RIBBED MORAINES IN SWEDEN - DISTRIBUTION PATTERN AND PALAEOGLACIOLOGICAL IMPLICATIONS

A detailed map of ribbed moraines in central and northern Sweden is pr esented. They are frequent features over much of the mapped area, and the distribution has well defined Limits. The most abundant ribbed mor aines are situated in close connection to areas that are inferred to h ave been cold-based during the retreat of the Late Weichselian Ice She et, while outer, fully warm-based areas, lack ribbed moraine. Observat ions have also been made on a more local scale, where cold-based patch es grade into ribbed moraines via a zone of a broken-up till cover. A model of origin is presented, which includes contracting of a frozen c are area of a retreating ice sheet. At the transition from non-sliding to sliding conditions there will be high stresses resulting in local extensional flow at the base of the ice sheet. A rising phase change s urface (frozen-thawed material) would cause detachment and 'boudinage- like' breaking up of a pre-existing till cover, thawed at the bedrock contact, but still frozen to the ice sole. The orientation of ice flow during the formation of the ribbed moraines closely matches the ice f low directions of the deglaciating Late Weichselian Ice Sheet. Thus, r ibbed moraines are considered to be deglaciation landforms, typical of shrinking cold-based zones of a retreating ice sheet.