Left: column density in a 0.6 × 0.5 × 0.5 pc³ box around the area in which the pillar will form at t ≈ 240 ky. Right: mass-weighted histogram of the line-of-sight velocity in the same box (similar to optically thin observational line spectra). Each spectrum is made on a square of 0.1 × 0.1 pc² drawn on the column density map. The spectra are drawn between − 4 and 4 km s^-1 in 80 bins (horizontal axis) and for mass between 10⁴ and 1 solar masses (vertical axis in log scale). The lateral shocks can be identified in the wide spectra leading to a very broad line width.