As winter snow accumulates, and one walks or drives over each layer of newly arrived snow, it develops into what we call hard pack.  Hard pack is a relatively innocuous substance, somewhere between ice and snow, that is, until it begins to get warm.  Then the hard pack develops a layer of ice on the surface, and becomes increasingly slick.  As break-up arrives the hard pack turns into slush, several inches thick or thicker dependent upon the amount of snow that winter and the amount of shoveling or plowing that did or did not get done.

If you were wondering, hard pack is most commonly considered BAD.  Especially during break up.