I triggered the avalanche from the top of the slab, where it was thin, by jumping up and down near a tree. The slab was composed of recently drifted snow. As it ran it entrained the faceted snowpack all the way to the ground, adding volume to the avalanche.
Snowpack
The snowpack was entirely faceted with deep ski penetration except for where I triggered an avalanche. This was the only I found with a slab. It was easy to see the recent drifting from the rippled appearance of the snow surface.