This is a thin, highly reflective sheet of clear ice that forms over a snow surface, often later in winter. 

Solar radiation produces some melting of the snow and with refreezing an ice sheet forms over the top of the crusted and granular snow surface. Acting like a mirror, this ice sheet amplifies the reflection coefficient so it can appear to be a very bright surface when viewed from an aircraft or from elevation some distance away.

(From Dr. Mark Seeley, Professor and Extension Climatologist/Meteorologist
Dept of Soil, Water, and Climate, University of Minnesota, St. Paul)