ice rind
From Glossary of Meteorology
(Also called ice crust.) A thin but hard layer of sea ice, river ice, or lake ice.
Apparently this term is used in at least two ways: 1) for a new encrustation upon old ice; and 2) for a single layer of ice usually found in bays and fjords where freshwater freezes on top of slightly colder seawater.
See ice crust.