Mudboils are round lo elongate, 1–3 m diameter bare soil patches that form on perennially frozen till, marine clayey silt, colluvium, or other poorly sorted sediments (muds) with significant silt and (or) clay content. In Keewatin these muds have low liquid limits (< 20%) and limited plasticity indices (< 10%). Natural moisture contents are very near the liquid limit so that the muds liquefy and flow readily in response to slight changes in moisture content or slight internal or external stresses. When the stresses cannot be readily relieved by downslope movement, mud may burst through a rigid surface layer or carapace, creating or maintaining a mudboil.Ideally, the structures of a mudboil include: (1) an outer turf or stone ring, the type of ring depending on the stoniness of the mud and severity of the climate; (2) a thawed mud substrate (TMS), lying between the permafrost table and a rigid surface layer; (3) a carapace, a semi-rigid, sandy, desiccated layer averaging 30–40 cm thick; and (4) a diapir, in the case of active mudboils, which marks a zone of intrusion of mud from the TMS through the carapace.Driving pressures for diapirism may result from hydrostatic or artesian pressures on a slope, from excess pore-water pressures due to rain or thawing ice lenses, or from loading by animals or man.The shapes of mudboils are controlled principally by slope, the steeper the slope, the more elongate the boil; the nature of their borders depends on the texture of mud, its thickness over bedrock, exposure to tundra fires, and on the suitability of local climate for growth of vegetation.The degree of activity of a mudboil is largely related to the amount of clay and silt in the mud, the steepness of the slope on which it is located, and the amount of moisture available to the mud during the thaw season.