[zamadatix]

The sand is going down. Each tick one bit of sand falls from the top left to the bottom of that valley. Over many ticks the sand from the top accumulates up the side of the valley, from over the "cliff". The strange bit is the pieces teleport in one tick one at a time. No matter how you do it you'll end up with some sort of tradeoff like that when using pixel grids instead of particles.