Here's a basic diagram of a typical slump block landslide, looking very much like the type of landslide that occurred here.
|From the United States Geological Survey (USGS)|
|Pre-landslide view. Note the whitish area at center which marks the area of the recent landslide. (Google Earth Image used for a noncommercial purpose)|
Let's look at the recent geologic history of the Puget Sound region. During the last Ice Age, perhaps 16,000 years ago, the Puget Lobe of the great continental ice sheet moved into our area from the north, part of the vast ice sheet that covered much of North America. The Puget Lobe essentially dammed up the river valleys along the front of the Cascade Range and even moved up these valleys. This is somewhat counter-intuitive because one pictures glaciers moving down a valley, not up. As these river valleys were dammed by the ice sheet, glacial ice margin lakes formed and in those lakes were deposited primarily clays and silts (Qglv - Advance glaciolacustrine deposits). Then the ice sheet moved up and overrode these lacustrine deposits forming another layer of deposits called till (Qgtv), basically a mish-mash of silt, clay, sand, and gravel that is ground and packed under a glacier as it moves. Both the till and advance glaciolacustrine deposits were densely packed under the hundreds of feet of glacial ice.
As the ice sheet receded some 13,000 years ago, it again left ice margin lakes with similar clay and silt deposits to the advance lacustribe deposits (Qgle - Recessional glaciolacustrine deposits). The final top layer of the glacial sediment cake were the recessional outwash deposits (Qgoe) composed mainly of sand and gravel disgorged from the melting and receding glacier. The recessional lacustrine and outwash deposits were never overridden and packed under glacial ice and are comparatively loose and unconsolidated.
As a refresher, the glacial deposits along the side of the river valley are from top to bottom (newest to oldest):
- Recessional outwash (sand and gravel; loose)
- Recessional glaciolacustrine deposit (silt and clay)
- Till (silt, sand, and gravel; dense)
- Advance glaciolacustrine deposits (silt and clay, dense)
|Landslide main scarp. Note that the trees are probably 50 feet, if not 100 feet tall. (Snohomish County)|