The particle column density averaged over the azimuthal direction, as a function of radial coordinate x and time t in orbits. The starting time was chosen to be slightly prior to the emergence of a pressure bump (compare with left-most and right-most plots of Fig. 4). The particles concentrate slightly downstream of the gas pressure bump, with a maximum column density between three and four times the mean particle column density. The particles are between 40 and 80 cm in radius (i.e. boulders) for our adopted disc model.