Evolution of the time-averaged surface density, Σ_p, in the simulation MspE1_M0.7St0.264. The time is given by a label in each panel. The colour palette is saturated in order to highlight how the overdensity of the filament encircling the pebble hole increases in time as more and more pebbles become trapped. A representative trajectory entering the overdense region is shown with a dotted curve and then truncated in order to keep the filament visible. The filament generates a strong negative torque (see Fig. E.1).