YOU'RE 40 FEET above the ground, clinging to an almost vertical slab of rock and being supported by your left big toe and a shaky faith in physics.

The possibilities pass before you as the lactic acid begins to burn in your left leg. You could keep climbing or you could lie back toward the ground and trust the rope around your waist, but the one thing you can't do is ask for anyone to help you up there. Now it's completely your game. A psychological infatuation with danger? Maybe. But this is rockclimbing, a sport that is becoming increasingly popular among people from all walks of life.

Around Washington, a city known more for a different sort of upward mobility, is one of the best technical climbing areas this side of the Mississippi: Carderock, Maryland is an ideal spot for both beginners and advanced climbers.

Since most of the routes at Carderock are less than 50 feet high, the area is well-suited for "top roping" (climbing with a rope which is tied around the climber, lopped up through a carabineer anchored at the top and back to the "belayer," who takes up the slack). Thus the beginner is able to get good and safe practice right from the ground.

For more advanced climbers, Carderock's excellent training because its mica schist rock is smoother and more difficult than the granite found in most popular climbing areas.

The drive from D.C. to Carderock is a sanity run in itself. Entering the park, one passes below a bridge and suddenly a canopy of trees opens above the narrow road toward the river. The silence is overwhelming and wonderfully un-citylike.

Doug Cosby, one of Carderock's most promising young climbers, finds a different sort of relaxation at the rocks. "I used to come out here a lot when I was having trouble in school or having other problems because you can often find yourself getting in trouble when you're a good ways up there on the rope." Subjecting oneself to anguishing circumstances might not be the most conventional way to relax, but it's certainly effective. Climbing demands 100 percent of your attention, leaving no time to worry about anything else. And the confidence that comes with a good climb can help you face problems that had seemed insurmountable.

Doug is what most folks would picture a typical climber: lithe young body with spidery legs, strong forearms and permanently blistered hands. He seems to have a natural talent for climbing, but in his early climbs he admits to having made quite a few near-fatal mistakes. The beginner, he says, needs "some sort of training. Either go to a school and get the basics down or go out with a good climber. The first few months are the toughest. It's a very safe sport if you approach it that way."

Almost any day one can find a few excellent climbers at Carderock practicing on "Mad Dog," which has been rated a 5.10/5.11 (5.0 is dead easy, 5.14 the toughest climb ever).

The Mad Dog experts are inspiring to watch but are not the best sources of advice for beginners; they tend toward arrogance and impatience with amateurs. John Gregory, who has climbed Carderock for 11 years, definitely doesn't fit that mold, though. He's a Heritage Foundation librarian by day, but come 5 o'clock he sheds his Britches' daywear, horn rims and Adam Smith necktie for a 150-foot nylon rope with a perlon belt, his tasseled loafers replaced by Sportiva Ballets. Eleven months of the year it's a daily routine for Gregory, who has become almost as permanent as the rocks themselves. He's always offering advice to beginners, helping someone tie a knot properly, pointing out the details that make an "impossible" climb do-able.

"One of the most striking things is that everyone is equal before the rocks," he says. "The rock doesn't care about your resume."

Beginners who have the most trouble, in his opinion, are often the ones who seem to progress most over the long run. That's hopeful advice for the beginners, but at Gregory's level of expertise climbing can be an unforgiving sport. One hundred feet above the ground with no rope for security and your palms sweating profusely, successfully moving that right foot just an inch higher is an intensely mental accomplishment. "You can't kid yourself," Gregory says. "You have to commit yourself to the rock."

But you don't need to be a super athlete to be a successful climber. It's a very graceful sport, sometimes demanding more coordination and balance than strength. A nicely maneuvered climb looks like a ballet. In this respect women beginners often have an advantage. Lacking in upper-body strength, many women learn to rely more heavily on their legs instead of trying to hoist themselves with their arms.

Climbing attracts all sorts of people. Recreational climber Reed Falwell was a Washington attorney for 15 years, then in 1984 moved to Rouse & Associates, where he's now a partner, and to a home less than five minutes from Carderock.

Reed's first climb was no beer-and-pretzel picnic in Carderock Park. With no climbing experience he embarked on a 180-mile climbing trek on the Tashi Labsha trail across the Himalayan mountains. Eleven years and many climbs later now, he's obsessed.

Reed is not exactly the type of guy who pursues things in moderation. "I simply refuse to get involved in anything other than my work that conflicts with my climbing," he says. "I guess that's a little bit obsessive."

The challenges of climbing and of the corporate life complement each other, Reed says. "I try not to make any differentiation between work and play," he says, "I'm as interested in climbing as I am in whatever I do to make money, but I don't make any distinction between the two, because I think that climbing or any other sport is very important to the process of working and vice-versa. So I really don't see any conflict at all. It's just a singular whole."

The faces of Carderock are young and old. They range from the avid outdoorsman to the corporate executive. And through the persistent preservation work of volunteers with the Conservation Project, the area has been able to grow with its climbers. NONMETAPHORIC UPWARD MOBILITY

Some of the area's best climbing courses:

ROCKVILLE RECREATION DEPARTMENT --

an "experience day" for absolute beginners at a cost of $14 and $16 for nonRockvillians. They also offer a more structured class that runs for four weeks in the fall at a cost of $40 or $47. Call Bob Plumb, 424-8000, ext.344.

POTOMAC APPALACHIAN TRAIL CLUB --

offers a range of instruction free to beginners. Courses set according to the demand. Call Stuart Pregnall, 543-3988.

WASHINGTON WOMEN OUTDOORS --

offers various instruction packages. 474-4403.

SENECA ROCKS, WEST VIRGINIA --

If you're really raring to go, you might head out to this area, which offers some pretty spectacular climbing. 304/567-2600.