Volume rendering of the logarithm of the density field in the 3D hydrodynamical simulations. Left panel: snapshot before the shell collision, about 3.5 Myr after the start of feedback. Right panel: snapshot about 1 Myr after the start of the shell collision (5.5 Myr after the start of feedback). The warm medium in this figure is made transparent to allow for a view of the dense clumps, as indicated by the gray/black curve along the colorbar. The mass density units assumed here are 10⁻²² g cm⁻³ (≈60 cm⁻³) and the size of the box is the total 200 pc simulated.