  In inviscid solutions of the forced Burgers equation the matter accumulates
in the shock discontinuities. We describe the limit motion of particles
everywhere including the shocks as the trajectories of a discontinuous velocity
field being a generalization of the gradient of the limit potential. The latter
is not differentiable but satisfies some convexity properties which guarantee
the existence of the gradient. It turns out that for such discontinuous
gradient ordinary differential equations there are natural existence,
uniqueness, and continuity theorems. The general results are applied for
investigation of formation and motion in plane of massive points which are
interpreted as various clusters in the adhesion model of the Universe.
