SENECA ROCKS, W.VA. -- For more than 400 million years, the Gendarme, a 20-ton slab of quartzite, perched precariously in a notch amid one of America's most spectacular rock formations, looking like it might fall at any moment.

Last Thursday, at 3:27 p.m., it did.

"New climbers always said 'It doesn't look very stable,' but we told them not to worry," said John Markwell, who operates the Seneca Rocks Climbing School and the Gendarme Climbing Shop here.

Today, viewed through a telescope outside the U.S. Forest Service's visitors center, the spot occupied for eons by the 25-foot-tall Gendarme, which stabbed into the gap in the Allegheny Mountains like a giant index finger, is wide open, a three-foot-wide gray slab that looks like the base of a sawed-off petrified tree.

There are exotic theories for the Gendarme's collapse -- reverberations from low-flying military jets, a sudden shift in temperature -- but Markwell believes "it probably just crumbled of old age."

Forest Service supervisor Jim Page attributed the Gendarme's collapse into rubble to "a natural phenomena."

Geologists have calculated that the 250-foot-thick vein of hard, quartz-grained rock, called Tuscarora sandstone, was formed about 440 million years ago, when the craggy summits that now overlook the Germany Valley here were under water.

The Seneca formation towers 1,000 feet above the ground and, until the Forest Service opened a hiking trail last month that goes within 50 feet of the summit, was the highest point east of Wyoming that could not be surmounted by walking.

Moments after the Gendarme came tumbling down, Markwell and several forest rangers, alerted by the noise, set off for the site, crossing the North Fork of the South Branch of the Potomac River.

Most climbers stop at Markwell's shop or the visitors center before beginning their ascent, but there is no requirement to sign in, "so we wanted to make sure no one was trapped out there," Markwell said.

No one was, though several climbers were in the area at the time.

One of them, Anthony C. Barnes, 30, a Washingtonian who had climbed Gendarme dozens of times, was perched on a nearby ledge, instructing three students in the art of rock climbing.

Barnes said that at first he thought the noise was from "one of the Navy fighter jets from Norfolk" that swoop over the valley in practice runs, occasionally breaking the sound barrier.

It was only when he saw "a huge cloud of dust and flying debris" that he realized "something tremendous" had happened, he said.

When the dust had cleared, "it was gone," Barnes said reverently, as though speaking of a departed loved one.

The exact time of the crash was noted by Markwell's 10-year-old son, Brock, who was in the parking lot of the Seneca Rocks Elementary School in the shadow of the summits.

"I heard a rock fall and then saw a big blur," the boy said. "It hit twice {on the way down} and then there was dust everywhere. I looked at my watch: it was 3:27. Then I yelled to my mom {who teaches kindergarten}, 'The Gendarme fell!' "

Gendarme, which is French for pinnacle or police, is a term used internationally by climbers to denote freestanding rock formations.

Seneca Rocks' Gendarme, also called "The Chimney" or "Gunsight," stood like a silent sentinel in a notch between twin peaks that rise vertically 1,000 feet above the river valley and 200 feet beyond the tree line. In 1943 and 1944, the site was a training ground for Army assault troops preparing for combat in the Alps.

The delicately balanced Gendarme, popular with Washington area climbers, had withstood an attempted dynamite attack by a local crazy a half century ago and decades of climbing by amateurs and experts. The first recorded ascent was made in September 1908.

There are more than 300 identified paths to the top of Seneca, and all but one require climbing, occasionally upside down. Professional equipment and guides are a must for many of the climbs.

The ascent to the summit is "a mountaineering experience unique in the East," according to Markwell, in a class with Yosemite and the Grand Tetons.

In the last decade, a half dozen climbers have been killed scrambling up and over Seneca Rocks' twin peaks. The two most recent fatalities occurred in 1985, Markwell said.

The greater danger is that of falling rocks.

Markwell recalled a major rock fall in the 1970s, shortly after he moved here from Ohio. "Once you hear that sound, you know it," he said.

As news of Gendarme's downfall spread through and beyond the Monongahela National Forest, of which Seneca Rocks is a part, sightseers flocked to the visitors center, where 1,300 people registered Sunday.

"We miss it," said Mildred Alt, who had a view of the Gendarme from the counter at the center, where she has worked for six years. "It was beautiful to look at."

The absence of Gendarme, which was the most visible symbol of Seneca Rocks, may spur a cottage industry in revisionist postcards, pennants and other souvenirs.

"A guy has already been in with a design for a T-shirt showing the fall of the Gendarme," Markwell said.