Hill Roars Down on Neighborhood : Geology: Nine homes are cut off and more are evacuated as 100-foot-high slope snaps trees and buckles street in Rowland Heights. Officials search for the cause.

Something was wrong with the hill. All the neighbors noticed.

For more than 20 years, the gently rolling slopes behind their homes in the unincorporated San Gabriel Valley community of Rowland Heights had brought them peace, full of wild mustard, oats and barley. But since November, the sandstone and siltstone hill had been sliding half an inch per day, enough to damage water mains and chip away at curbs.

On Friday morning, the 100-foot-high hill inexplicably roared to life, with a vengeance. It snapped tall pine trees and buckled Shepherd Hills Road, pushing the pavement 10 feet high. A chasm 50 feet wide and a quarter-mile long opened in the slope.

On Shepherd Hills Road, families in nine homes at the end of a cul-de-sac could get out only by foot; the road was impassable by car. On Morning Sun Avenue, which intersects the road, four families voluntarily evacuated their houses at the suggestion of Los Angeles County Public Works officials.

County geologists, who said there was little they could do, estimated Friday morning that the hill was sliding an inch every hour. But it appeared that the visible slippage had stopped in the afternoon.

“There’s no guarantee it won’t move again,” Dave Smith, a city of Diamond Bar consultant, said at a meeting of homeowners Friday night. “We haven’t seen any movement at the [base] for the better part of the afternoon. There’s no guarantee it won’t move again but it appears to have reached a resting point.”

State Sen. Richard Mountjoy (R-Arcadia) said late Friday that he asked Gov. Pete Wilson to declare the area a disaster. And officials said they will begin moving dirt at the top of the hill Saturday morning in an effort to take pressure off the base.

That was little comfort for homeowner Nellie Ayala-Reyes, 42, as she surveyed the damage in her back yard, including a crumpled white picket fence and buried hot tub. Awe-struck, she shook her head at the force of the slide. Like other homeowners, she worried that her insurance policy will not cover the damage.

“Mother Nature at work,” said Ayala-Reyes, a USC administrator. “I’ve always had respect for nature. I’ve always known it could do things.”

County public works officials had no damage estimate. On Friday, geology experts climbed the hill to take soil samples and other tests in an effort to pinpoint the cause of the slide and figure out how to shore up the hill.

“There’s no way of holding back this slide,” said Donna Guyovich, a spokeswoman for the County Department of Public Works. “It’s spooky.” As she talked, trickles of pebbles slid down the hill, an uneasy reminder of the morning rumble.

Officials said they were not immediately certain who is responsible for the hill, which lies in the city of Diamond Bar. Several parties have been entangled in the hill’s problems: the county, Diamond Bar, Walnut Valley Unified School District and a private developer. The ridge above the houses is in Diamond Bar, although the homes are on county land. Wednesday, the Diamond Bar City Council gave preliminary approval for Sasak Corp. developers to build a 21-home subdivision on a six-acre piece of the hill--even though homeowners had protested that the unstable hill should not be developed.

Homeowners blame the school district for the hill’s collapse.

Walnut Valley Unified School District is building a middle school on 78 acres next to the Sasak development. In November, contractors for the district moved 1 million tons of dirt to make room for the new school, creating a huge man-made mountain. Neighbors believe the activity put pressure on a shaky hillside, which already was loose from spring rains.

Until then, “we never had a pebble come down in 20 years,” said homeowner Bob Roberts, 47, whose house is trapped in the cul-de-sac.

Walnut Valley school district officials deny that their bulldozing affected the hill. “We’ve been assured by our geologists that our operation has nothing to do with the slide,” said Clayton Chaput, an assistant superintendent. On Friday, homeowner Yung Kim, 58, was awakened at 3 a.m. by the eerie, creaking sound of the hill sliding--and then a crash as the hill came down on her concrete retaining wall and spa deck. She thought her four-bedroom house was a goner. Later that morning, as she started to pack clothes and other belongings for evacuation, she called her grown daughter.

“Hey, can you come take your piano?” she asked her.

Times staff writer John Hubbell contributed to this story.