Geologist: Ancient Whatcom County slides dwarf the Oso disaster

THE BELLINGHAM HERALDApril 12, 2014 

BELLINGHAM - Geologist Don Easterbrook scoffed when he heard news reports that called the Oso slide on the Stillaguamish River the biggest in the United States.

The Stillaguamish slide on March 22, 2014, killed more than 30 people, dammed the river and shut down a state highway. Its estimated volume was about 10 million cubic yards of debris.

Easterbrook says a slide that slipped off of Church Mountain and rumbled seven miles down the north fork of Whatcom County's Nooksack River contained about 285 million cubic yards. It was about 1.6 miles wide and 312 feet thick. When you drive out Mount Baker Highway through the town of Glacier, you are driving on top of that slide.

Easterbrook, retired from the faculty at Western Washington University, believes the Church Mountain slide is the largest non-volcanic landslide in the U.S., although it occurred long before this country was imagined. Carbon 14 dating of a log embedded in the debris provided an estimate that the Church Mountain slide occurred about 2,500 years ago - almost in the current events category for a geologist.

But if there had been any newspapers back then, the slide would have competed for front page space with the latest dispatches from the Peloponnesian War.

Was anyone in the path of that massive collapse - a band of Salish hunters or fishers? There is no way to know.

West of Church Mountain is the site of another ancient slide where rocks slipped off Van Zandt Dike and rumbled across the valley floor to stick a toe into the bed of the Nooksack River. The river channel cuts through that toe today. Easterbrook said the Van Zandt slide was smaller than Church Mountain, but still massive: It covers about five square miles.

When did that one occur? Easterbrook isn't sure. A log extracted from a swamp atop the slide has been dated to 1,500 years ago, but Easterbrook said the tree might have grown and died there long after the slide occurred.

Easterbrook has noted debris from two other old slides - smaller, but still massive - at other points along the Nooksack and its forks: Racehorse Creek, along the Nooksack's north fork south of Kendall, and Slide Mountain, about two and a half miles upstream from Maple Falls.

Unlike the Oso slide on the Stillaguamish, all four Nooksack events evolved sudden and unpredictable collapses of rock formations.

The Oso slide occurred on a steep hillside of unstable sediments left behind by a glacier. Other big slides had occurred in the same area repeatedly, as recently as 2006. Easterbrook said he and other regional geologists were well aware of the area's potential for frequent collapses of the loose material along the Stillaguamish.

Big rockslides like the Church Mountain event are impossible to predict, Easterbrook said. He thinks it likely that all four of the big rockslides in this area have something to do with the many small earthquakes that occur in the Deming area. About 400 have been detected since monitoring began in earnest in 1969.

"There's no reason for that bedrock to slide, except for earthquakes," Easterbrook said. "I think they're probably all related."

Could a similar colossal rock collapse happen again here?

"The answer is, we don't know, because these quakes are still going on," Easterbrook said. "Church Mountain could go again. It's a big, steep mountain face. ... There's no guarantee that it won't happen again, but the probability is pretty low in any given lifetime."

Whatcom County does have one area similar to the Stillaguamish slide zone: an area known to geologists as the Clay Banks, between Deming and Nugents Corner. Thick clay deposits there were left behind by a glacier about 12,000 years ago, and big blocks of clay tumble into the river now and then as water undercuts the bluff.

"It (the clay) soaks up water and then it loses all of its internal strength, and it's worse because the river is pushing at the base of it," Easterbrook said. "That thing has been moving back for 70 years."

A few homes above the river were abandoned to their fate years ago, and at least one of those homes has tumbled down the steep bank. At this point, the unstable area doesn't appear to pose a threat to any existing dwellings, but geologists and county officials are keeping an eye on it.

On Feb. 21, 2014, the Clay Banks area experienced a slide that was big enough to block the river for a few minutes.

Reach John Stark at 360-715-2274 or john.stark@bellinghamherald.com . Read the Politics Blog at bellinghamherald.com/politics-blog or get updates on Twitter at @bhampolitics.

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