Thursday, March 8, 2012

Slides, Mud, Blackberries and Whidbey Formation

As a follow up to yesterday's post on warning signs and unstable slopes, I do spend a fair bit of time at work checking out unstable slopes. This slope provided a fair bit of adventure that tested my mud climbing skills and blackberry avoidance techniques.

Recent slope failure on Salish Sea shoreline

Doesn't take a geologist to know that the above pictured slope is unstable. My job was to assess the risk of additional landslides, how big the slides might be, could homes be at risk and what if anything could or should be done. My primary concern was the presence of a non glacial unit on the bluff called the Whidbey Formation. This formation was deposited by rivers sometime between glacial periods approximately 100,000 years ago. The way I think of the Whidbey is by picturing all the current rivers flowing into Puget Sound from the Cascade Range, Olympic Range, and Coast Range of B.C. (Fraser) progressively filling in Puget Sound. Throw in some big volcanic mud flows and it gets easy to picture.

The Whidbey Formation always makes me nervous. It has been highly compacted by glacial ice having been over ridden by glaciers twice since deposition. As such the Whidbey is very compact. However, this non glacial deposit contains a silt/clay unit that not only contains silt and clay sized particles but also contains clay mineralogy. It also has some inherent structural weaknesses associated with its burial, uplift and exposure. And those very compact silt/clay units allow groundwater to perch on top of them causing saturation of the overlying sediments. The Whidbey is associated with a number of very large landslide complexes around Puget Sound.

As I approached the landslide site, I observed the bluff was underlain by alternating silt and clay typical of one of the Whidbey units.

Compact silt/clay unit of Whidbey Formation

Though compact, the unit readily parts along the silt/clay contacts

The unit also readily turns into soup as I found out with an inadvertent foot step into the soup

Whidbey silt/clay with loose saturated material above

In this particular case, the Whidbey silt/clay unit did not extend to the slope I was exploring. What Whidbey was present was the main channel river unit consisting mostly of gravel. The slope was also underlain by a two glacial drift units. All and all good news from a stability stand point with slides limited to shallow surface failures due to the very steep slope. And I managed to get up and down the slope without any misadventures and only a few holes in my skin from the blackberry brambles. I will say I was a bit muddy afterwards.


Gerard said...

Really enjoy your blog. I learn something from every post. I was wondering what caused the uplift. Is it just the ice being gone?

Dan McShane said...

Gerad: Your asking a fundamental question tnat I ask as well. Some isostatic uplift due to erosion may take place. But there are also several fault zones cutting across the Salish Sea/Puget Sound area and it is in areas where uplift has taken place that the Whidbey is observed.

Anonymous said...

Thanks for the info about the Whidbey formation. Couldn't quickly find anywhere else to learn a bit more about its characteristics. You explained it very clearly!

Anonymous said...

How would you compare the Whidbey Formation to the Lawton Formation? As an interglacial deposit from 100,000 years ago, wouldn't the Whidbey had been subsequently consolidated by the more recent Vashon glaciation?