Visitor Matt Ballard of Tennessee, U.S. climbs a boulder near Clanwilliam in South Africa July 18, 2011. Ballard is one of many climbers from around the world, drawn to the sleepy town in South Africa's remote Cederberg mountains as initiates of the new sport of bouldering. REUTERS/Anton Ferreira

They carry mattresses on their backs, and in bars they are the ones gesticulating like crazed semaphore operators.

They are initiates of the sport of bouldering, and they have invaded the sleepy town of Clanwilliam in South Africa's remote Cederberg mountains in their hundreds, bringing an economic shot in the arm to the region 250 km north of Cape Town famous for producing rooibos herbal tea.

"The whole climbing thing has caught us a bit by surprise," said Thys Kruger, owner of De Pakhuys outside Clanwilliam, one of a half dozen farms offering no-frills accommodation for climbers.

"It's grown tremendously from 2004, when we had maybe 34 climbers. This year I'm expecting in excess of 500, mostly international climbers. Some of the top guys in the world climb here," Kruger said.

"When I bought the farm, I thought 'what can I do with all these rocks?' Now I'm farming the rocks."

Kruger estimated that climbers would pump from 4-5 million rand ($591,000-$738,394) into the economy of Clanwilliam this year.

Climbers flock to the bouldering mecca, dubbed Rocklands, during the southern hemisphere winter. Cold weather is essential, because sweaty fingers cannot grip the rock.

Jason Crase took a two-month break from the San Francisco climbing gym where he works to explore the zone of spectacular rock formations, caused by glacier action aeons ago.

"During the northern hemisphere summer, this is where you come to climb," he said. "There's a couple of other places you can go in our summer, but not with the concentration of good climbing that they have here."

CRASH PAD, CHALK BAG

Crase and other climbers spend the entire duration of their trips in the Cederberg - safaris and beaches hold little attraction for them.

Bouldering -- or clambering over rocks and rock faces generally not much higher than 8 metres (yards) without aids -- has been part of the training regime for climbers for more than a century. But its evolution as a sport unto itself is a modern invention embraced across Europe, North America and now South Africa.

The only equipment needed is a pair of special shoes, a bag of powdered chalk to improve finger grip, and a thick foam mattress or "crash pad" to place on the ground as a safety cushion.

Climbers start conquering a boulder at ground level, searching for cracks and ledges that will provide finger- and footholds. A typical boulder "problem" takes little more than a minute or two to complete.

"It is trying to search for movement," Crase said.

"Everyone has their own thing they're looking for in a boulder problem, and movement is a huge part of it. People get really excited about doing certain moves, in that way it's like gymnastics."

David Majure, a cardiologist from San Francisco, described bouldering as "a readily apparent challenge that you can conquer in a relatively short period of time".

When climbers get together they regale each other with graphic body-language descriptions of memorable boulders, repeating the arm movements they used to conquer a tricky problem.

"You can always pick out a group of climbers when you walk into a bar because they're doing that with their hands," Majure said, shooting his arms into the air as though looking for elusive handholds.

Paul van Hoesslin, a web developer from Cape Town, said the sport had a zen-like appeal.

"You become one with the boulder," he said.

"The rock teaches you how to become humble. You sit back and go, 'what am I actually supposed to do? How do I do this? What am I actually after? What's the goal?'"

Many of the climbers at Rocklands describe the sport as an addiction. Some will spend up to a year studying a single "project" - how to move along a line of handholds across a particular boulder and complete the problem.

Austrian climber Manuel Ladner said success was "like winning the lottery".

"You don't need anything else," he said. "You're really proud."