An Ancient Underwater Forest in Lake Sammamish

Reproduced from Washington Geology, Vol. 26, No.2/3 - September 1998 Washington State Department of Natural Resources
Division of Geology and Earth Resources

Greenwood Point is located at the southern end of Lake Sammamish less than a mile south from the projected trace of the Seattle fault (Bucknam and others, 1992) and associated nearby structures. Two embayments along the north shoreline of the point, clusters of tilted tree snags offshore, and depth-finder sonar profiles of the lake bottom provided evidence that the shoreline collapsed and slid into the lake, carrying an ancient forest with it. If a great earthquake occured about 1,100 years ago, as suggested by other studies referenced above, then it is reasonable to speculate that the shoreline could have collapsed as part of a seismically induced landslide.

Logan and Walsh (1995) reported radiocarbon ages of 1,450 +/- 40 and 1,330 +/- 50 yr B.P. (Beta 80713 and 80719, respectively) for wood recovered from snags that protrude from the surface of Lake Sammamish near Greenwood Point. The ages are probably about 200 years too old because they came from the inner (older) parts of the trees (Logan and Walsh, 1995). In a cooperative effort with Gordon Jacoby of Lamont-Doherty Earth Observatory, Columbia University, we employed divers to recover wood from trees that were rooted on the surface of the submarine landslide. Radiocarbon analysis revelaed that one of the trees in the landslide surface drowned about 1,050 +/- 60 years ago. The sample was foor a root of the tree about 15 rings from the bark, and the radiocarbon age should be very close to the time of the tree's death.

For more info on this landslide, click here.

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