  Torsional oscillator experiments on solid 4He show frequency changes which
suggest mass decoupling, but the onset is broad and is accompanied by a
dissipation peak. We have measured the elastic shear modulus over a broad
frequency range, from 0.5 Hz to 8 kHz, and observe similar behavior -
stiffening and a dissipation peak. These features are associated with a
dynamical crossover in a thermally activated relaxation process in a disordered
system rather than a true phase transition. If there is a transition in the DC
response, e.g. to a supersolid state, it must occur below 55 mK.
