  A continuum theory is used to predict scaling laws for the morphological
relaxation of crystal surfaces in two independent space dimensions. The goal is
to unify previously disconnected experimental observations of decaying surface
profiles. The continuum description is derived from the motion of interacting
atomic steps. For isotropic diffusion of adatoms across each terrace, induced
adatom fluxes transverse and parallel to step edges obey different laws,
yielding a tensor mobility for the continuum surface flux. The partial
differential equation (PDE) for the height profile expresses an interplay of
step energetics and kinetics, and aspect ratio of surface topography that
plausibly unifies observations of decaying bidirectional surface corrugations.
The PDE reduces to known evolution equations for axisymmetric mounds and
one-dimensional periodic corrugations.
