A rock slide slid into the upper end of Diablo Lake in eastern Whatcom County. (I love reminding people that Diablo Lake is in Whatcom County). The slide blocks the access road to the Ross Lake Dam power house.
The slide took place in vertically foliated banded biotite schist and is part of Skagit gneiss complex. This formation consists of very highly metamorphosed sandstone that has been heated pressurized during deep burial during the Cretaceous. These rocks were buried as much as 30 km below the surface approximately 75 million years ago. The rock failure appears to be the result of a combination of vertical foliation with cross cutting horizontal joints. And of coarse the slope is a cliff and gravity and some freeze thaw will cause things to fall down. It won't be the last in this area of very steep slopes.