  The angular-momentum distribution of classical Cepheids in the outer Milky
Way disc is bi-modal with a gap at
$L_\mathrm{gap}=2950\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{kpc}$,
corresponding to $R=13\,\mathrm{kpc}$, while no similar feature has been found
in the general population of disc stars. We show that star formation in
multiple spiral arm segments at the same azimuth leads to such multi-modality
which quickly dissolves and only shows in young stars. Unlike other
explanations, such as a 1:1 orbital resonance with the Galactic bar, this also
accounts for the observed steepening of the stellar warp at $L_\mathrm{gap}$,
since the adjacent spiral arms represent different parts of the warped gas
disc. In this scenario the gap is clearly present only in young stars, as
observed, while most purely stellar dynamical origins would affect all disc
populations, including older disc stars.
