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