Normalised surface density Σ/Σ_c0 in the (lnR, θ) plane for the same runs and times as shown in Figure 6. The radial range in this figure is chosen to highlight the annulus from 2 − 4 kpc where feather formation is strongest. We can see the feathers appear as sloped lines that are periodic in azimuth, with length, thickness, orientation and spacing that vary with ℳ_c. The associated time evolution movies are available online.