Tuesday, April 10, 2012

Lily Point III: West Bluff Landslide

A previous post on Lily Point discussed the bluff north of the point lily-point-point-roberts-part-ii-north. The bluff to the west is also attention getting.

Approaching the West Bluff from the Lily Point I noted lots of trees with roots depsoited along the up strand of the beach.

West Bluff and the source of the trees

I do not recall seeing latoral drift tree sorting before. The big trees brought down the landslide are lag driftwood with the smaller trees having been transported to the Lily Point sand and gravel spit.

The toe of the slide extends all the way down this view of the beach

View of the toe of the slide with burried trees at the top of the beach

The slide mechanics were obscured. Its a huge slide area that extends all the way to the top of the bluff with a vertical headwall scarp. Hints can be seen at the exposure at the North Bluff; layers silts and clays. These units are most frequently the cause of bib rotational slides along Salish Sea bluffs. Hugh Shipman refernced the presence of these uits at Gravel Beach Blog and Dames and Moore noted the presence of lake sediments at the base of the bluff in a report they wrote regarding the slide in the 1990s: before the slide made another big movement in 1999. However, the collapsed material from above the silt/clay units had buried the silt/clay unit at least for the time being. One way this bluff is different besides the big slide than the North Bluff is presence of a thicker sequence of glcial marine sediment along the upper bluff slope. Most of the slide debris sediment at the time of my visit consisted of drift.

Block of glacial drift with cobbles and pebbles embedded in silt/clay
Glacial drift

Note the boulder within the top soil overhang above the top pf the slide scarp. Not sure how that rock is still hanging there!

View across the headwall scarp.

View of Lummi Peak, Lummi Island on the left and Mount Constitution, Orcas Island on the right from the top of the head wall scarp

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