Swift Creek Landslide - Dynamics

Dynamics

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 scarping, 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 Pleistocene Glaciation. Loose unconsolidated sediment is piled on top of the Chuckanut Formation and is causing a very unstable environment.

This also has an effect on the rate at which this mass is moving. Since there isn't a bond between the conglomerate bedrock and the serpentinite above, the Swift Creek Landslide is moving at a much faster rate.