  The abundance scatter of heavier r-process elements (Z > 56) relative to Fe
([r/Fe]) in metal-poor stars preserves excellent information of the star
formation history and provides important insights into the various situations
of the Galactic chemical enrichment. In this respect, the upper and lower
boundaries of [r/Fe] could present useful clues for investigating the extreme
situations of the star formation history and the early Galactic chemical
evolution. In this paper, we investigate the formation of the upper and lower
boundaries of [r/Fe] for the gas clouds. We find that, for a cloud from which
metal-poor stars formed, the formation of the upper limits of [r/Fe] is mainly
due to the pollution from a single main r-process event. For a cloud from which
metal-poor stars formed, the formation of the lower limits of [r/Fe] is mainly
due to the pollution from a single SN II event that ejects primary Fe.
