There's also bit in next week's Radio Times (page 130) with a photo of a standing stone -

"Kenneth Steven is a poet and he travels along the longest enclosed glen in Scotland, Glen Lyon... He starts at Fortingall, where an ancient yew tree has stood for thousands of years. It's older than Christendom, says his guide and companion Norman Haddow, a drystone dyker... Their journey is marked out by ancient stones, including one which young men once tested their strength."