A longer, but more correct phrase, is “perennially frozen ground” (77).The difficulties of the present terminology have been discussed by Bryan (4; 6) who proposed a new set of terms.The southern margin of permafrost is known only approximately, and additional isolated bodies are being discovered as more detailed work is undertaken.The southern margin of permafrost has receded northward within the last century (47).Size, shape, and orientation of the ice crystals differ widely.Discordant structures in sediments around large masses of ice are evidence of growth (38; 77; 78). In the continuous zone of permafrost, the upper limit (permafrost table, 41) is generally within a few inches to 2 feet of the surface.
Permafrost may be absent in the tops of some well-drained low hills.The term permafrost has been widely adopted by agencies of the United States Government, by private organizations, and by scientists and laymen alike.Its use is continued in this article as it is simple, euphonious, and easily understood by all.Ground perennially below freezing but containing no ice has been called “dry per– mafrost” (41).Permafrost composed largely of ice is abundant, particularly in poorly drained, fine-grained materials.
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Unfortunately, the bulk of the literature is in Russian and unavailable to the average reader; some of it has been summarized by Muller (41).