"The reason we like orienteering," Marit Beecroft said as she consulted the map and compass that dangled from her neck, "is because it's the thinking man's sport."

With that she took off at a steady sprint through the brians and brambles of the Quantico Marine Base, heading due east along a creek bed after the next station on the orienteering course of the day.

Fifteen steps later she was sprawled on the rough ground, five foot ten of Norwegian-born working mother splattered among the leaves and blown-down tree limbs.

She uttered not a sound; simply gathered herself upright, brushed a few stickers from her tattered blue nylon Scandinavian orienteering suit and scrambled on. Later she confessed: "I couldn't find my running shoes this morning, so I borrowed a pair of my son Erik's. They're two sizes too big. That's why I keep falling down."

This is your average Sunday with the Quantico Orienteering Club, a conglomeration of housewives, Marines, sedentary businessmen, Boy Scouts and assorted others who gather every other weekend or so to do battle with the woods of Prince William County, Virginia.

Orienteering is not a gentle wander through the woods on a compass course. It's race, and a highly competitive one for some of the national-class runners of the club. The idea is to scrape one's way to 10 preordained sites in the forest as fast as possible, punch in at each of the sites and get back to the starting point first.

Half the battle is wits: What route makes sense over the lay of the land to the next marker. The other half is [WORD ILLEGIBLE] , battling through the brush.

Beecroft , whose daughter Kristin won last year's national race for 15- to 18-year-old women, consented to my following her around on one of the Quantico races last weekend. "Don't worry," she said for starters, "I'm just out for fun. I don't take it seriously. It's mostly walking."

We picked the second-hardest of four courses - the orange. The hardest is the red, and there's a white course along roads for raw beginners and a yellow for novices.

The orange was a 4 1/2-kilometer jaunt over several hundred acres of rolling forestland. We started at the call of the starter, who shouted our number and handed us a map. We raced to a master map, copied it and set out into the woods for site No. 1.

The pace was a moderate jog, which seemed reasonable at that point. Occasionally we'd stop for a map check, to make sure we were headed right. Beecroft read the topographical map, spotting creeks, rises and man-made obstacles like fences.

In 10 minutes' time we'd found our first marker, a small square of nylon hung from a tree. Inside was a plastic punch like a train conductor's, and Beecroft punched the No. 1 square in her race card with the punch, to prove we'd indeed been there. Then we head for No. 2.

It went on that way for exactly 97 minutes and 5 seconds, which may not sound like much but is a very long time to be running through the woods with tree limbs smacking you in the face.

Along the way we were buoyed to find some of the 75 or so other orienteerers tromping through the forest many looking even more baffled than we. At one point we were paralleled by a young man with think glasses who was walking at a fast pace and scanning some low piney woods for his next marker. I watched as he plunged along, and to my shock saw him walked smack into a pine trunk and go crashing on his keister.

No harm done. He was up and forging on in seconds.

Doesn't anyone get hurt? Not really, Beecroft said. Once there was a fellow who impaled a leg on a sharp branch, but he pulled it out and kept going until loss of blood forced him out of the race.

Nothing serious.

Our 97 minutes 5 seconds placed us fifth out of 17 who ran the orange course, which was not bad at all. "We did very well," said Beecroft. "We only got lost twice."

Those times we weren't really lost, just briefly confused. The worst was No. 10, the marker for which was stuck down in a piney depression. We were orienteering over land the Marines use for war games, and to get to No. 10 we had to cross a deep ditch for foxholes and strands of concertina wire, which is the stuff the military uses to keep snipers out of a base. It makes for rough walking.

We found that last marker when Beecroft's other daughter, Marit, showed up and pointed it out. Her mother had just whispered to me that if we came on it first, "Don't tell Marit."

Quantico Orienteering is the only club of its kind in the area. Its membership is open to everyone, and practically anyone can complete one or another of the four fresh courses mapped for every meet. Materials needed: Compass, available for rent, at the race for 50 cents, and map, sold by the club for $1.50 at the race. That's all.

The idea, says Beecroft, is "to get out of the house and do something different. I work at a desk five days, and on Saturday I do the housework."

That said, she headed for the car, slipped a pair of jeans over her scratched and bloodied legs, fluffed here sweet-matted hair, gathered her kids and headed back to civilization.