The locations of the two shock fronts (forward shock: filled squares; reverse shock: open diamonds) for galactic distances z ≤ 100   kpc and compression ratios of the gas as a function of time. Already at early times (t ≤ 20  ×  10⁶years) the shock propagation almost becomes constant. The solid line denotes the compression ratio of the forward gas subshock, the dashed line depicts the total compression ratio including the contribution form the cosmic ray precursor. The dashed-dotted line plots the gas compression ratio of the reverse shock growing in time (see text for more details).