crystosphene

\\ˈkristəˌsfēn\ noun (-s)

Etymology: cryst- (irregular from Greek krystallos ice) + -o- + Greek sphēn wedge — more at crystal, spoon

: a buried sheet of ice under the tundra of northern America formed by the freezing of spring water which rises from the rock beneath alluvial deposits or under swamps and spreads laterally at the zone of freezing