At the cold extremes of the globe, permafrost forms in groundwater systems (Figure 44). Permafrost can be soil, rock or sediment that is saturated or unsaturated, but by definition is frozen for more than two consecutive years.
Where the surface is free of ice, permafrost occurs beneath the “active layer” which is soil, rock or sediment that freezes and thaws each year. Active groundwater flow occurs above and below the permafrost layer which is essentially impermeable. This obstacle to downward drainage causes the surface to be boggy. When substantial volumes of water flow into the groundwater system, areas of unfrozen ground occur within the permafrost (Figure 45).