  We present a new, high-resolution chronographic (age) map of the Milky Way's
halo, based on the inferred ages of ~130,000 field blue horizontal-branch (BHB)
stars with photometry from the Sloan Digital Sky Survey. Our map exhibits a
strong central concentration of BHB stars with ages greater than 12 Gyr,
extending up to ~15 kpc from the Galactic center (reaching close to the solar
vicinity), and a decrease in the mean ages of field stars with distance by
1-1.5 Gyr out to ~45-50 kpc, along with an apparent increase of the dispersion
of stellar ages, and numerous known (and previously unknown) resolved
over-densities and debris streams, including the Sagittarius Stream. These
results agree with expectations from modern LambdaCDM cosmological simulations,
and support the existence of a dual (inner/outer) halo system, punctuated by
the presence of over-densities and debris streams that have not yet completely
phase-space mixed.
