Velocity shear across dense clumps at t = 6.800 Myr: (a) horizontal component of the gas velocity, v_z, is shown in the upper half-plane and gas density (log₁₀ ρ/g cm⁻³) in the lower half-plane. Dense clumps with ρ >  10⁻²² g cm⁻³ are indicated by the black contours, and the seven almost-vertical lines show the slices where the v_z is plotted in panels (b) and (c). Panel (b) shows v_z in the colour-coded lines across the three downstream dense clumps, with distance measured from the large-z end of the line, to show the velocity shear at the edge of the clumps. Panel (c) shows the same but for the upstream dense clumps.