Recent experiments in the actinide region, using Gammasphere and the evaporation residue detector HERCULES, have covered the territory between N = 126 and the center of static octupole deformation at N = 134. The 220Th nucleus and the neighboring 218Ra and 219Th nuclei mark the emergence of quadrupole-octupole collectivity in this mass region. Their octupole bands have B(E1)=B(E2) ratios which are typical for the region, but the level spacings do not concur with a rotational-like behavior. In addition, a spin-dependent staggering of the B(E1)=B(E2) ratios is evident. These features can be described, based on a phonon picture, by a constant-frequency tidal-wave mode for a reflection-asymmetric nuclear surface.