Sunday, March 23, 2014

Geology of Stilliguamish Blocking Slide

This post is more on the lines of what is known about the geology of the slide area. This landslide area is well known and has been a big problem for the past roughly 20 years. This was very much like the Ledgewood Slide on Whidbey Island. When I heard about a blocking slide on the Stilly, I had two candidate locations in mind and this was the first one. I say this not to brag, but to emphasize that this particular slide is well known amongst geologists as well as fisheries and forestry policy wonks. I do not do much work in the Stilly, but last summer had some work near here and took a little side trip to check it out. My thought at the time was "Yikes, this is a bad slide."

                               Geologic Map of the Sauk 1:100,000 Quadrangle.pdf

The above map shows the larger scale geologic features of the landslide area that blocked the Stilliguamish. Ql on the map noted by the yellow area with red dots are landslides. As can be seen there are a number of areas designated as landslides. A word of caution though. Not all landslide designations are the same and there is a bit of inconsistency on maps including this one. This is a 1:100,000 scale map so there is a lot of detail that can not be presented. That is true of the landslide units as well as other units on the map. This map is a big picture map that is a compilation of decades of work by many different geologists. Part of the Sauk map utilizes mapping I did in 1990 miles from this location. The area I mapped covered 150 square miles - I guarantee there was a lot of detail I did not cover, but even some of the detail I had on my map could not be included at the sale of this map sheet. 

The geologic unit that is within the landslide area is mapped as Qvr (mustard green?). Qvr stands for Vashon recessional sediments. These sediments were deposited during the late stages of the last glacial period. During the late stages of the ice age the valley was filled with glacial derived meltwater sediment. River gradients were different at that time due to higher local sea level as result of the mass of the Puget ice lobe having pushed the crust downward. As the land rebounded the river gradients became steeper and the river incised down through the former valley fill creating steep side slopes in unconsolidated sediment - a recipe for landslides. That is particularly true when the river is hard up against the base of the steep slope as was the case at this recent big slide (and on the recent slide on the Nooksack).

I should add that one of the interesting features of the North Fork of the Stilly is that this valley was for a time occupied by the Sauk River (glacier-peak-and-sauk-stiiliguamsih).

A more detailed map of the slide area was completed by Dragovich and others (2003)

This map also shows extensive landslide areas. On this map the designation is Qls. The current slide is just part of a larger area along the valley that has indications of landslides.

If you click and enlarge the image, there are designations of units within the recent slide. The map indicates the slope is underlain by advance glacial lake sediments (Qgle). These sediments were deposited when the outlet to the valley was blocked by the advancing ice in the Puget Ice lobe that dammed the river forming a lake. The next unit up the slope is Qgtv. This is glacial till and was deposited directly by glacial ice when he ice advanced up the valley and covered the area approximately 16,000 years ago. Both of these units will be very hard and compact due to the mass of glacial ice that had compacted them. But the real trouble is from the units above these. As the glacial ice receded, a new lake formed by the blocking ice and a recessional glacial lake formed (Qgle). This unit was never compacted and on top that unit is a sandy layer. Picture the valley being filled clear across by these units and the floor of the valley being a couple or three hundred feet higher than it is today. The river (first the Sauk and NF Stilly combined and then latter just the NF Stilly) down cut and incised through these sediments. These two late glacial units are particularly susceptible to slope failures and have failed on multiple occasions at this location (https://slidingthought/north-fork-of-stillaguamish/ and as well as elsewhere along the sides of the NF Stilly valley. 

A final note. The work done on the 2003 map by Dragovich and others with the Washington State Department of Natural Resources is the kind of basic research that is so informative. The mapping program at the DNR has been outstanding and has been funded by both State and Federal funding. This kind of mapping is enormously important. How it is translated to public benefit is another matter.


Unknown said...

You mention that you had two candidate locations. May I ask where the other location is? What do you think are the chances of a similar slide taking place there, especially given the continued rain in the forecast?

Anonymous said...

As a geologist, I really appreciate the expert analysis. Thanks for taking the time to work this up and post it.
M.E. Renz
Columbus, Ohio

Anonymous said...

Youdaman Dan