Distribution of the gas mass as a function of the line-of-sight velocity for the snapshots in Fig. 13. Top: the spectra are taken when the pillar is forming and the shell curved, just before the shell has collapsed in on itself. The blue- and red-shifted components can be clearly identified in both cases and are associated with the curved shell. Bottom: after the collapse, when the pillar is in its growing state, the spectra display a single peak.