A highly mobile layer, periodically thawing, located above the permafrost in tundra regions, and ranging in depth from a few centimetres to 3 m. Its thickness depends on slope angle and aspect, drainage, rock and/or soil type, depth of snow cover, and ground-moisture conditions; clearing vegetation will increase the depth and mobility of the active layer. See Zhang et al. (2006) Nat. Snow & Ice Data Center on active layers in the Russian Arctic, and M. Lecki (2006). The mobility of the active layer is due to the tundra vegetation, which scarcely binds it together, so that it moves on slopes as gentle as 2°. Thawing may occur daily or only in summer; on refreezing, the active layer may expand, especially if silt-sized particles predominate.

Periglacial processes in the active layer include frost heaving, frost thrusting, ice wedging, gelifluction, and the formation of patterned ground.