  We study out-of-equilibrium dynamics in the quantum Ising model with
power-law interactions and positional disorder. For arbitrary dimension $d$ and
interaction range $\alpha \geq d$ we analytically find a stretched exponential
decay with stretch power $\beta = d/\alpha$ for the global magnetization and
ensemble-averaged single-spin purity in the thermodynamic limit. We reveal
numerically that glassy behavior persists for finite system sizes and
sufficiently strong disorder. We conclude that the magnetization decay is due
to interaction induced dephasing while entanglement builds up at a smaller rate
evident from the decay of single-spin purity, thus providing a microscopic
understanding of glassy dynamics in disordered closed quantum systems.
