  We explore the initial moments of impact between two dense granular clusters
in a two-dimensional geometry. The particles are composed of solid CO$_{2}$ and
are levitated on a hot surface. Upon collision, the propagation of a dynamic
"jamming front" produces a distinct regime for energy dissipation in a granular
gas in which the translational kinetic energy decreases by over 90%.
Experiments and associated simulations show that the initial loss of kinetic
energy obeys a power law in time, $\Delta E=-Kt^{3/2}$, a form that can be
predicted from kinetic arguments.
