  We experimentally probe nonlinear wave propagation in weakly compressed
granular media, and observe a crossover from quasi-linear sound waves at low
impact, to shock waves at high impact. We show that this crossover grows with
the confining pressure $P_0$, whereas the shock wave speed is independent of
$P_0$ --- two hallmarks of granular shocks predicted recently. The shocks
exhibit powerlaw attenuation, which we model with a logarithmic law implying
that local dissipation is weak. We show that elastic and potential energy
balance in the leading part of the shocks.
