Tuesday, November 9, 2010

Slide Mountain Landslide on the North Fork Nooksack River

Last week before heading to eastern Washington I had a project up the Nooksack River Valley in Whatcom County. Before I left I had noticed in LIDAR imagery that the North Fork Nooksack valley floor just past Maple Falls appeared very lumpy. There are lots of old very large landslides on the steep mountain slopes in the North Fork Nooksack valley. A large landslide has been recognized on the north side of the of the aptly named Slide Mountain and was included on a geologic map of the area by Moen in 1961. The bedrock failure took place within the Chuckanut Formation. The Chuckanut Formation consists of layered sand stone, mud stone and occasional coal seams. The layers dip steeply to the north on the north end of Slide Mountain.

LIDAR of north end of Slide Mountain and lumpy ground on valley floor

Google Earth oblique view looking up the North Fork Nooksack valley to the east with failure area on Slide Mountain marked.

So while driving up the valley I slowed down and noticed that indeed the ground along this stretch of highway is a bit lumpy with boulders strewn about in the woods including the one shown below.

One of numerous boulders on the valley floor

Area along highway east of Maple Falls where boulders from the slide can be seen

Based on the scattered boulders that extend across the valley, this slide covered the entire valley floor and likely at least temporarily backed up the river. The large boulders from the slide are limiting the channel migration such that the river is entrenched along this reach of the river versus the multiple braided channels more typical upstream and downstream. Along portions of the river bank it appears that the river bank is lined by rip rap boulders to prevent erosion, but in this case the rip rap is natural.

Rocks from landslide line river bank slowing erosion

Another look at the LIDAR shows that the prior to the slide the river had carved into the glacial sediments on the north side of the valley, but now the river is entrenched into the slide deposit. I am not aware of anyone having dated the landslide, but if buried trees could be found beneath the slide deposit, a date could be derived.

One added treat while walking and crawling along the river bank, I spotted this palm frond fossil on one of the sandstone boulders.

Palm frond fossils in boulder within the landslide deposit 


Sam Crawford said...

Palm fronds. Planet must have been quite a bit warmer.

PS: that's some serious erosion of a car key. Well worn.

Dan McShane said...

Leaf fossils in the Chuckanut indicate some variableness in temperature. There is evidence that indeed the Earth was warmer for at least part of the Eocene and that warm period correlates with higher CO2.
There are some other factors. At the time of the deposition a large embayment was located top the south of the area extending into eastern Washington so the area may have been more exposed to a moderating body of water. In addition the area has been transported northward along strike slip faults.