The walls are a single row of stones, big stones, but with holes built in to the wall, so the wind blows through
BCNU - Crescent

Oh there's loads of different styles - a friend of mine spent months travelling round Britain looking at different walls for a handbook he was re-writing on walling. He's got about a 1000 photos!

The single stone walls are called boulder dykes and are stronger than they look- not that I've built any! If you want to know more look at the dry stone walling association web-site: www.dswa.org.uk (sorry I don't know how to make it a link). There's also a special millenium display of lots of types of walls that are going to be built on 29 April- 1st May at the National Stone Centre in Wirksworth in Derbyshire.