Age-metallicity maps of four selected BBs from each halo. We have normalized the total mass in our solar neighbourhood volume on the estimated mass density (in unevolved stars) by Fuchs & Jahreiß (1998). This results in a present day total stellar mass in halo stars in our selected volume of ~ 5.6 × 10⁷M_⊙, as indicated in the upper left corner of the leftmost panels, which show the summed age-metallicity maps of the four selected BBs. The mass that each of the BBs contributes to the solar neighbourhood volumes is also indicated in the upper left corner of each of their panels, as is a percentage showing the fraction of the total stellar halo building block (after our modifications to match it to the green lines in Fig. 3) to which this mass corresponds.