A 50-foot-tall stone retaining wall along the Henry Hudson Parkway (search) in upper Manhattan collapsed with a big boom on Thursday afternoon, burying parked cars under a massive mound of dirt, trees and debris.

No one was injured, Mayor Michael Bloomberg (search) said. And police said they did not believe anyone was buried in the rubble.

Amy Allison, who lives in the apartment building closest to the wall collapse, said she was at home when she heard a "huge crash."

"I looked out my window, and there was a huge landslide starting to happen," she said. "I saw all of a wall of dirt just tumbling onto the highway."

Bloomberg said at a press conference at the site hours later that 150 feet of the 600-foot-long wall had collapsed but the rest appeared stable.

"It was just fortunate that nothing worse happened," he said.

The mayor said police on the scene for a small initial collapse saw the larger slide about five minutes later.

Officer Edward O'Connell said half the wall was blocking Riverside Drive, which runs along the wall and parallel to the parkway, when he arrived and there were no people or cars in the path of the bigger landslide. A car on the parkway was hit by debris but drove away, he said.

As firefighters removed debris and tree branches from the parkway's northbound lanes, neighborhood residents gathered along other parts of the retaining wall to see what happened. Several described the wall as shoddy and old and said they had complained about it.

The wall, now owned by the Castle Village housing complex, was built between 1905 and 1908 along with Paterno Castle, a mansion owned by real estate developer Charles Paterno (search), Department of Transportation Commissioner Iris Weinshall said. The wall was sold to Castle Village in 1938, she said.

An investigation will determine what led to the collapse, but water damage could have been a cause, city officials said.

"For years, water has been seeping through, and they have been fixing it," Weinshall said.

Sarah Morgridge, who works for local Councilman Robert Jackson, said workers for Castle Village had been trying to reinforce the wall recently.

"There was a major corrective action for over the last six months," she said. "This is definitely not a case of neglect."

City traffic and Castle Village officials had already scheduled a meeting for Friday to discuss the wall. A telephone message left with Castle Village shortly after the collapse was not immediately returned.

The closest building to the retaining wall, 1380 Riverside Dr., was evacuated. That building appeared to be in stable condition, Department of Buildings Commissioner Patricia Lancaster said at the news conference. The department was doing an inventory of the area.

Bloomberg praised the work of the multiple city agencies responding to the scene, noting that police department helicopters had carried firefighters and other emergency workers.

"It just goes to show the cooperation that we had," he said.

Earlier in the week, police and fire officials clashed over how to respond to major emergencies, specifically biological, chemical or radiological attacks, and Bloomberg had warned that anyone who didn't follow the city's new protocol — which gives police the lead role — would be out of a job. A top fire chief had criticized the protocol, saying firefighters are better trained and equipped to handle certain emergencies.

The mayor said that the six-lane parkway, Route 9A, remained closed northbound but that the southbound lanes had been reopened. He said the northbound lanes would be closed for "a while."

The Henry Hudson Parkway, named for the explorer and built in the 1930s, runs along Manhattan's West Side beginning at 72nd Street and travels through the Bronx to the Westchester County line. The Manhattan portion of the highway was temporarily closed during the 1970s because it was in disrepair.