  Thin polymer films have striking dynamical properties that differ from their
bulk counterparts. With the simple geometry of a stepped polymer film on a
substrate, we probe mobility above and below the glass transition temperature
$T_{\textrm{g}}$. Above $T_{\textrm{g}}$ the entire film flows, while below
$T_{\textrm{g}}$ only the near surface region responds to the excess
interfacial energy. An analytical thin film model for flow limited to the free
surface region shows excellent agreement with sub-$T_{\textrm{g}}$ data. The
system transitions from whole film flow to surface localized flow over a narrow
temperature region near the bulk $T_{\textrm{g}}$. The experiments and model
provide a measure of surface mobility in a sample geometry where confinement
and substrate effects are negligible. This fine control of the glassy rheology
is of key interest to nanolithography among numerous other applications.
