  We study the formation and final structure of the rare earth peak
($A\sim160$) of the $r$-process nucleosynthesis. The rare earth peak forms at
late times in the $r$-process after neutron exhaustion (neutron-to-seed ratio
unity or R=1) as matter decays back to stability. Since rare earth peak
formation does not occur during \nggn \ equilibrium it is sensitive to the
strong interplay between late time thermodynamic evolution and nuclear physics
input. Depending on the conditions the peak forms either because of the pattern
of the neutron capture rates or because of the pattern of the separation
energies. We analyze three mass models under different thermodynamic
conditions. We find that the subtleties of each mass model, including
separation energies and neutron capture rates, influence not only the final
shape of the peak but also when it forms. We identify the range of nuclei which
are influential in rare earth peak formation.

Nuclear Theory;Solar and Stellar Astrophysics