In the image below, you can see the plume rising what I would guess is a couple thousand feet above valley level. Typically the effluent from a power plant is warmer than the environment when it is released from the stack. As a result, it is positively buoyant and rises. The plume cools, however, as it rises due to both expansion and in some instances the entrainment of environmental air. The smoke layer seen below forms at what is known and the equilibrium level, the level at which the plume has cooled to the temperature of the environment.
The sounding for yesterday afternoon shows an ideal environment for a pancaked smoke layer like the one shown above as it was absolutely stable from the valley floor to about 700 mb, which is near the crest of the Oquirrhs. The equilibrium layer was lower than the Oquirrh crest, so the smoke spread in a layered, laminar fashion due to the strong atmospheric stability.