  We identify two distinct low-energy sectors in the classical isotropic
antiferromagnetic Heisenberg spin-S chain. In the continuum limit, we show that
two types of rotation generators arise for the field in each sector. Using
these, the Lagrangian for sector I is shown to be that of the nonlinear sigma
model. Sector II has a null Lagrangian; Its Hamiltonian density is just the
Pontryagin term. Exact solutions are found in the form of magnons and
precessing pulses in I and moving kinks in II. The kink has `spin' S. Sector I
has a higher minimum energy than II.
