Time evolution of a) the vertically integrated dust-to-gas ratio and b) surface density of planetesimals. The outer parts of the disc are depleted of dust because they are drift dominated. At the same time, there is a temporary pile-up of dust in the fragmentation-dominated inner disc, which enables planetesimal formation via the streaming instability. The dashed line in panel b) corresponds to the surface density of solids in the MMSN model.