Dagnabit!!! I knew I had something!!!

I have a jar full of those glass retro-reflector beads used to make paint stripes on roads visible at night in your headlights. Anyway, I like to turn the jar on it's side and slowly rotate it to watch the pattern of collapse as the slope exceeds the maximum angle of repose.

The interesting thing I noticed is if I watch the back side, I see that a split second before the slope gives way, about 2-3 mm of depth suddenly compacts about 10-20%.

Brian