  We report on measurements of the angular dependence of the irreversibility
temperature $T_{irr}(\theta) $ in $YBa_2Cu_3O_{7-\delta }$ thin films, defined
by the onset of a third harmonic signal and measured by a miniature Hall probe.
From the functional form of $T_{irr}(\theta)$ we conclude that the origin of
the irreversibility line in unirradiated films is a dynamic crossover from an
unpinned to a pinned vortex liquid. In irradiated films the irreversibility
temperature is determined by the trapping angle.
