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Swift Creek Landslide


The Swift Creek Landslide is an active, slow moving landslide located in western Washington, USA, due east of Everson on Sumas Mountain. Asbestos laden sediment originating from the landslide has recently generated much interest in this area. Asbestos is a known carcinogen. EPA sampling has documented asbestos in sediments in Swift Creek and in downstream Sumas River. Average asbestos levels in Swift Creek dredged material exceed the level that, in construction materials, triggers worker safety requirements and material handling and disposal regulations.

Landslides in the Pacific Northwest have a number of causes. One is the geology of the area; layers of loosely consolidated sediments, such as sand, overlay strongly consolidated (less permeable) layers of sediments such as clays. That, in addition to the large amount of precipitation received in Washington state each year, makes for prime landsliding conditions. Earthquakes are thought to help trigger landslides in the Puget Sound, as well as things such as logging and wave erosion. The Swift Creek Landslide is thought to be a smaller reactivation of an older and larger landslide in that area; perhaps an earthquake set the older, then static landslide into motion.

This landslide is roughly half a kilometer in width, and 1.5 kilometers long. The depth of the landslide is approximately 100 meters. The velocity of the landslide varies, but the fastest moving part is at the toe, which moves upwards of 10 meters annually. There is a lack of vegetation on the toe of the landslide because it is over steepened and constantly sliding, providing no stable area for trees to grow. The trees that are growing on the sides and front of the landslide are tilted and disturbed due to the constant motion of the land. However, there is a section of trees in the middle of the slide that are not disturbed and seem to be moving as a connected unit riding on top of the sliding ground. At the head of the landslide, there is pronounced , and on the sides of the slide, there is transform faulting. At about the middle of the slide you can see where, just above the topples of the slide, the trees are moving in motion with the landslide. The soil under the trees is compact; however, just away from the outside of the root ball you can see cracks, which is an indicator of how fast this landslide is moving. If this landslide was moving slowly the rain fall would move this soft sediment into the cracks and fill them. We visited this site the morning after rainfall and the cracks were pronounced up to 1 cm in width. The main ingredient in this landslide is Serpentinite,which is formed from serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The asbestos mineral that forms from this serpentinite is chrysotile.The possible reason why there was conglomerate bedrock underneath the serpentinite could be because of the glacial till or delta deposits flowing out of the Cascade mountain range as it was forming millions of years ago. After this or while this was all happening, the North American plate and the Pacific Plate were colliding causing uplift that eventually formed Sumas Mountain. As the weathering of this place was happening, the landslide started to slip away from the face of the mountain to its current position. About the time that all this was happening, 45 mya, the Chuckanut Formation was ceasing to gain deposits and the volcanic activity known as the North Cascades was in its youth. This area was also heavily affected by the Glaciation. Loose unconsolidated sediment is piled on top of the Chuckanut Formation and is causing a very unstable environment.


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