According to an Oregonian article by Richard Hill (2002):

"... Radiocarbon dates taken in 1958 from drowned trees indicated that the slide occurred between 1250 and 1280. A quarter-century later, a radiocarbon date of wood samples taken from within and below the landslide deposit put the date at about 1100. Four years ago, Pringle and Robert L. Schuster of the U.S. Geological Survey had radiocarbon dates taken of a buried Douglas fir that indicated the tree died between 1500 and 1760. That would place the slide close to the earthquake in 1700 that devastated the Northwest coast. Counting the tree rings, with each ring representing one year of the tree's life, they estimated that the tree died in about 1699. But the mystery of the slide's date has taken new twists recently. Last year, Nathaniel D. Reynolds, then a graduate student at Washington State University in Vancouver, used a technique called lichenometry to estimate the age of the Bonneville Landslide. The dating method uses the growth rate of specific lichen species as an indicator of the age of the surface the lichen is growing on. Lichens, slow-growing organisms formed from an association between a fungus and an alga, can be used for dating earthquakes and landslides because they quickly colonize fresh rockfalls that occur in the wake of a quake. Once established, they form at a constant rate if left undisturbed. Reynolds, now with the U.S. Forest Service, said his study indicates the landslide probably happened between 1670 and 1760. "These results demonstrate that the Bonneville Landslide may have occurred more recently than previously believed," Reynolds said in a recent article in Washington Geology. The dates "provocatively bracket" the powerful offshore Cascadia Subduction Zone earthquake of 1700, he said, but he cautioned that the study doesn't prove that the quake caused the landslide. The research plot thickened with the recent surprising discovery of tree samples cut from the landslide site in 1934 by the late Donald B. Hamilton of Johns Hopkins University and the University of Minnesota before Bonneville Dam was completed. Pringle and Reynolds, along with colleagues Jim E. O'Connor, a hydrologist with the Geological Survey in Portland, and Alex C. Bourdeau, an archaeologist with the U.S. Fish and Wildlife Service in Sherwood, had wondered where the samples were. The Western Forestry Center, where some of the samples had been kept, burned to the ground in 1964. ... "So the four of us went over there and they drag these slabs out," Pringle said. "We couldn't believe it. They had four slabs -- two were from living old trees and two were from this submerged forest of the Columbia. We were just drooling." O'Connor and Bourdeau took samples for radiocarbon testing, and Pringle took samples for tree-ring dating. Pringle found that the sample from a submerged forest tree appears to have died the same year -- 1699 -- as the buried tree that he and Schuster had studied. "I was amazed when I found that these two trees from different sites had died the same year," Pringle said. "It was a victorious moment. And that 1699 date, almost the same as the 1700 earthquake date, just stopped me cold." Despite the finding, however, the mystery of the landslide date remains unsolved. Although the tree-ring and lichen studies point to a slide date around 1700, the radiocarbon dates O'Connor obtained from the tree samples found at the World Forestry Center indicate that the trees died about 1500. "So now we have conflicting evidence," Pringle said. "We have our work cut out for us in trying to resolve these ambiguities from the different dating techniques." Source: Richard Hill, 2002, Landslide Sleuths, from the Oregonian, Portland, Oregon, May 15, 2002, and presented on the U.S. Geological Survey Earthquake Hazards Website, 2004