The Gevsjön Formation (calcareous phyllite — garbenschiefer) of the Tännforsfältet Köli is divided into three tectonic units, separated by two important mylonite zones. These mylonite zones exhibit an earlier ductile phase of deformation and a later brittle phase with brecciation of the mylonitic foliation. Associated with this brecciation pseudotachylites have been recognized at four localities. A fifth locality is so far not directly related to any mylonite zone, but occurs close below the upper mylonite zone. The change from mylonite formation (ductile deformation) to the development of breccias and pseudotachylites represents a change in physical conditions during translation of the nappes. Early movement occurred at higher and the later movement at lower temperatures, implying that the thrusting involved progressive translation of the Köli Nappes to higher structural levels.