Some Observations on Boulder-Cored Frost Boils

Abstract

resulting from vertical frost heaving of individual boulders. They were observed in various stages of development on two glacier forelands in Jotunheimen, southern Norway. In their final stage the features comprise a boulder core partially exposed at the surface but retaining a sediment/vegetation capping. Surrounding the boulder is a shallow trough or moat beneath which is a distinct concentration of stones and gravel. Separating this aureole of clastic material from the sides of the boulder is a gap up to 5 cm wide. It is concluded that the 'frost puir mechanism of frost heave is most likely to bring large boulders to the surface. During winter freezing a boulder acts as a heat sink, leading to radial advance of the freezing plane into the surrounding till. Once the boulder has stabilized, the resulting lateral frost sorting concentrates coarser elements of the till diamicton around the embedded portion of the boulder. The formation of boulder-cored frost boils on till surfaces exposed by post-AD 1750 glacier retreat indicates the rapidity of their development and demonstrates the advantages of glacier forelands for the study of periglacial processes and landforms. BOULDER-CORED MOUNDS up to about 1.5m in diameter, and in various stages of development, were observed at sites near the glaciers Boverbreen and Storbreen, Jotunheimen mountains, southern Norway (Fig. 1). These small-scale landforms were found in poorly-drained tills deposited within the 'Little Ice Age' limits of the glaciers. Frost heaving of individual boulders towards the surface is considered to be the main process responsible for their formation. Although up-heaved boulders that have disturbed or partly emerged from the ground surface have been reported in the literature (Dionne, 1966; Washburn, 1969; Price, 1970), little attention has been given to developmental sequences. We describe here a complete sequence, including previously unrecorded forms characteristic of the later stages. Inferences are made concerning the vertical and horizontal frost sorting processes involved in their formation, the significance of which are discussed in the broader context of frost heave, with particular reference to the so-called 'frost push' and 'frost pulL mechanisms. Nature of the sites At Boverbreen, boulder-cored frost boils are associated with a complex 'Little Ice Age' end moraine system (Fig. 1, inset A). The site is immediately adjacent to a lake, dammed by the moraines, which maintains a high moisture content and a high water table in the till. The lake level lies within 0.5m of the ground surface at this site. At Storbreen the boils occur between the two outermost 'Little Ice Age' end moraines, at the foot of the proximal slope of the outermost moraine (Fig. 1, inset B). This site, crossed by ephemeral streams, is characterized by pools of standing water in wet weather. The site at Storbreen was deglaciated after ice retreat from the glacier limit of about AD 1750 (Matthews, 1974, 1977). It is probable that the Boverbreen site