A 100-foot-long section of a seven story high stone retaining wall collapsed yesterday afternoon in Washington Heights along Riverside Drive, dumping tons of dirt, boulders, and two large oak trees over an on-ramp and onto the northbound lane of the Henry Hudson Parkway.

Improbably, no injuries were reported, fire officials said.

The collapse, which happened around 4 p.m., buried at least one parked car on Riverside Drive and halted rush-hour traffic just north of the George Washington Bridge.

The 600-foot-long retaining wall, which was built in 1908, hemmed in the five-building Castle Village cooperatives, whose engineers had decided as recently as two weeks ago that the wall had become structurally unsound.

Engineers of Castle Village cooperatives, Langan Engineering, had concluded that the wall was no longer structurally sound, one resident, Allan Brent, told The New York Sun. The co-op’s management company, Goodstein Management, sent notices to residents alerting them of a plan to shore up the wall by tethering it with cable to the bedrock upon which the five 14-story buildings were built, Mr. Brent said.

Residents of 1380 Riverside Dr., which practically abuts the retaining wall and was the only building to be evacuated last night, noticed the wall had begun to bulge in recent weeks.

“It didn’t take an engineering degree to see that the wall was going to do something,” one resident, Robin Marin, said. “But I didn’t think it was going to be this major.”

Mr. Marin sat on a wall overlooking the Hudson River yesterday with his wife, Gina Hartwig-Marin, and their cat, Zoe, who was also evacuated.

“I’m sure it could have been prevented,” Mr. Marin said. “It was such an obvious failure waiting to happen.”

A blue scaffold adorned with razor wire erected at the bottom of the retaining wall had been there for years, he said, but nothing had been done to stabilize the wall.

Some residents said that debris from the wall would sometimes fall onto their parked cars, and that fear of the wall’s imminent collapse led them to stop parking nearby.

The wall fell yesterday after a 10-foot-wide chunk was dislodged, according to one witness, Carlos Pellecier, who is the doorman at 1380 Riverside Drive.

Minutes after police arrived, around 4:19 p.m., the earth began to rumble and the entire 100-foot length of wall came tumbling down with a boom.

“The lieutenant who came out to survey the wall yelled, ‘It’s moving!’ ” Mr. Pellecier said. “Then the earth just fell.”

Another witness, Rudy Claudio, had been enjoying the view of the Hudson River moments earlier when he turned around to see the earth slide off the hill.

“I was terrified,” said Mr. Claudio, who works as a security guard at Yankee Stadium. “It was like a Godzilla movie. Everything was just avalanching and coming down.”

Emergency workers, fearing remaining sections of the wall might collapse, stopped their search until the area could be stabilized. Search dogs, however, did not pick up scents of anyone trapped in the debris, firefighters said.

Shortly after the wall’s collapse, Mayor Bloomberg arrived by helicopter. He said the northbound lane of the West Side Highway north of the bridge would be closed at least through the weekend.

A drainage problem for the co-op, which sits above Riverside Drive with impressive views of the George Washington Bridge to its south, had long been an enigma for the building’s engineers. Over time, shifting dirt and wayward water patterns had begun to push the wall out, residents said.

The commissioner of the Department of Buildings, Patricia Lancaster, said the wall – which is the last remnant of the Castle Paterno – had been repaired at least once before, sometime in the 1980s. Her agency will investigate the cause of the collapse, and the City Council’s Committee on Transportation plans to hold a hearing soon on the collapse.

Late last summer, engineers had begun excavating the small grassy area by the retaining wall and filling it with gravel to improve drainage, residents and neighbors said. The area, still cordoned by orange gates, had just been reseeded, according to Mr. Brent, who said he is an engineer.

“That’s why nobody got hurt,” he said. “Normally, there would have been dozens of children playing in the yard.”

Mr. Bloomberg, too, expressed relief that no one was hurt.

“It could have been worse,” he said.

The mayor congratulated emergency response teams that blanketed the neighborhood at 181st Street, including 40 fire units, scores of police, the city’s Office of Emergency Management, and the American Red Cross of Greater New York. For the mayor, the emergency was a textbook example of the cooperation among the city’s emergency response agencies.

For residents of a of a six-story co-op building, at the corner of W. 181st St. and Riverside Drive, which sits on the bottom of a hill, the collapse fueled fears over another retaining wall near by that they have said is near collapse. One board member, Della DeKay, noted that at their May 4 meeting, the members discussed the possibility that a wall on the west side of Riverside Drive could collapse.

The co-op board, she said, filed a lawsuit several years ago to force the city to make repairs to the wall. She said that the lawsuit was dismissed and that officials had said the wall was not the city’s property. State authorities, too, refused to take responsibility for the wall and for the problems with the bedrock underneath it, she said.

Ms. DeKay said her co-op has spent about $1 million on engineering services in an effort to stabilize the earth surrounding their building, whose walls show cracks caused by the shifts in the ground. Langan Engineering has placed sensors inside the building to monitor the earth’s movement. The engineering firm produced a report three years ago on the stability of the building, recommending that gravel be pumped underneath the building to support the foundation.

The Castle Paterno was built, along with the retaining wall, in 1907 by an Italian immigrant and builder, Charles Paterno, who used it as his personal mansion. It included a mushroom cellar, a pool, and a 1,600 square–foot master bedroom.

About 30 years later, he tore down the castle and replaced it with the Castle Village apartments. Only the original retaining wall remained – until yesterday.