When a ridge above the north bank of the North Fork Stillaguamish River gave way and swept across into a neighborhood on the opposite bank, it exposed the layers of soil beneath, and probably provided a clue to the geologists who will later analyze the slide area.
"It'd be interesting to know whether that's glacial material or if that's hard bedrock," said Eric Cheney, professor emeritus at the University of Washington's Department of Earth and Space Sciences.
Looking at photos of the stratified layers, Cheney guesses that glacial deposits made up much of the surface soils, although only firsthand examination will reveal a better picture.
From a geologic perspective, the Oso mudslide was probably typical.
"It's probably fair to say this has been going on for millennia, It's just the past couple hundred years there's been lots of people living in the area as well," Cheney said.
Indeed, the hillside that collapsed on Saturday bore scars from earlier slides, including a smaller 2006 episode that also blocked the river with debris.
The geology of the Cascades was shaped over two million years by advancing and retreating glaciers.
Glaciers deposited sediments in the river valleys, and when the ice melted, the rivers carved out new beds in the sediments, which are a loose mixture of sand, gravel and stones called glacial till.
The result was geography that is less stable and more prone to slides.
Soil composition is just one factor in determining how slide-prone an area is. Also important are recent precipitation and the amount of human activity in the area, including logging, which often removes support for the already loose soils.
Doug Lenker, a logger from Darrington who knows the area well, said the hill that slid away probably had not been logged for a number of years, since there were some second-growth trees on the slopes.
"Any active logging was way further out to the west," he said. "There were no new roads or anything."
A few hours after the slide Saturday, he went up the hill following old logging roads that run from behind. He made it to the crest where the hill sloughed away.
"It just opens up into nothing," Lenker said. "The bank drops probably 300, 350 feet down."
The climate of the Pacific Northwest leaves much of the ground saturated in the winter months, and the Darrington area typically gets significantly more rainfall than lower lying areas.
The last rainfall in Snohomish County was Wednesday and the last heavy rainfall was March 16. Earlier this month, flooding briefly closed part of Highway 530.
Because much of the river valley soil is glacial till resting atop harder layers of clay and bedrock, water tends to saturate the less dense sandy layers. That makes a slide more likely.
"Once the water gets into it and the pore space between the grains, it tends to liquefy it and it tends to flow," Cheney said.
More people moving into areas prone to sliding has long been a concern in Snohomish County. In a 2005 report, county officials identified the ongoing spread of residential, agricultural, commercial and industrial development into outlying areas as probably the greatest significant risk factor for damage from mudslides.
The Snohomish County Natural Hazards Mitigation Plan identified several areas of special concern, including the hilly areas next to the North Fork Stillaguamish River, where the slopes are prone to slides.
The county estimated at the time of the report that 28,500 people lived in areas across the county susceptible to slides.
Other areas of concern include the South Fork of the Stilly, parts of the Skykomish River valley, the foothills of the Cascades in general, and the bluffs overlooking Puget Sound from Everett to Edmonds.
Those areas have seen a lot of slide activity. During the winter storm of 1996-97, slides caused damage in the county estimated at more than $30 million.
The Jan. 15, 1997 slide in Woodway just north of Richmond Beach dropped about 50 lateral feet of land down onto the railway, washing a freight train into Puget Sound.
The plan didn't anticipate a slow-moving slide in Everett that in recent years began devouring homes in the Valley View neighborhood. Homeowners are suing the city, blaming a 2004 drainage project. City engineers say the problems were caused by a combination of natural topography, fill dirt placed during construction, groundwater movement and heavy rainfall during the winter of 2010 to 2011.
The slide that pummeled an Oso neighborhood Saturday occurred in the same location as a smaller slide that let loose Jan. 25, 2006 after a series of winter storms soaked the area with rain.
While the damage was less than Saturday's slide, the 2006 slide released enough debris to dam the river just downstream from the Steelhead Drive neighborhood.
That year, neighbors worked together to move belongings out of harm's way while crews of jail inmates were brought in to help fill sandbags.
The river eventually carved a new path around the tangle of mud and logs. Emergency officials brought in heavy equipment to help speed along the process.
Two weeks later and about a mile away from Steelhead Drive, more problems surfaced with the valley's unstable soil. Highway 530 suddenly began cracking and the pavement sank six inches along a 250-foot stretch.
There were fears that the roadway could be closed for months, but close inspection deemed it safe enough for use while repairs were made. The highway's slumping came after the upper valley again was hit by heavy rains.
While Highway 530 is closed indefinitely, at some point the emergency responders will go home and the engineers will be called in to try to clean up the mess.
When that might be is unknown. David Lervik, a civil engineer with experience in soil stabilization, said his concern would be that the hill would continue to slide.
"The top of the hill could keep sloughing back unless all the hydrostatic pressure has been relieved," Lervik said.
Looking at photos of the slide, he said there are still probably some unstable areas in the hill. Future slides would be likely until the soils reached a new state of equilibrium, he said.
"I'm not sure there's anything we can do to stop it. It's pretty big," Lervik said. "When I saw pictures of it, it reminded me very much of St. Helens."
Scott North contributed to this report.
Chris Winters: 425-374-4165 or firstname.lastname@example.org.
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