  We propose a collective model formalism which describes the strong parity
shift observed in low-lying spectra of nuclei with octupole deformations
together with the fine rotational band structure developed at higher angular
momenta. The parity effect is obtained by the Schroedinger equation for
oscillations of the reflection asymmetric (octupole) shape between two opposite
orientations in an angular momentum dependent double-well potential. The
rotational structure is obtained by a collective quadrupole-octupole rotation
Hamiltonian. The model scheme reproduces the complicated beat staggering
patterns observed in the octupole bands of light actinide nuclei. It explains
the angular momentum evolution of octupole spectra as the interplay between the
octupole shape oscillation (parity shift) mode and the stable
quadrupole-octupole rotation mode.
