  We present an examination of the metallicity distribution function of the
outermost stellar halo of the Galaxy based on an analysis of both local (within
4 kpc of the Sun, ~16,500 stars) and non-local (~21,700 stars) samples. These
samples were compiled using spectroscopic metallicities from the Sloan Digital
Sky Survey and photometric metallicities from the SkyMapper Southern Survey. We
detect a negative metallicity gradient in the outermost halo (r > 35 kpc from
the Galactic center), and find that the frequency of very metal-poor ([Fe/H] <
-2.0) stars in the outer-halo region reaches up to ~60% in our most distant
sample, commensurate with previous theoretical predictions. This result
provides clear evidence that the outer-halo formed hierarchically. The
retrograde stars in the outermost halo exhibit a roughly constant metallicity,
which may be linked to the accretion of the Sequoia progenitor. In contrast,
prograde stars in the outermost halo exhibit a strong metallicity-distance
dependence, indicating that they likely originated from the accretion of
galaxies less massive than the Sequoia progenitor galaxy.
