Saturday, February 20, 2016

An Unstable Day

I visit a lot of steep potentially unstable slopes for my work. Yesterday was a day of seeing very recent landslides and one in action.  
When I arrived at my first site, I noted a couple of city workers down the street from my location. A couple of boulders had come loose and landed on the city street below.   
After a late afternoon site visit to another site, I ended the day taking a hike along a shore reach I had not previously visited on Whidbey Island. I heard some deep thumping noises that sounded like artillery. While Whidbey Island is known for loud noise ("the sound of freedom" from the jets at the naval airbase), this was not a sound I had heard before.
It got louder as I headed north and then I saw the source: 
The large cracked block dropped off the bluff face when I was about 200 meters away. The bluff was acting like a glacial ice front in the sea, periodically calving off blocks of soil. This reach of bluff is all very compact glacial drift from the last glacial period - a rather thick section of till. As such the blocks are very cohesive and hard - almost like concrete.

However, the till does weather and becomes soft with water. When I walked over the toe of the area of collapsed blocks of till I encountered very fluid mud.

The source of water and the calving process were fairly evident on this wet day. Water has been flowing behind the dialation cracks on the till bluff face. Not a surprise given the amount of rain.
On my way back I took a picture in the dim light of a hanging bit of till on the bluff face.
 A minute later the slab had peeled off the bluff.

Alas I was not patient (it was late and raining hard) enough to keep the camera ready or quick enough to capture the event with the camera. I will simply say it was a cool thing to witness the slab rotate and fall off the bluff face.

1 comment:

Anonymous said...

Glad you caught all that, but gladder that you didn't get too close.