A little after 4 a.m. Monday, Diane and John McConnell were up and getting an early, soggy start on the work week when they heard a deep rumbling outside their home on Glass Run Road in Baldwin Borough.
John ran outside and found his back yard and part of his garage buried under mud, rock and debris that until moments before had formed part of a seemingly solid hillside on an adjacent property. The force of the landslide toppled some of the garage's 16-inch concrete blocks onto his wife's 2001 Nissan Sentra, ripped off its bumper and pushed it halfway through the closed garage door.
"I was too late to do anything," he said. "There had been smaller landslides before. I probably spent $5,000 since we moved in seven years ago cleaning them up, but they never did any damage. They were just a nuisance. It's a natural thing. The hillsides get wet and then they let go."
After last weekend's saturating rains, it was not surprising that several of the region's hillsides yielded to gravity's pull, falling onto the McConnells' property and blocking Route 28 between the 31st and 40th street bridges and Route 51 in Stowe near the Fleming Park Bridge.
Southwestern Pennsylvania's rolling hills have a long geological reputation of not living up to their elevated topographic names. The region is one of the most landslide-prone areas of North America because of its steep hills and soft and slippery underlying rock layers.
And the widespread regrading and filling of hills and valleys for housing and business development over the past 50 years, plus the use of large excavating equipment to slice through hillsides for roads, have made the problem worse by opening up the clay stone layers to inundation from storms.
Add to the mix that the region has more freeze-thaw cycles during the winter than any other in the country and you've got a sure-fire recipe for the hillside rocks to roll.
Although recent subfreezing weather has stabilized the ground, the next slide is only a warming trend away.
"We'll have more problems as the result of the freezing and thaw cycles. That tends to shear off the hillsides like a knife," said Dick Skrinjar, Pennsylvania Department of Transportation spokesman. "This is probably the beginning of a series of episodes over the next several weeks."
That's because the subfreezing weather has caused the water in the saturated slopes to turn to ice and expand, breaking the natural cohesiveness of the hillside soils and rocks, said Charles Shultz, a geology professor recently retired from Slippery Rock University.
"The abundance of landslides is standard operating procedure in the Pittsburgh area," he said. "When the water freezes, it heaves up and puffs out the ground perpendicular to the slope of the road cut. When thaws occur, so do landslides."
Land has been answering to gravity in southwestern Pennsylvania on a regular basis since the last Ice Age. U.S. Geological Survey landslide maps document more than 15,000 ancient and geologically recent major landslides in Allegheny and Washington counties. And in one small, movement-prone section of Greene County, a USGS map identifies more than 2,100 landslides.
American Indian tribes acknowledged the local landscape's unsteady nature. They called the river now known as the Monongahela the "Menaungehilla," which means "river with the sliding banks," or "high banks that break off and fall down."
Such earth movement happened then and happens now because, in many places, the bedrock consists mainly of soft shales and clay stones that weather rapidly when exposed and become slippery when large doses of water from snowmelt or spring rains are added. The so-called "Pittsburgh red beds," a 40- to 60-foot thick layer of mostly reddish, greenish and grayish clay stones and shales are the best known among the highly mobile bedrocks.
According to Shultz, the region is also the site of the oldest known landslide on Earth, the Bakerstown Landslide, near Route 8 in Richland, in northern Allegheny County.
"In all the slopes north of Downtown there are a lot of ancient landslides dating to 25,000 years ago," Shultz said. "When humans go in and start digging around, that tends to reactivate the old slides. When they built Route 28 near Kittanning and Natrona Heights, they made a lot of steep road cuts and opened a lot of red beds."
Skrinjar said that the whole length of Route 28 from the Interstate 279 interchange to Harmarville is a slide-prone area.
"It's just a case," he said, "of the valley soils that have been carved into hills along the rivers wanting to go back to where they were before."

A slide waiting to happen: landslide graphic and risk regions in Pa.

First Published: January 11, 2004, 5:00 a.m.