Frost Heave

Freezing progresses down from the surface.  Meanwhile, the ground expands upwards, lifting a stone by first freezing onto the upper surface.  The freezing front progresses downwards and space is created below the stone as it is lifted.  Ice crystals grow into this space and push up the stone.  Thawing progresses down from the surface causing contraction and lowering of the surface level but the stone is still supported by the ice crystals which have collected beneath it.  Thawed sediment collapses around the stone supporting it.  The stone is held in thawed sediment while ice crystals melt and space is filled by collapsing sediment.

Patterned ground is formed as stones collect, or stone polygons when ice wedges (see thermal contraction) are present.  These stones remain in place when the gradient is lower than 6.  Shapes elongate on steeper slopes to form stone stripes or garlands.