  Combining model experiments and theory, we investigate the dense phases of
polar active matter beyond the conventional flocking picture. We show that
above a critical density flocks assembled from self-propelled colloids arrest
their collective motion, lose their orientational order and form solids that
actively rearrange their local structure while continuously melting and
freezing at their boundaries. We establish that active solidification is a
first-order dynamical transition: active solids nucleate, grow, and slowly
coarsen until complete phase separation with the polar liquids they coexists
with. We then theoretically elucidate this phase behaviour by introducing a
minimal hydrodynamic description of dense polar flocks and show that the active
solids originate from a Motility-Induced Phase Separation. We argue that the
suppression of collective motion in the form of solid jams is a generic feature
of flocks assembled from motile units that reduce their speed as density
increases, a feature common to a broad class of active bodies, from synthetic
colloids to living creatures.
