Two types of structures represent the results of former intensive frost action in central Montana. Involutions, which consist of local deformation and interpenetration in stratified materials, are common throughout the area and occur in a variety of forms. They are believed to have developed by differential freezing and growth of ground-ice, in a seasonally thawed zone overlying perennially frozen ground. Wedge structures, found at a few localities, consist of wedge-shaped vertical fissures in weathered bedrock. The fissures form polygonal networks and have been filled by slumping of surface materials. They are believed to have been opened by ice veins developed when the ground became perennially frozen. A mean annual temperature at least 8° C. below that of the present would be required for the development of perennially frozen ground. This period of colder climate probably occurred at the time of the last glaciation of adjoining areas, which may belong to the Middle Wisconsin substage.
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J.P. Schafer (1949) studied this question.