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Hikers discover massive landslide

If a tree falls in a forest and no one is around to hear it, does it make a sound? That's hard to say. But what if an entire stand of timber, 30 acres in size, suddenly dropped hundreds of feet into thin air? Scientists say you can check the seismic record.

A massive chunk of land appears to have given way spectacularly sometime over the past winter from the south flank of Greenleaf Peak, three miles north of North Bonneville. No one knows for sure when it happened in the remote forest, but hikers recently documented the aftermath.

"This was all rock," said Don Nelson, a Vancouver hiker who recently visited the area twice. "It's nothing like the oozing mudslide in Stevenson."

Scientists familiar with the area said the rock slide appears to be old lava atop an underlying formation of sedimentary rock that naturally dips toward the south.

Bob Norris, a U.S. Geological Survey seismologist in Seattle, said it's possible that a slide of that magnitude landed with a high-impact crunch big enough to be detected by the array of seismometers around the region. He said he'll dig back through the archive of seismic records to find out.

Other scientists speculated that the slide may have been triggered by the intense rainstorm that clobbered Western Washington in December.

Accessible primarily from a loop off the Pacific Crest Trail that runs along the spine of the Cascade Range, the landslide in the state-owned forest is too remote to affect neighboring landowners. The landslide is part of the 2,837-acre Table Mountain Natural Resources Conservation Area.

"We've lost a little bit of owl habitat, but it's pretty minimal in the scope of things," said Carlo Abbruzzese, regional natural areas manager for the state Department of Natural Resources.

Nelson, who recently hiked through the site, estimated that the slide skidded almost a half-mile and dropped about 1,000 feet in elevation.

"It's so big, it really gives one pause as to what could happen," he said. "It could potentially cut loose a big enough slide to do serious damage."

Big as it is, scientists said it's merely a sliver of the gargantuan complex of material that clogged the Columbia River about 550 years ago. Remnant material from one of the largest of those ancient slides, known as the Bonneville complex, is still apparent where the Columbia narrows at the Bridge of the Gods.

"It shows that landslides are part of the suite of geological processes in the Pacific Northwest, and the gorge has a long history of them," said Pat Pringle, a Centralia College geology instructor who has studied the area for several years. "It's part of our geological heritage."

It also illustrates another feature obvious to anyone living in the gorge: While Oregon boasts spectacular cataracts such as Multnomah Falls, people on the Washington side have to grapple with landslides.

The reason lurks beneath the surface.

There, an undulating body of sedimentary rock slopes from north to south underneath the river at a grade variously described as between 2 percent and 7 percent. The Eagle Creek formation, named for the Oregon creek where these rocks are well-exposed, covers roughly 30 miles of the gorge east of Cape Horn.

It's topped by flows of basaltic lava dating back millions of years.

As water percolates down through the basalt on both sides of the river, it can saturate the Eagle Creek rocks or run into claylike material that acts as a sort of greased plain.

Where the formation is closer to the surface, on the north side of the river, material can easily slide away in a landslide. In Oregon, the Eagle Creek formation is deeper, and it slopes away from the river. The overlying basalt is much thicker and less inclined to slide away.

Nelson first noticed something amiss on Greenleaf Peak during a hike in early January.

"I happened to see what looked like a fresh clearcut," he said, noting that it was covered by a thick blanket of snow. "I took a picture and forgot about it until about two weeks ago."

That's when he noticed a comment on a local hiking Web site, referring to a large landslide visible near Table Mountain. Nelson said he looked at his picture, double-checked the location, "and realized there's been a major event."

The landslide, roughly the size of Vancouver's ballyhooed waterfront redevelopment district, swept up fir trees Nelson estimated at more than 100 feet tall.

He spotted at least one older tree measuring close to 6 feet in diameter.

Russell Evarts, a U.S. Geological Survey research geologist who is in the process of mapping the gorge, said geologists intend to examine the remote slide more closely. It's a rare opportunity to see such a large cross-section of material normally covered by vegetation.

"It does provide a scientific opportunity, absolutely," Evarts said. "Geologists tend to get excited about disasters because we learn so much from them."