  We report on the crossover from the thermal to athermal regime of an
artificial spin ice formed from a square array of magnetic islands whose
lateral size, 30~nm~$\times$~70~nm, is small enough that they are
superparamagnetic at room temperature. We used resonant magnetic soft x-ray
photon correlation spectroscopy (XPCS) as a method to observe the time-time
correlations of the fluctuating magnetic configurations of spin ice during
cooling, which are found to slow abruptly as a freezing temperature $T_0 = 178
\pm 5$~K is approached. This slowing is well-described by a
Vogel-Fulcher-Tammann law, implying that the frozen state is glassy, with the
freezing temperature being commensurate with the strength of magnetostatic
interaction energies in the array. The activation temperature, $T_\mathrm{A} =
40 \pm 10$~K, is much less than that expected from a Stoner-Wohlfarth coherent
rotation model. Zero-field-cooled/field-cooled magnetometry reveals a freeing
up of fluctuations of states within islands above this temperature, caused by
variation in the local anisotropy axes at the oxidised edges. This
Vogel-Fulcher-Tammann behavior implies that the system enters a glassy state on
freezing, which is unexpected for a system with a well-defined ground state.
