The apparent similarity of glacial and periglacial hummocky landscapes often leads to errors in delimiting former glaciated and perennially frozen areas by means of aerial mapping. To avoid the misinterpretation a set of morphological criteria is suggested, based on inherent difference between relief development by thermokarst sinking in permafrost terrains, on one hand, and by glaciokarst piercing of stagnant glaciers, on the other. Inverted glacial hillocks have an irregular shape, being produced by piling of superimposed filling of multi-floored glaciokarst cavities. Flatter knolls of periglacial terrains, originating from dust-filled surface thaw lakes, are fairly regular in plan and often show a preferred orientarion.