Groundwater flowing through discontinuous bodies of ice-rich permafrost thermally erodes tunnels, cavities, caverns and pits. Near Fairbanks, surface water entering small cracks and tunnels, augmented by meltwater from ground ice, percolates downward through ice-rich silts towards a water table 5-30 m beneath the surface. The percolating water often enlarges depressions initiated by thermokarst subsidence, forming steep-walled, sinkhole-like features (1.5-6 m deep and 1-10 m across) known as thermokarst pits (Pewe 1954, 1982; Higgins et al. 1990). The pits develop within 3-30 years after vegetation clearance. Tunnels extend from the base of some pits, and current marks and waterlain silt on some tunnel floors indicate intermittent underground streamflow. In some pits, the floor of the boreal forest — held together by roots — is suspended over cavities 0.5 m deep formed by thermokarst subsidence beneath the root layer (Osterkamp et al. 2000). Caverns and pits sometimes fill with sediment to form casts that can be identified in stream banks and mining cuts in Alaska.
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