Washington State Department of Ecology
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Puget Sound Geology

Upper layers
Glacial till forms a dense, concrete-like cap. Fractures in till allow water to flow down to lower layers.  
 
layers
common soil layer sequence
Middle layers
Water soaks down through the middle sand and gravel layers.  
 
Bottom layers
Lakebed silts, clays, and older dense sediments make up the lower layers. As water collects on top of the less permeable materials, a zone of weakness is formed.
 
Common soil layer sequence
layers of till and sand on top of impermeable silts and clays

The geology of Puget Sound bluffs varies from one location to the next. Although the generalized geologic conditions described here occur broadly and demonstrate common landslide-prone features, they do not apply to all shorelines and landslides are certainly not limited to these circumstances. The sequence of geologic layers is usually more complex than what is illustrated here.
 
links

Landslides, 1996/97, Department of Natural Resources.
Puget Sound Bluffs: The Where, Why, and When of Landslides Following the Holiday 1996/97 Storms.
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