  In order to characterise the cloud structures responsible for the formation
of high-mass stars, we present Herschel observations of the DR21 environment.
Maps of the column density and dust temperature unveil the structure of the
DR21 ridge and several connected filaments. The ridge has column densities
larger than 1e23/cm^2 over a region of 2.3 pc^2. It shows substructured column
density profiles and branching into two major filaments in the north. The
masses in the studied filaments range between 130 and 1400 Msun whereas the
mass in the ridge is 15000 Msun. The accretion of these filaments onto the DR21
ridge, suggested by a previous molecular line study, could provide a continuous
mass inflow to the ridge. In contrast to the striations seen in e.g., the
Taurus region, these filaments are gravitationally unstable and form cores and
protostars. These cores formed in the filaments potentially fall into the
ridge. Both inflow and collisions of cores could be important to drive the
observed high-mass star formation. The evolutionary gradient of star formation
running from DR21 in the south to the northern branching is traced by
decreasing dust temperature. This evolution and the ridge structure can be
explained by two main filamentary components of the ridge that merged first in
the south.
