Column density of the dense gas only (n_H > 100 cm⁻³) integrated along the z-direction in the pure hydrodynamic runs of the simulations. The left panel shows a 3.5 Myr snapshot and the right panel shows a 5.5 Myr snapshot (after the shells have collided). The axes are in parsecs. The R₁ and R₂ labels refer to the radii ranges plotted in Figure 8. These figures are for the pure hydrodynamic run, but there is no significant difference to the self-gravitating run.