The rock's face was gray. I suppose mine was too, as I dangled 40 feet above the Potomac shoreline on a couple of ropes, a swami belt and something called a Swiss seat. I inched down the rope, rappelling at a snail's pace. None of that graceful gliding from ledge to ledge you see in rock-climbing movies. No sir. It was my first descent. I was slightly nauseated and very terrified.

That was the beginning of the most rigorous physical test I've ever endured - a two-day course in beginning rock-climbing, given by a local outward-bound sort of outfit called "Discovery, Inc." Discovery, which specializes in outdoor "adventure" courses for schools and colleges, opens its programs to the public each autumn in workshops in such sports as spelunking, white-water canoeing and rock-climbing. (For information on coming workshops, call 471-1014.)

The workshops cost about $30 and provide an opportunity to sample an outdoor sport you may always have wanted to try but had no one to teach you. I'll admit there were times that weekend when I wished I were sampling something else. Like that first rappel, or when the instructor's hand steadying my right foot on a thimble-size nub of wet rock, while I searched for a toe-hold with my left, was the only thing averting a fall. A very short fall, on the end of a rope with a tensile strength of 5,000 pounds, but nobody wants to fall, not even six inches.

There were times when all of us wanted to give up. But rock-climbing is like childbirth - once you've started there's no backing out. Highly complex, it demands planning and, obviously, an extensive safety system. Instructor Mike Murray taught us that first.

For 3 1/2 hours the seven of us stood under a shelter at Great Falls Park and worked on knots - watermen's knots, double overhand knots, bowlines on bights, figure-eights. We learned about ropes our swami belt, the girth attaching a climber to his safety rope.

We learned about carabiners (pronounced carabeeners), those large metal links that open and close to take on loops of rope or webbing, and we made Swiss seats to attach ourselves to rappelling ropes. We learned to "belay" or hold the safety rope for one another, and we memorized the terse dialogue climbers and belayers use to tell one another what's going on. Finally we learned climbing techniques on eight-foot boulders.

Then it was the real thing.We inched up 40-foot cliffs, our cheeks against the lichen, the roar of the river below drumming in our ears. "Get your nose over your toes," Mike would yell if he saw us hugging the rocks. The rule is to keep your weight over your feet for maximum purchase, which often means leaning away from the rocks.

It rained much of the time. "We rocks will help you perfect your techniques," Mike assured us. He'd climbed the Grand Tetons in the rain, so he figured we could handle the Great Falls gorge in a drizzle.

We all worked hard, struggled until our arms and legs shook, although I could never figure if the shaking was caused by fatigue or fear. And when it was over we'd learned a lot - not just about rock-climbing but about our own bodies and our heads.

I was exhausted but proud of the nerve I'd never dreamt I had. For a 34-year-old mother of two, it was something of a rite of passage. Now that I've emerged intact, except for aching muscles and a rope burn in an unmentionable place, I think I'll buy a good pair of binoculars, turn it into a spectator sport, and hope my kids discover rock-climbing when they're younger.

If you want to try it, take a course if possible, or go with an experienced climber. The Appalachian Trail Club often offers instruction for members or prospective ones. Call 638-5306, between 7 and 10 p.m., Monday through Friday. Fred Norris of the Potomac Valley Climbing School offers day-long classes. Call 333-3398.

To strengthen your arms do chin-ups, palms away from you. Any leg-strengthening exercises will help.

Wear loose-fitting pants or hiking shorts (but not too short - when you're rappelling, the ropes between your legs can give you a nasty burn). Good light-weight lug-soled hiking boots are the best footwear, and a crash helmet is essential. Leather work gloves help ease the friction during a rappel.

If you want to invest in equipment, a crash helmet will run between $15 and $20. You'll need a 150-foot length of nylon rope ($45), plus a couple of 14-foot lengths of tubular nylon webbing (about $2.50 a length) for a swami belt and Swiss seat. You can get by with two locking-D carabiners ($4.25 apiece) and you'll need a carabiner and break-bar assembly for rappelling (about $5).

If you want to watch before you try it, climbers are nearly always at Great Falls on weekends. Just walk downstream of the picnic area to the cliffs, and you'll see them. Carderock in Maryland and Seneca Rocks, W. Va., are also popular.