Figure 1: Schematic sketch of the temporal and spatial evolution of the folding instability: The wedge, moving from right to left, displaces the material in the machined solid, causing a compressive stress in front of the wedge. ($1$) Due to this compressive stress, the grains with a larger deformability already form bumps several hundreds of microns in front of the wedge. ($2$) When different bumps are close enough, they often interact to form a fold. ($3$) Under the wedge the fold is flattened to finally form a tear ($4$) when it reappears from underneath the wedge. Thin red lines represent streak lines indicating laminar flow away from the surface.