Distribution of the surface density Σ of star clusters in the 2nd Galactic quadrant versus their distance d_XY from the Sun projected onto the Galactic plane. The density distribution of all clusters is given in black in both panels. In the upper panel we show the distribution of young clusters (log t < 7.9) in blue, in the lower panel that of older clusters (log t ≥ 7.9) in red. The dotted lines indicate the completeness limit, the dashed horizontal lines correspond to the average density at d_XY ≤  2 kpc for a given group of clusters.