The sheeting of rocks and their disintegration, thought to be due to thermal expansion, at least on small structures. Waragai (1998) Arct. Alp. Res. 30 finds that high rates of change in rock temperature may produce exfoliation sheets in periglacial environments. Le Pera and Sorriso-Valvo (2000) Geomorph. 34, 3–4 think exfoliation boulders, now found at the ground surface, may have been formed at depth.