Density cut of the formation of a pillar in an idealized simulation of a clump embedded in a homogeneous medium (left: Tremblin et al. 2012a) and in a turbulent medium at Mach 1 (right: Tremblin et al. 2012b). The black arrows represent the velocity field of the neutral gas. The compressed layer can be identified with the velocity field and the high density (around 10⁴ cm^-3 in yellow). Top: the snapshots are taken when the pillar is forming, just before the compressed layer has collapsed in on itself. The high curvature of the compressed layer can be clearly identified in both cases. Bottom: after the shell has collapsed, the pillars are formed.